Weighting Material in Drilling Fluids: Sagging Mechanisms, Influencing Factors, and Prevention Strategies

Abstract

Drilling high-temperature and high-pressure (HTHP) formations requires high-density drilling fluids, commonly formulated using weighting materials such as Barite, hematite, Micromax, ilmenite, magnetite, and manganese tetroxide. However, these materials are prone to sagging, resulting in density variations that compromise wellbore stability, pressure control, and overall drilling safety. This review addresses the critical challenge of weighting material sagging by summarizing recent advances in understanding sagging mechanisms and mitigation strategies. The sagging behavior of weighting materials is strongly influenced by factors such as density mismatch, particle size distribution, fluid viscosity, flow regime, temperature, pressure, and wellbore inclination. Recent findings demonstrate that sagging can be mitigated by blending different weighting agents to optimize particle distribution and surface characteristics, thereby enhancing dispersion and reducing settling. In addition, reducing particle size improves suspension stability and increases yield strength, which supports particle suspension. Surface modification enhances the dispersion of weighting agents in drilling fluid, and thereby mitigating sagging. Furthermore, antisagging additives increase viscosity and form gel networks that entrap particles, minimizing aggregation and enhancing stability. Despite these advancements, challenges remain regarding environmental impact, additive compatibility, and high production costs. Future research should focus on developing biodegradable, multifunctional additives, employing advanced material engineering for tailored particle design, and implementing real-time monitoring systems for effective sag control. This review fills a critical gap by integrating materials science, fluid dynamics, and sustainability, offering a comprehensive foundation for next-generation drilling fluid design.

1. Introduction

In modern oil and gas exploration, drilling fluid is an indispensable and important part, which is not only the core element to ensure the safety and efficiency of drilling, but also plays a key role in the interpretation of the bottom formation, the stability of the well wall, and the drilling speed. In high-temperature and high-pressure (HTHP) formations, in order to balance the pressure of formation and prevent the instability of the wellbore, high-density drilling fluid is needed. Particularly, high-density drilling fluids play a critical role in ensuring well control and operational safety in HTHP environments. In such conditions, formation pressures are extremely high, and inadequate mud weight can lead to severe risks such as wellbore instability, influx of formation fluids, and blowouts. By increasing the density of the drilling fluid, a sufficient hydrostatic pressure is generated to balance or slightly exceed the formation pressure, thereby maintaining wellbore stability and preventing kicks. Moreover, high-density drilling fluids help to minimize the risk of differential sticking by providing effective support to the wellbore wall. They also contribute to efficient cuttings transport under challenging HTHP conditions, where high viscosity and density are needed to overcome extreme downhole stresses. Therefore, weighting agents are employed to increase the density of the drilling fluid. Commonly used weighting agents include Barite, Micromax, hematite, magnetite, ilmenite and manganese tetoxide.

The addition of these weighting agents can increase the density of drilling fluid, whereas the high density of these weighting materials makes them easy to precipitate in drilling fluid, resulting in some adverse consequences. First of all, the deposition of weighting materials can cause changes in the rheological properties of drilling fluid, reducing its performance, and thus affecting the normal progress of drilling operations. The deposited weighting materials can increase the viscosity of drilling fluid at the bottom, resulting in a decrease in circulation speed, which may lead to problems such as hole collapse and pipe stuck, thereby seriously affecting the safety and efficiency of drilling operations. Second, increased weighting material deposition can also lead to damage to drilling equipment. When the weighting materials are deposited on the internal surface of the drilling equipment, they can form deposits that reduce the efficiency of the equipment and even lead to equipment failure and damage. This not only increases the equipment repair and replacement costs, but also can cause operational stagnation and delays in drilling activities. Third, aggravating material deposition may also cause adverse effects on formation environment. Weighting materials deposited in the formation may cause blockage and damage to the formation, affecting the normal operation of oil exploration. This impact may extend to harm groundwater resources, further destabilizing the ecological environment. Finally, the deposition of weighting materials may also lead to a decline in the permeability of drilling fluid, increasing the formation’s adsorption and penetration of drilling fluid, thus increasing the cost and difficulty of drilling operations. In summary, the deposition of weighting materials in drilling fluid poses numerous hazards, including deterioration of the rheological properties of drilling fluid, drilling equipment damage, adverse effects on formation environment, and increased cost and difficulty of drilling operations. Therefore, during drilling operations, it is necessary to take effective measures to prevent the deposition of weighting materials to ensure the smooth progress of drilling operations and the safety of formation environment. Despite decades of field experience and laboratory research, sagging remains inadequately controlled under dynamic downhole conditions, especially in deviated and horizontal wells where flow behavior is complex.

In recent years, various methods have been used to address the settling issue and improve the dispersion and stability of the drilling fluids. The first approach involves the combined use of different weighting agents. For instance, Basfar et al. studied the combination of Barite and ilmenite and found that it improved the stability of drilling fluid and alleviated the depression trend. The second method is to reduce the particle size of weighting materials. For example, Mohamed et al. studied the influence of Barite particle size on settling tendency in . They found that reducing particle size improved drilling fluid stability and mitigated sag tendency, as well. The third strategy is to modify the surface property of weighting agents. For example, Yang et al. surface modified Barite with hydrophilic copolymers and applied the modified Barite as the weighting agent in water-based drilling fluid. The grafted hydrophilic polymer on the surface of Barite increased the electrostatic force and steric resistance by forming a hydration film, thereby enhancing the dispersion stability. Additionally, the use of additives (e.g., suspending agents and rheological modifiers) has proven effective. Temple et al. proposed a new method to reduce the depression tendency of Barite in reversed-phase emulsified mud by adding polyalkyl methacrylate with low molecular weight. This formulation effectively decreased sag tendency without elevating drilling fluid viscosity, and thereby has a promising application in drilling operations. , Overall, the application of these methods helped improve the stability of the weighting material in drilling fluid and maintained their consistency and performance. , Although various mitigation strategies have been explored, existing solutions often suffer from limitations like poor thermal stability, incompatibility with other fluid components, environmental concerns, or high costs.

A notable previous review by Adjei et al. focused on the challenges of Barite sag in drilling fluids, particularly under dynamic conditions and in highly inclined or horizontal wells. Their work emphasized the influence of rheological properties and wellbore geometry on Barite settling and explored emerging tools such as machine learning for sag prediction and monitoring. While informative, that review primarily concentrated on evaluation techniques for Barite sagging and remediation methods, including increasing the viscosity of the drilling fluid to improve suspension stability and using a combination of nontraditional weighting systems to reduce the tendency to sink, offering limited insights into alternative materials or broader mitigation strategies. The present review provides a comprehensive and comparative assessment of multiple weighting materials, such as hematite, ilmenite, and manganese tetroxide, and systematically evaluates recent advances across four major sag control strategies: different weighting agent blending, particle size reduction, surface modification, and the use of antisagging additives. Moreover, this review incorporates sustainability considerations, economic challenges, and future perspectives, offering a more holistic foundation for improving the design and performance of HTHP drilling fluids.

The main content of this review is shown in Figure . The review begins by introducing the types and physiochemical properties of weighting agents used in drilling fluids. Then, the sagging mechanism and influencing parameters, including density, particle size and its distribution, viscosity, flow rate, temperature and pressure, as well as wellbore inclination angle, are outlined. Next, various methods to prevent the sagging of the weighting agent in drilling fluid are summarized, including compounding different weighting agents, reducing the particle size of the weighting agent, modifying the surface of the weighting agent, and adding a series of rheological and suspension additives. Finally, the development opportunities, main challenges, and future directions in this field are discussed.

1.

An illustration of the sagging issue of weighting materials in drilling fluids covered in this review, including the types of weighting materials, influencing factors, and remediation methods.

To ensure transparency and reproducibility, a systematic literature search was conducted using Web of Science, Scopus, ScienceDirect, and Google Scholar with keywords such as “weighting material,” “sagging,” “HTHP drilling fluids,” “anti-sagging additives,” and “drilling fluid stability.” Peer-reviewed English-language articles published between 2020 and 2025 were included, focusing on sagging mechanisms, material properties, or mitigation strategies under high-temperature and high-pressure (HTHP) conditions. Titles, abstracts, and full texts of over 300 initial records were screened, resulting in approximately 90 relevant publications. These were categorized by weighting agent type, sagging causes, and mitigation approaches, and evaluated by static and dynamic sag coefficients, forming the basis of this review.

2. Fundamentals of Weighting Agents

As the drilling depth increases, the pressure of the formation gradually increases. In deep well conditions, to maintain the pressure balance at the bottom of the hole and prevent hole collapse, it is necessary to use high-density drilling fluids. The addition of weighting materials to the drilling fluid is an efficient method to improve the density of the drilling fluid, thereby preventing hole collapse and maintaining the integrity of the wellbore. Weighting agents are divided into two categories, i.e., natural weighting agents (e.g., hematite, Barite, magnetite, and manganese tetroxide) and artificial weighting agents (e.g., Micromax and ilmenite). The topography of the weighted material is shown in Figure . The physicochemical properties and applicable strata of various weighting materials are summarized in Table .

2.

SEM images of weighting materials: (a) barite, (b) hematite, (c) micromax, (d) ilmenite, (e) magnetite, (f) manganese tetroxide. This figure was reproduced with permission from ref Copyright 2020 Springer Nature. This figure was reproduced with permission from ref Copyright 2021 American Chemical Society. This figure was reproduced with permission from ref Copyright 2021 Springer Nature. This figure was reproduced with permission from ref Copyright 2021 Springer Nature.

1. Summary of Physicochemical Properties and Applicable Strata of Weighting Materials.

types	density (g/cm3)	Mohs hardness	temperature tolerance (°C)	applicable strata
barite	4.2–4.5	3–3.5	1538	conventional oil and gas reservoirs
				shallow to moderately deep formations
				low to moderate pressure formations
hematite	4.9–5.3	5.5–6.5	1565	deep HPHT formations
				geothermal wells
micromax	4.7–4.9	5.5	1500	depleted reservoirs
				low-permeability and fractured formations
				sensitive or production zone
ilmenite	4.5–5.1	5–6	1843	high-temperature and high-density wells
				sandstone and carbonate reservoirs
magnetite	5.2	5.5	575	HPHT wells and magnetically sensitive operations
				formations requiring magnetic detection or advanced logging tools
manganese tetroxide	5.03	5.5	535	high-angle, extended reach wells
				formations with high sag risk or requiring low equivalent circulating density

2.1. Barite

Barite is a colorless, white or light-yellow crystal, flake or columnar, with its main component being barium sulfate. The topography of Barite is shown in Figure a. It possesses a relatively high density, generally ranging between 4.2 and 4.5 g/cm³, and belongs to the natural weighting agent. The chemical stability of Barite is high. It has good corrosion resistance and is not easy to be eroded by chemical substances such as acid and alkali. In addition, Barite exhibits superior temperature resistance. Its melting point is about 1538 °C, and it is thereby easy to melt or decompose at high temperatures. Therefore, it can be used in high-temperature environments. Barite is a ferromagnetic material with good magnetic stability. At room temperature, it is magnetized by an applied magnetic field. Moreover, the magnetism of Barite usually remains stable at higher temperatures, offering a distinct advantage in magnetic applications. Due to the above-discussed excellent performance at high temperatures, Barite is the most widely used weighing material at present.

2.2. Hematite

Hematite is a reddish-brown powder with electrical conductivity and a specific gravity of 4.9–5.3 g/cm³. It is also a natural weighting agent. The main component of hematite is iron oxide, usually in the form of Fe₂O₃. It is relatively stable under general chemical conditions and is resistant to erosion by chemical substances such as acid and base, as well. However, under certain strong acid or strong base conditions, hematite may dissolve or react. Its color is usually red or brownish-red due to the oxidized state of the iron within it. Hematite exhibits diverse crystal forms, such as hexagonal plate crystals, needles, fibers or blocks. The topography of hematite is shown in Figure b. Because hematite has a high hardness, generally ranging between 5.5 and 6.5, it can withstand pressure and wear during drilling, helping to maintain the performance of drilling fluids. Hematite also exhibits superior stability at high temperatures with a high melting point of about 1565 °C. Hematite is an antiferromagnetic material that typically lacks spontaneous magnetism at room temperature. However, when it is subjected to an external magnetic field, hematite can be magnetized, exhibiting ferromagnetism. However, once the external magnetic field disappears, hematite loses its magnetization. Hematite is widely distributed around the globally, and its widespread availability and relatively low cost make it a cost-effective choice for weighting agents.

2.3. Micromax

Micromax is a reddish-brown powder with a specific gravity range of 4.7 to 4.9 g/cm³, available in both solid and liquid forms. Because its particles are relatively finer, about 1 μm in diameter, Micromax is less abrasive and less prone to pitting. Micromax particles are spherical, which gives them a lower plastic viscosity, and thereby minimizing friction resulting from particle interactions. The topography of Micromax is shown in Figure c. Micromax generally has high chemical stability, resisting reactions or decomposition under normal chemical conditions. It is highly resistant to chemical attacks from substances like acids and bases, ensuring long-term stability. It has a high melting point of about 1500 °C, thus it does not usually melt or decompose under normal conditions. Micromax typically lacks inherent magnetic properties. Therefore, it does not affect the magnetism of drilling fluid. Despite Micromax has several advantages, it is more expensive than other weighting materials. Therefore, it is generally used as a compound weighting material.

2.4. Ilmenite

Ilmenite is mainly composed of two oxides, i.e., iron oxide (Fe₂O₃) and titanium oxide (TiO₂). The proportion can vary, but usually contains a higher proportion of iron oxide. The color is usually black or dark brown, and its appearance can be blocky, granular, or lumpy. The topography of ilmenite is shown in Figure d. Ilmenite has a specific gravity of 4.5–5.1 g/cm³. Titanium dioxide, one of the main components in ilmenite, is a chemically inert compound that resists reaction or decomposition under normal chemical conditions. However, in specific high-temperature or strong acid–base environment, ilmenite may occur partial dissolution or oxidation reaction. It has a high hardness and density. Ilmenite has a high melting point of approximately 1843 °C and is suitable for a variety of drilling fluids. Ilmenite typically behaves as an antiferromagnetic material. While its magnetic properties are weak at room temperature, they become more pronounced at lower temperatures. Nevertheless, as temperature increases beyond a certain threshold, the magnetic properties of ilmenite gradually diminish or disappear. Furthermore, ilmenite has a high hardness and density. To avoid the wear of ilmenite, it is advisible to optimize the size distribution, ensuring that large particles do not exceed 3%. Therefore, microilmenite or its combination with Barite is commonly used to improve mud stability.

2.5. Magnetite

Magnetite is black and has a metallic luster. The crystals take the shape of cubes or octahedrons. The topography of magnetite is shown in Figure e. Its density ranges from 5.15 to 5.20 g/cm³. The main mineral in magnetite is Fe₃O₄, which is a black to brown ore. In addition to Fe₃O₄, magnetite may also contain other iron minerals, such as α-Fe₂O₃ and α-FeO­(OH). The chemical stability of magnetite mainly depends on its composition and the surrounding environmental conditions. Magnetite is stable at room temperature, but may dissolve or react under strong acid or alkali conditions. In oxidizing environment, magnetite can undergo oxidation reaction. Magnetite is a mineral with strong magnetic properties. It usually shows strong magnetism at room temperature. However, as the temperature increases, its magnetism weakens until it eventually loses its magnetism at a certain temperature. This is because that magnetite decomposes into iron oxide at high temperatures. Magnetite has a low hardness, which makes it relatively easy to process and handle. However, magnetite particles are usually coarse, it can affect the flow and viscosity of drilling fluids, resulting in handling and pumping problems. Moreover, the sedimentation rate of magnetite is relatively high, which may lead to particle deposition in circulation and reduce the performance of drilling fluid. Therefore, drilling fluid weighting is generally carried out after compounding magnetite with other weighting materials. In specific drilling projects where magnetism is a consideration, such as magnetic surveys or under special downhole conditions, the addition of magnetite can impart liquids with some magnetic properties to meet specific surveying or navigation needs.

2.6. Manganese Tetroxide

Manganese tetroxide usually appears as a black or dark-brown powdery substance. The topography of manganese tetroxide is shown in Figure f. The density is about 5.026 g/cm³ and the melting point is about 535 °C. At room temperature, manganese tetroxide demonstrates relative chemical stability, as well. Furthermore, manganese tetroxide serves as a significant magnetic material, displaying robust magnetism at room temperature. However, it loses its magnetic properties at a specific temperature known as the Curie temperature at about 530 °C. Manganese tetroxide can be used as a natural weighting agent in drilling fluids in the petroleum industry to increase the density of drilling fluids to meet the needs of specific formation conditions. By adjusting the amount of manganese tetroxide, the properties of drilling fluid can be adjusted to meet the requirements of downhole operations. However, when using manganese tetroxide, its environmental and health effects need to be considered.

3. Sagging Mechanism and Influencing Parameters

Weighting agents play a vital role in controlling the hydrostatic pressure of drilling fluids, enabling the safe and efficient drilling of high-pressure formations. However, despite their importance, maintaining a stable suspension of these high-density solids remains a significant challenge, especially under demanding downhole conditions. The stability of weighting agents in drilling fluids is governed not only by their inherent propertiessuch as density, particle size, shape, and surface characteristicsbut also by their interactions with the surrounding fluid medium. These interactions, in turn, are strongly influenced by environmental conditions such as temperature, pressure, and flow dynamics. Understanding how these factors interplay is crucial for diagnosing and mitigating sagging behavior. Therefore, the following section delves into the mechanisms responsible for sagging and the parameters that govern this phenomenon in various drilling scenarios.

3.1. Sagging Mechanism

The drilling fluid is a stable, laminar fluid state in accordance with Stokes’ law, where the particle diameter of the weighting material is smaller than the characteristic scales of eddy and turbulence in the fluid. As the angle of liquid column varies, the sag mechanism of weighting material becomes different. The settlement mechanism diagram is shown in Figure .

3.

Experimental setup for static sag test: (a) vertical and (b) inclined (45°); illustration on settling phenomena under static condition: (c) vertical; (d) inclined. This figure was reproduced with permission from ref Copyright 2020 Springer Nature. This figure was reproduced with permission from ref Copyright 2019 The University of Oklahoma.

In the vertical pipe settlement, the pipe is divided into three areas because of the settling of the weighting material, i.e., the clarification area, the obstruction settlement area, and the sedimentation area. Details are shown in Figure a,c. The clarified region contains a small amount of weighting material particles, and the sedimentation rate of the weighting material at this stage follows Stokes’ law

$$v=\frac{{(d_{\mathrm{s}})}^2({\rho }_{\mathrm{s}}-{\rho }_{\mathrm{f}})g}{18{\mu }_{\mathrm{f}}}$$ 1

where v is settling velocity of a particle; d _s is weighting material particle diameter; ρ_s is weight material particle density; ρ_f is fluid density; g is acceleration due to gravity; μ_f is fluid viscosity. It can be seen from the Stokes’ law that the settling velocity of the weighting material particles in the clarified region are related to the size of the particles, the density difference between weighting material and fluid, and the viscosity of the fluid. In the area of obstructed settlement, the number of particles significantly increases compared to the clarified area. As the weighting material particles settle in the fluid, they may interact with other particles or solid particles in the fluid to form aggregates or deposits. When particles aggregate, the sedimentation velocity increases with the increase in particle size. The sedimentation area mainly consists of deposited weighting material particles, which gradually accumulate to form a granular deposit layer as they settle to the bottom. This accumulation may cause an increase in the density of the bottom area and create a certain sedimentation effect. At the same time, as the bottom deposit becomes compacted, the excess fluid is slowly discharged, which affects the sedimentation rate of the particles in the sedimentation area.

When the pipe is in the inclined position, the weighting material particles fall faster because the distance from the top to the end of the inclined pipe is shorter than the vertical position. As shown in Figure b,d. In inclined pipes, weighting material particles may be more likely to deposit on the downstream side of the bottom of the pipe, because the particles are moved downstream along the bottom of the pipe by gravity and the tilt force of the pipe. In addition, in inclined pipes, the fluid velocity may produce uneven distribution in the cross-section of the pipe due to the combined action of gravity and the tilt force of the pipe. This uneven distribution may affect the settling and movement characteristics of weighting material particles.

3.2. Influencing Parameters

3.2.1. Density Difference

Density difference refers to the variance between the density of the weighting materials and the density of the drilling fluid. Since the density of weighting materials is much higher than that of drilling fluid, spontaneous drooping occurs. The occurrence of this phenomenon is closely related to several key mechanisms. First of all, gravity is one of the main driving forces that aggravate material droop. Because the density of weighting materials is higher than that of drilling fluid, the weighting material particles will settle down under the action of gravity. Due to the high-density difference, the weighting material particles experience greater gravitational force, resulting in a relatively rapid sagging speed. Second, buoyancy also contributes to the aggravation of weighting material sag. The drilling fluid around the weighting materials generates buoyancy, partially counteracting the effect of gravity. However, weighting material sag still occurs because the buoyancy is not sufficient to fully balance the sinking trend of the weighting material particles. Therefore, to reduce the adverse effects of weighting material sag, it is usually necessary to adjust the density of drilling fluid. Increasing the density of drilling fluid can not only reduce the density difference with weighting materials, it can also improve buoyancy and slow down the sinking speed of the weighted material, thus reducing the occurrence of sagging phenomenon. Understanding these mechanisms is essential for drilling engineers to optimize drilling fluid formulation and adjust drilling parameters to minimize the interference of weighting material sag on the drilling process and ensure the successful completion of drilling operations.

3.2.2. Particle Size and its Distribution

The particle size and distribution of weighting materials have a profound effect on their drooping behavior, primarily through the interaction between particles and deposition behavior. First of all, particle size directly determines its movement characteristics in drilling fluid. According to Stokes’ law, the sedimentation rate is proportional to the square of the particle radius. In general, the larger particles sink faster due to their mass and inertia, and are more likely to deposit rapidly near the bottom of the hole or the wall of the hole, increasing the possibility of depression, while the smaller particles are more affected by the drilling fluid due to their smaller inertia, and the subsidence rate is slower. However, while the addition of small particles helps to improve sedimentation stability, it also increases the viscosity of the suspension, which can lead to poor fluidity. Second, particle size distribution also determines the nonuniformity of sedimentary behavior. In drilling fluids, larger particles tend to be deposited first to form a dense layer, while smaller particles may be suspended for longer. This uneven deposition increases the concentration of larger particles in the lower part of the sedimentary layer and exacerbates the subsidence phenomenon, especially the high-density particles such as Barite. In summary, the particle size and distribution of weight gaining materials directly determine their deposition and settlement behavior in drilling fluid, which in turn affects the bottom hole pressure control and wellbore stability. Therefore, when designing the drilling fluid formulation, the particle size and distribution of the weighted material must be fully considered to minimize the adverse effects of subsidence on drilling operations.

3.2.3. Viscosity

The viscosity of drilling fluid is very important to the sagging process of weighting material, and its influence mechanism mainly involves fluid dynamics and particle deposition. When the viscosity of the drilling fluid increases, it has a significant effect on the sinking rate of the particles. First, the high viscosity of the drilling fluid increases the resistance between the particles and the fluid, hindering the movement of the particles within the fluid, and consequently slowing the sagging rate. Second, the high-viscosity drilling fluid forms a denser flow field structure around the particles, leading to more complex interactions between the particles. This can lead to aggregation or repulsion between particles, which can affect the rate and manner of particles deposition. However, the viscosity of the drilling fluid should not be too high. High viscosity can induce difficulty in dispersing weighting material particles, leading to uneven distribution of particles in drilling fluid. In addition, the high viscosity of the drilling fluid also increases the friction between the drilling fluid and the wellbore wall, thereby affecting the drilling rate and time. Therefore, the above factors should be comprehensively considered when formulating drilling fluid to ensure the superior suspending stability of weighting material and fast drilling rate.

3.2.4. Flow Rate

Changing the flow rate of drilling fluid can profoundly affect the sagging behavior of the weighting material, which involves complex fluid mechanics and particle transport mechanisms. First, with the increase of flow rate, the impact force exerted by drilling fluid on the weighting material increases, which promotes the sagging speed of particles. When the flow rate increases, the shear force on the particles in the drilling fluid also increases, which weakens the interaction force between the particles and reduces the possibility of aggregation, thus alleviating the tendency of settlement. On the contrary, when the flow rate decreases, the relative motion between the particles decreases, and they are more likely to gather together through electrostatic forces, van der Waals forces and other interaction forces, forming larger particles, which are more susceptible to the influence of gravity and settlement. This change may affect the motion trajectory and deposition position of the particles, thus affecting the droop characteristics of the weighing material. In addition, the change of flow velocity also affects the rheological properties of drilling fluid. At high flow rates, drilling fluids may behave more like Newtonian fluids where the shear stress is proportional to the shear rate. At low flow rates, drilling fluids may exhibit the characteristics of non-Newtonian fluids, and the relationship between shear stress and shear rate is more complex. In conclusion, the increase of drilling fluid flow not only speeds up the sedimentation rate of the weighted material, but also may change the sedimentation mode and distribution of particles. This effect is caused by changes in particle motion dynamics and fluid environment caused by increased flow velocity. Therefore, in the drilling process, the influence of flow rate on the sag behavior of aggravating materials must be fully considered to ensure the smooth progress of drilling operations.

3.2.5. Temperature and Pressure

Temperature and pressure are another two important factors affecting the sagging behavior of weighting materials, and their changes have a significant impact on the deposition rate and distribution of particles. The influencing mechanism mainly involves thermodynamic and physicochemical processes. First of all, temperature changes affect the viscosity and density of drilling fluid, thereby affecting the sinking speed of particles. In general, an increase in temperature decreases the viscosity of the drilling fluid, reducing the resistance between the particles and the fluid, thus increasing the sagging rate of the particles. On the other hand, the increase in temperature reduces the density of drilling fluid, thus reducing the buoyancy of particles and further promoting the sagging of particles. In contrast, lower temperatures make the drilling fluid more viscous, increasing the resistance between particles and fluid, resulting in a slower sagging rate.

The change in pressure also has an important impact on the sagging behavior of the weighting materials. With the increase in pressure, the density of drilling fluid increases, which increases the buoyancy force on the particles and slows down the sagging rate of the particles. In addition, the increased pressure also changes the gas solubility and foam formation in the drilling fluid, which affects the flow of the drilling fluid and the way of particle deposition.

In summary, the changes in temperature and pressure have significant effects on the sagging behavior of the weighted materials. Temperature fluctuations directly influence the viscosity and density of drilling fluid, thereby affecting particle sinking speed. Pressure changes mainly affect particle sedimentation behavior by altering the density and fluidity of drilling fluid. Therefore, it is necessary to fully consider the influence of temperature and pressure on the sagging behavior of the weighting materials in the drilling operation to ensure the safe and effective drilling operation.

3.2.6. Wellbore Inclination Angle

Wellbore inclination angle is another important factor affecting the sedimentation behavior of weighting materials in drilling fluids. Changes in inclination can significantly influence the sedimentation rate and distribution of particles. The underlying mechanisms primarily involve variations in the hydrodynamic and gravitational forces acting on the particles within the fluid.

First, the wellbore inclination angle alters the gravitational component acting on the weighting material. In vertical wells (with an inclination of 0°), gravity acts directly downward, promoting relatively uniform particle settling. However, as the wellbore inclination increases, the effective gravitational component along the axial direction of the wellbore decreases. This reduction slows the settling velocity of particles along the wellbore, resulting in a more dispersed distribution of the heavier substances over a longer axial distance within the drilling fluid.

In addition, changes in wellbore inclination can significantly affect the flow dynamics of the drilling fluid. In inclined wells, the flow pattern deviates from the direct vertical flow observed in vertical wells. Variations in flow paths and velocity profiles can result in complex fluid–particle interactions. For instance, in highly deviated or horizontal wells, the drilling fluid tends to flow along the lower side of the wellbore, causing greater accumulation of weighting materials in that region. This uneven distribution may lead to localized zones of high-density materials, potentially causing issues such as differential sticking or uneven pressure distribution along the wellbore.

In summary, the wellbore inclination angle has a substantial impact on the sedimentation behavior of weighting materials in drilling fluids. Changes in inclination affect both the gravitational forces acting on particles and the flow dynamics of the fluid, thereby influencing the sedimentation rate and particle distribution. Therefore, to maintain wellbore stability and drilling efficiency, the effects of inclination angle on the behavior of weighting materials must be carefully considered during drilling operations.

4. Approaches to Prevent Sagging

A comprehensive understanding of the sagging mechanism and the influencing parameters provides a foundation for developing effective countermeasures. Since sagging is inherently a result of complex interactions between fluid rheology, particle dynamics, and wellbore conditions, any preventive strategy must be tailored to address these interrelated factors. Strategies to mitigate sagging aim to either modify the properties of the weighting agents themselves or alter the characteristics of the fluid medium to enhance suspension stability. To prevent the sagging of weighting agents, a wide spectrum of strategies has been applied, primarily including (1) combing different weighting agents, (2) reducing the particle size of weighting agents, (3) surface modification of weighting agents, and (4) adding antisagging additives. The combination of different weighting materials and the reduction of the particle size of weighting materials can effectively fill the voids within the drilling fluids, reduce the loose structure, and minimize the possibility of settlement. In addition, they can help reduce the binding force and friction between particles, thereby further preventing settlement. Surface modification of the weighting material may induce alterations in surface energy, surface properties, and surface charge of the weighting materials. Changes in surface energy can affect the interaction between the particles and the surrounding media, thus impacting the dispersion and sedimentation of the particles. An alteration in surface property from hydrophobicity to hydrophilicity facilitates easier dispersion in the water-based drilling fluids, rather than precipitation. Changing the surface charge may result in a change in the electrostatic interaction between the particles, which affects the colloidal stability, as well. Furthermore, the addition of sagging inhibitors (e.g., suspending and rheological additives) into the drilling fluid can keep the particles well-suspended and prevent precipitation. Although this method is straightforward and cost-effective, its applicability is limited to specific wells. Under high-temperature and high-pressure conditions, certain polymer-based sedimentation inhibitors may degrade, leading to poor suspending capacity. Table is a summary of the literature on various methods for preventing the sedimentation of weighting materials.

2. Summary of Literature on Various Methods for Preventing the Sedimentation of Weighting Materials.

	original weighting agent	alternative weighting agent	sagging tendency	ref
(1) using the combined weighting agents	barite	micromax	With an increase in the loading of Micromax from 0 to 15 and 30 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.590/0.620 to 0.550/0.570 and 0.502/0.504, respectively. In addition, the dynamic VSST value decreased from 1.3 ppg to 1.03 ppg and 0.5 ppg.
	hematite	ilmenite	With an increase in the loading of Ilmenite from 0 to 25 and 50 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.54/0.56 to 0.52/0.53 and 0.50/0.51, respectively. In addition, the dynamic VSST value decreased from 1.89 ppg to 0.7 ppg and 0.24 ppg.
	barite	magnetite	When magnetite was used instead of Barite, the static sag factor under vertical and inclined (45°) conditions changed from 0.55/0.56 to 0.51/0.52, respectively. The dynamic VSST value changed from 1.23 ppg to 0.82 ppg
	hematite	manganese tetroxide	With an increase in the loading of Manganese tetroxide from 0 to 10, 15, and 20 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.54/0.56 to 0.54/0.55, 0.53/0.53 and 0.51/0.51, respectively. In addition, the dynamic VSST value decreased from 1.89 ppg to 1.5 ppg. 1.2 ppg and 0.7 ppg.
	barite	ilmenite	With an increase in the loading of Ilmenite from 0 to 20 wt %, 40 wt %, 60, 80, and 100 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.568/0.597 to 0.534/0.544, 0.507/0.512, 0.503/0.507, 0.501/0.503 and 0.501/0.501, respectively. In addition, the dynamic VSST value decreased from 1.50 ppg to 1.00 ppg, 0.50 ppg, 0.06 ppg, 0.04 ppg and 0.04 ppg.
	barite	micromax	With an increase in the loading of Ilmenite from 0 to 25 and 50 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.58/0.6 to 0.514/0.518 and 0.505/0.51, respectively. In addition, the dynamic VSST value decreased from 2.1 ppg to 0.52 ppg and 0.05 ppg.
	hematite	ilmenite	With an increase in the loading of Ilmenite from 0 to 20, 40, and 50 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.56/0.58 to 0.55/0.57, 0.54/0.55 and 0.51/0.52, respectively. In addition, the dynamic VSST value decreased from 1.4 ppg to 1.2 ppg. 1.0 ppg and 0.05 ppg.

	original weighting agent	treatment	sagging tendency	ref
(2) reducing the particle size of weighting material	barite	ball milling	the fluid stability increases with the decrease of particle size.

	original weighting agent	modification method	sagging tendency	ref
(3) surface modification of weighting material	barite	free radical polymerization	conventional barite particles settle completely to the bottom in 5 min, while the modified Barite has a full settling time of 6 h

	weighting material	antisagging additives	sag tendency	ref
(4) adding Antisagging additives	barite	perlite	perlite additive was added to the drilling fluid in various concentrations, 0.0–3.0 lb/bbl. perlite enhanced the stability of the mud by reducing the static and dynamic sag factors
	barite	vermiculite	when the loading of vermiculite increased from 0 to 1 lb/bbl, 2 lb/bbl, 3 lb/bbl, and 4 lb/bbl, the static sag coefficients decreased from 0.55/0.57 to 0.54/0.55, 0.53/0.54, 0.526/0.53 and 0.51/0.52 at vertical and inclined (45°) conditions
	ilmenite	fly ash	with the presence of 3 lb/bbl fly ash, the static sag coefficient decreased from 0.504/0.521 to 0.508/0.517 at vertical and inclined (45°) conditions, and the dynamic VSST value decreased from 1.65 ppg to 0.51 ppg.

4.1. Combining Different Weighting Agents

Combining different weighting agents is an effective strategy to mitigate the sagging issue in drilling fluids. This approach involves mixing two or more weighting agents with complementary properties to enhance the overall stability and suspension characteristics of the drilling fluid. There are two specific effects that contribute to the enhanced stability of weighting agents in drilling fluids using this strategy, as follows. First, the combination of different weighting agents can change the overall particle size and distribution of weighting agents. Particularly, reducing the particle size of the weighting material can cause strong Brownian motion and random collision of particles, thus reducing the influence of gravity on the precipitation of solid particles. The presence of weighting material with smaller particle size and higher surface area also leads to greater lift and drag effects, rather than cohesion and settlement. Second, the combination of different weighting agents can induce alternations in the surface properties of weighting agents. Changing the surface properties of the weighting material can enhance the colloidal stability of weighting agents as discussed previously. Therefore, the compounding of different weighting agents can improve the dispersion of the weighting agents in the liquid phase, reduce the possibility of aggregation and precipitation, and thus improve the stability of the drilling fluid.

Among a wide range of weighting materials, Barite is commonly used as a weighting agent in drilling fluid because of its adjustable density range, excellent chemical stability, high temperature resistance, good suspension dispersion, and cost-effectiveness. However, sagging is a serious problem for Barite when using in drilling fluid. This problem usually occurs when solid particles separate from the liquid phase and precipitate. The resultant changes in fluid density due to Barite settling can cause serious well control problems and disrupt drilling operations. This phenomenon occurs in both directional and vertical Wells, most commonly under dynamic conditions with low shear rates. To address the challenge of Barite sagging, compounding Barite with other weighting agents is one of the most effective methods. Among different weighting agents, Micromax has been extensively compounded with Barite. Micromax has a higher specific gravity, and its fine particles make them less abrasive and sagging. Compared to other weighting agents, Micromax additives are more reservoir-friendly and cause less damage to the formation. The combination of Micromax and Barite is applicable in both water-based and oil-based drilling fluid systems for different formation types and operating conditions. This combination provides great flexibility and can be used in combination to reduce filtration and maintain fluid stability simultaneously. Several factors contribute to the improved performance of drilling fluid with the Micromax additive. First of all, Micromax particles are fine and uniform, which can cause strong Brownian motion and random collision of particles, thus minimizing the influence of gravity on solid particle settlement and improving the stability of drilling fluid. Second, Micromax particles have a higher specific surface area gravity, resulting in greater lift and drag, rather than condensation and precipitation. Finally, Micromax particles have a higher surface charge, which increases colloidal stability. For instance, Basfar et al. performed the static sag test by pressurizing the drilling fluid to 300 psi in aging cell and aged in an oven for 24 h at 250 °F. It was found that the static sag factor of the drilling fluid exhibited a significant downward trend as the Micromax loading increased from 0 wt % to 25 and 50 wt %, under both vertical and inclined (45°) conditions. Specifically, the static sagging coefficient under vertical conditions decreased from 0.58 to 0.514 and 0.505, while under inclined conditions, it decreased from 0.60 to 0.518 and 0.51. Furthermore, under atmospheric pressure and at 120 °F, the dynamic sagging test was also conducted. It was observed that the value of viscometer sag shoe test (VSST) of the drilling fluid decreased from 2.1 ppg to 0.52 ppg and 0.05 ppg, as the Micromax loading increased from 0 wt % to 25 and 50 wt %. Micromax particles filled the voids between Barite particles, forming a tighter packing structure. This increased the yield stress of the fluid, and thereby decreased the sedimentation rate of Barite. The combination of Micromax and Barite significantly enhanced the overall performance of the drilling fluid.

In addition to Micromax, ilmenite was also mixed with Barite to mitigate the sagging issue. On the one hand, the reasonable mixing of Barite and ilmenite can optimize the particle distribution and morphology, leading to reducing the fluid loss in the formation. This helps reduce formation penetration, maintains drilling fluid performance, improves drilling efficiency, and enables more precise control of drilling fluid density. As shown in Figure a. Due to the different density ranges and properties of Barite and ilmenite, the weighting effect can be adjusted more flexibly to meet specific drilling needs under different well types and formation conditions. Mohamed et al. found that with increasing of ilmenite load from 0 to 20, 40, 60, 80, and 100 wt %, the static sag factor under vertical and inclined (45°) conditions decreased from 0.568/0.597 to 0.534/0.544, 0.507/0.512, 0.503/0.507, 0.501/0.503, and 0.501/0.501, respectively. As shown in Figure b,c. In addition, the dynamic VSST values decreased from 1.50 ppg to 1.00 ppg, 0.50 ppg, 0.06 ppg, 0.04 ppg, and 0.04 ppg. The combination of Barite and ilmenite proved effective in significantly inhibiting settlement.

4.

Prevention of sagging through combining different weighting agents: (a) Bridge mechanism between mud samples treated with Barite/ilmenite mixture and other samples; (b,c) the influence of loading of weighting materials on sag tendency under static conditions 0°and 45° (b), and under dynamic conditions (c). This figure was reproduced with permission from ref Copyright 2018 Elsevier. This figure was reproduced with permission from ref Copyright 2019 Multidisciplinary Digital Publishing Institute.

Hematite and ilmenite are relatively environmentally friendly because they are natural ores and do not contain harmful chemicals. This makes them more popular in some environmental areas. Basfar et al. mixed hematite and ilmenite to minimize the sagging issue of hematite. They found that when the load of ilmenite increased from 0 wt % to 20, 40, and 50 wt %, the static sag coefficients at vertical and inclined (45°) conditions decreased from 0.56/0.58 to 0.55/0.57, 0.54/0.55 and 0.51/0.52, respectively. In addition, dynamic VSST values decreased from 1.4 ppg to 1.2 ppg, 1.0 ppg and 0.05ppg. Furthermore, the combination of hematite and ilmenite also improved the rheological properties and filtration properties of drilling fluids. The addition of 40 wt % ilmenite increased the yield point from 15 lb/100 ft² to 26 lb/100 ft², while the plastic viscosity did not increase significantly, only from 15 cP to 16 cP. In addition, the addition of 40 wt % ilmenite to hematite improved the gel strength and storage modulus of drilling fluid, thereby enhancing the elasticity of drilling fluid. As a result, the drilling fluid structure is more stable.

In addition to ilmenite, manganese tetroxide was used to mitigate sagging issue of hematite in drilling fluids, as well. Manganese tetroxide is a relatively high-density material. The addition of manganese tetroxide to drilling fluid can effectively increase the density of the fluid, which is critical to maintain appropriate fluid density under different formation conditions. High-density drilling fluids help prevent blowouts, maintain wellbore stability, and improve drilling efficiency. The amount of manganese tetroxide can be flexibly adjusted to meet the density requirements under specific well depth and formation conditions, providing tunability to adjust well fluid properties. It is worth noting that manganese tetroxide is a relatively widespread and readily available substance, which reduces costs and improves availability. Basfar et al. combined manganese tetroxide with hematite and evaluated their stability through a series of static and dynamic sagging tests. With an increase in the loading of manganese tetroxide from 0 to 10, 15, and 20 wt %, the static sag factors under vertical and inclined (45°) conditions decreased from 0.54/0.56 to 0.54/0.55, 0.53/0.53, and 0.51/0.51, respectively. In addition, the dynamic VSST values decreased from 1.89 ppg to 1.5 ppg, 1.2 ppg, and 0.7 ppg. Furthermore, the combined use of hematite and manganese tetroxide helps form a high-density drilling fluid, improving mud rheological performance, suspending ability, and effectively sealing hole cracks, leading to enhanced wellbore stability.

In actual drilling applications, the selection of an appropriate combination of weighting agents needs to be adjusted according to specific drilling conditions. For example, in a high-temperature and high-pressure environment, the thermal stability and chemical stability of weighting agents are key factors; In horizontal Wells or Wells with high slope, the suspension capacity and rheological properties of the drilling fluid are even more important. In addition, factors such as the density requirements of drilling fluid, well depth and formation characteristics will also significantly affect the selection of weighting agent combinations. The combination of different weighting materials can not only adjust the density of the drilling fluid to adapt to different formation conditions, but also effectively prevent the sedimentation problem of weighting agents. However, at present, most studies have only focused on the combination of two weighting agents. Future research should further explore the combined use of three or more weighting agents to achieve a better synergistic effect. At the same time, it should be noted that relying solely on the combination of weighting agents cannot solve all problems. In some cases, in order to better adapt to different formation conditions, it is also necessary to optimize the particle size and surface properties of the weighting agent. Therefore, it is suggested that future research should be more comprehensive, combining multiple strategies to enhance the overall performance of drilling fluids, thereby developing more targeted and adaptive drilling fluid formulations.

4.2. Reducing the Particle Size of Weighting Materials

In addition to the combination of different weighting materials, reducing the particle size of the weighting material is also an effective way to alleviate their sagging issue. On the one hand, reducing the particle size improves the dispersion of the weighting materials in drilling fluids, thereby improving the suspension stability of the weighting materials in the drilling fluid. On the other hand, it is found that reducing the size of weighting materials can improve the yield strength of the drilling fluids, which is advantageous for the suspending of weighing materials, as well. In addition, as the average size of weighting materials decreases, the collisions between Barite particles and chelating agents increase, which increases the solubility of Barite during the filter cake removal.

Some physical methods can be used to reduce the particle size of the weighting material, including ball milling, air grinding, ultrasonication, differential centrifugation, and microfluidization. Details are shown in Figure a–e. The ball milling method is a method of grinding materials by using a ball mill. As shown in Figure a, in the ball milling process, the Barite particles roll in the grinding cylinder together with the grinding medium (usually steel balls), resulting in the crushing and thinning of the particles. This can effectively reduce the size of the particle. Air grinding is by suspending particles in the air stream and impacting particles in the high-speed air stream, so as to achieve the purpose of particle refinement. As shown in Figure b, this is a method suitable for fine powders and can be carried out without involving liquids. Ultrasonic crushing is the use of ultrasonic vibration to break the particles and make them smaller. As shown in Figure c, this method is effective for the control of high particle size. Differential centrifugal method is to peel the particles from the centrifugal force field of high-speed rotation by using a centrifuge, resulting in particle refinement. As shown in Figure d, this method is especially suitable for fine powders. Microfluidization simultaneously uses hydraulic shear, shock, friction, impact, strong turbulence, and cavitation to reduce size of particles under the action of a high-pressure pump (500–50,000 psi). As shown in Figure e. it forces the feed through an interaction chamber composed of microchannels, producing very fine particles. The impact energy from the particle collisions dissipates as heat and requires cooling. Through the above methods, the particle size can be effectively reduced and the suspension stability of weighting materials in drilling fluid can be improved.

5.

Preventing sagging by reducing the particle size of the weighting agent: (a–e) illustration of the ball milling, air grinding, ultrasonication, differential centrifugation, and microfluidization; and (f,g) the effect of Barite particle size on density difference and sag factor. This figure was reproduced with permission from ref Copyright 2021 Multidisciplinary Digital Publishing Institute. This figure was reproduced with permission from ref Copyright 2021 Powder Metallurgy Technology Editorial Department. This figure was reproduced with permission from ref Copyright 2017 Elsevier. This figure was reproduced with permission from ref Copyright 2020 Inderscience Enterprises Ltd.

For instance, Mohamed et al. prepared different fluid samples using Barite with different particle sizes as weighting materials. The sag test was then performed using the vertical static experiments at 230 °F and 500 psi. It was observed that the fluid stability gradually increased with the decrease of particle size, as shown in Figure f,g. In another example, Bu et al. found that high-density water-based drilling fluid with different particle sizes of weighting materials exhibited varying viscosity effects, and selecting an appropriate grading range effectively improved the rheological and suspension properties of high-density drilling fluids. Coarse particles combined at high temperatures to form hairballs, with coarse particles acting as the core cells. When the content of coarse particles was higher than a certain degree, these hairballs formed structures through interaction, resulting in increases in apparent viscosity and suspending effect of drilling fluids.

However, when the particle size of the weighting material is significantly reduced, the fine particles are easy to form a colloidal system within the drilling fluids, which can notably increase the viscosity of the drilling fluid and further increase the difficulty of drilling operation. Additionally, it necessitates more complex formulation process and equipment input, which increase the cost of drilling fluid. To address these challenges, the surface treatment of the weighting materials can be carried out while reducing the particle size. This can involve methods such as surface chemical modification or physical coating to adjust the surface properties of the weighting materials as needed. The comprehensive use of these methods can effectively enhance the suspending capacity while maintaining appropriate viscosity of drilling fluids.

4.3. Surface Modification of Weighting Agents

Modifying the surface of weighting materials is another effective method to mitigate the sagging issue of weighting materials and improve the suspension stability of drilling fluids. The surface properties of the weighting material affect its dispersion in the drilling fluids. For water-based drilling, weighting materials with abundant hydrophilic functional groups on the surface are easier to disperse, leading to enhanced colloidal stability. In contrast, for oil-based drilling fluids, weighting materials with hydrophobic surface properties are preferred. Furthermore, the interaction between weighting materials and other components in the drilling fluids plays critical role in the suspension ability. For instance, the formation of a 3D networked structure via surface interaction is beneficial for the suspension stability. Therefore, by controlling the surface properties of the weighting material, its dispersion and suspension properties in the drilling fluid can be improved, ensuring the smooth drilling process and reducing potential problems.

The surface modification methods of the weighting agent mainly include physical and chemical methods. Physical methods involve surface treatment using mechanical methods, such as abrasive and sandblasting to change the roughness and morphology of the surface of the weighting agent and increase the contact area with other components in the drilling fluid. Chemical methods include the use of surface modifiers, such as surfactants and silanes, to form a layer of molecular film on the surface of the weighting agent via chemical bond to enhance dispersion and stability. After chemical modification, functional groups, such as carboxyl group and amino group, are introduced onto the surface of weighting materials to increase their interaction ability with other components of water-based drilling fluid and improve suspension ability. Among different methods, chemical vapor deposition (CVD) is one of the most efficient methods, which involves heating the surface reaction area and injecting gaseous precursors to form a dense and uniform coating on the surface of weighting materials, leading to a change in the surface properties. These methods can be applied individually or in combination to modify the surface properties of weighing materials for the specific drilling conditions and requirements.

For instance, to address the sagging issue of Barite and improve the suspension stability of water-based drilling fluid, Yang et al. chemically modified microcrystalline Barite (MB). First, they added the BaCl₂/ 2- (methylacryloxy) ethyl] dimethyl - (3-sulfopropyl) hydroxide mixture to the MB suspension and stirred the mixture vigorously in a three-necked flask. Then, K₂S₂O₈/KOH solution was dropped and heated at 60 °C for 5 h to graft 2- (methylacryloxy) ethyl] dimethyl-(3-sulfopropyl) hydroxide on the surface. Finally, 2-acrylamide-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, N-vinylpyrrolidone and initiator K₂S₂O₈ were added in sequence, and the mixture was heated at 70 °C for 12 h to achieve homogeneous polymerization, as shown in Figure a. The obtained samples are centrifugated, washed, and dried at 50 °C to a constant weight, and then crushed into powder to prepare white microfine Barite particles (mMB). It was found that under normal conditions, MB particles settled completely to the bottom within 5 min, as shown in Figure b. However, the complete settling time for mMB extended to 6 h, as shown in Figure c, showing that the suspension stability of the mMB was significantly improved. This is because the grafted hydrophilic polymer formed a hydrated film, which increased the electrostatic force and steric resistance, thus enhancing the dispersion and suspending stability.

6.

Prevention of sagging through surface modification of weighting agents: (a) Schematic illustration of the modification route of mMB, (b, c) the sagging phenomena of MB (b) and mMB (c) in aqueous solution. This figure was reproduced with permission from ref Copyright 2024 Elsevier.

To overcome the agglomeration and rapid sedimentation problems of API Barite in oil-based drilling fluids, Yang et al. adopted a surface chemical grafting strategy, in which a lipophilic polymer was covalently anchored onto Barite particles to prepare modified API Barite (mAB). The procedure involved first dispersing API Barite in a weakly alkaline aqueous phase, followed by the successive addition of BaCl₂ and the bifunctional monomer [2-(methylacryloyloxy)­ethyl] dimethyl (3-sulfopropyl) ammonium hydroxide (MSAH). A deposition reaction was then initiated at 60 °C to generate vinyl active sites in situ on the particle surface. Subsequently, the temperature was raised to 70 °C, and a mixed monomer solution of styrene, isooctyl methacrylate, and vinyl acetate was introduced dropwise. Radical polymerization was carried out under the initiation of K₂S₂O₈, yielding a white powder mAB with a polymer coating rate of approximately 3%. The product was collected by centrifugation, washed with alcohol, dried at 50 °C, and ground for use. Static sedimentation experiments showed that unmodified Barite completely settled to the bottom of white oil within 1 min, whereas the complete sedimentation time of mAB was extended to more than 45 min. The underlying mechanism is that the grafted long-chain alkyl groups impart significant lipophilicity to the particles, reducing surface energy. Meanwhile, the polymer shell provides steric hindrance and weak cross-networking in the oil phase, effectively suppressing particle collision and agglomeration. In addition, the shell increases thickness and reduces equivalent density, thereby enhancing suspension stability. As a result, after aging at 180 °C for 16 h, mAB was still able to maintain the static sedimentation factor of oil-based drilling fluid within 0.53, achieving long-term stable suspension of Barite in the oil-based system.

While chemical modification of weighting materials offers significant advantages in improving suspension stability and overall performance of drilling fluids, it is crucial to consider the associated cost implications. Chemical modification processes often involve additional steps, specialized equipment, and the use of various reagents and initiators, all of which contribute to higher production costs. Moreover, environmental concerns play a significant role in evaluating the feasibility of chemical modification processes for weighting materials in drilling fluid formulations. The use of chemical modified weighting materials in drilling fluids may introduce new substances into the environment during drilling operations, and therefore sourcing suitable reagents for chemical modification should be taken into account.

4.4. Adding Antisagging Additives

Adding antisagging additive is one of the most simple and effective methods to improve the stability of drilling fluids. , Thickening agents are the most used antisagging agents, which play an important role in the stability of drilling fluids and have a direct impact on the success of the drilling process. The mechanism of its influence on the suspension stability of drilling fluid is as follows. First, thickening agents increase the viscosity of the drilling fluid. This higher viscosity helps in suspending weighting materials within the fluid. As a result, the weighting agent particles are less likely to settle to the bottom of the wellbore or on the pipe wall. Second, some thickening agents can form a gel-like structure when added to the drilling fluid. This gel network helps to trap and suspend the weighting material particles, preventing their settling. The gel structure also contributes to the overall stability of the drilling fluid by providing resistance to fluid movement and preventing fluid loss into the formation. Third, thickening agents may also interact with the surface of weighting material particles, forming a protective layer around them. This interaction helps to prevent particle aggregation and settling. Additionally, the thickened fluid matrix acts as a barrier, preventing direct contact between particles and reducing the likelihood of agglomeration.

Vermiculite is a type of rock belonging to silicate minerals, mainly composed of kaolinite and Illite. It is usually gray, white, or brown in color and has a powdery or granular structure, as shown in Figure a,b. In addition, vermiculite helps control the permeability of the drilling fluid to the rock formation, preventing the water in the liquid from penetrating into the rock formation, thus reducing the sagging of the weighting materials. Therefore, the use of vermiculite as an antisag inhibitor in drilling fluid improves the efficiency and safety of drilling. Ahmed et al. introduced vermiculite as a novel antisagging additive and found that vermiculite was sufficient to delay the sagging of Barite and reduce the settlement coefficient to a safe value under both static and dynamic conditions. The liquid samples were first placed in an autoclave cell at 250 °F and 300 psi. After 24 h of aging treatment, it was observed that as the vermiculite loading increased from 0 to 1 lb/bbl, 2 lb/bbl, 3 lb/bbl, and 4 lb/bbl, the static sag coefficients under vertical and inclined (45°) conditions decreased from 0.55/0.57 to 0.54/0.55, 0.53/0.54, 0.526/0.53, and 0.51/0.52, respectively. As shown in Figure c,d, a dynamic sag test was conducted at a rotational speed of 100 rpm for 30 min using a viscometer at 120 °F and atmospheric pressure to assess the dynamic sagging trend. The dynamic VSST values were found to decrease progressively from 2.5 ppg to 2.2 ppg, 1.7 ppg, 1.3 ppg, and 1.0 ppg. On one hand, vermiculite particles filled the voids in the drilling fluid, forming a network structure that prevented the settling of weighting materials. On the other hand, vermiculite increased the fluid’s viscosity, thereby enhancing its carrying capacity and resistance to sagging of weighting agents. It was found that the addition of vermiculite led to a 36% increase in yield point (YP), a 14% decrease in plastic viscosity (PV), and an increase in the YP/PV ratio from 1.61 to 2.55. Additionally, the drilling fluid exhibited a stronger gel structure, higher gel strength, improved viscoelasticity, and enhanced suspension stability of Barite.

7.

Prevention of sagging through adding antisagging additives: (a,b) Molecular structure and SEM image of vermiculite, (c,d) influence of vermiculite loading on the sag factor at static 0° and 45° conditions (c) and at dynamic condition (d). This figure was reproduced with permission from ref Copyright 2022 Elsevier. This figure was reproduced with permission from ref Copyright 2022 Elsevier.

Perlite is a porous sedimentary rock, usually composed of minerals such as calcium carbonate (calcareous perlite) or silicates (siliceous perlite). In drilling fluids, perlite can be used as an antisagging additive because of its porous structure and uniform particle size, which allows it to effectively fill voids in drilling fluids and prevent the sinking and deposition of weighting materials. In addition, perlite can absorb water in drilling fluid, thereby increasing its weight and density, improving its suspension in liquid, and effectively inhibiting the drop of weighting materials. Finally, perlite has a certain mechanical strength and can withstand a certain pressure and shear force, making it difficult to break or disintegrate during drilling. Basfar et al. added perlite to the formula of hematite water-based drilling mud and found that the addition of perlite had an antisagging effect on hematite water-based drilling fluid. The addition of perlite with a concentration of 4 lb/bbl improved the stability of the mud. They found that when the loading of perlite increased from 0 to 1 lb/bbl, 2 lb/bbl, 3 lb/bbl, and 4 lb/bbl, the static sag coefficients decreased from 0.54/0.56 to 0.53/0.55, 0.53/0.54, 0.52/0.54 and 0.51/0.52 at vertical and inclined (45°) conditions, respectively. Dynamic VSST values decreased from 1.8 ppg to 1.6 ppg, 1.3 ppg, 0.9 ppg, and 0.7 ppg. In addition, the addition of perlite resulted in a comprehensive improvement on the PV, YP, YP/PV ratio, and gel strength. Moreover, the presence of perlite improved the filtration performance, leading to enhanced wellbore stability.

In addition to vermiculite and perlite, Oni et al. found that fly ash can be also used as an antisag additive in water-based drilling fluids. Fly ash is made up of tiny, fluffy, spherical, solid, and glassy (amorphous) particles. It was found that the sagging phenomenon was dependent on both the loading of fly ash and temperature. At 250 °F, with the presence of 3 lb/bbl fly ash, the static sag coefficient decreased from 0.504/0.521 to 0.508/0.517 at vertical and inclined (45°) conditions, and the dynamic VSST value decreased from 1.65 ppg to 0.51 ppg. At 400 °F, the static sag coefficient at vertical and inclined (45°) decreased from 0.527/0.532 to 0.519/0.525. The dynamic VSST value decreased from 1.79 ppg to 0.68 ppg. Fly ash significantly enhanced the suspension performance of the weighting material under both low and high temperature conditions.

Although traditional antisagging additives can effectively prevent the sedimentation of weighting materials in drilling fluids, reduce friction, and protect the formation, they typically exhibit high chemical stability and are difficult to degrade rapidly in the natural environment. These additives may be discharged or leaked into soil and water bodies along with the drilling fluids. Upon accumulation, they can alter soil properties, inhibit microbial activity, and adversely affect plant growth and the ecological balance of the soil. When released into aquatic environments, they may exert toxic effects on aquatic organisms, promote eutrophication, stimulate excessive algal growth, deplete dissolved oxygen, and ultimately cause the suffocation and death of aquatic life. In addition, the production processes of conventional antisagging additives may generate harmful byproducts, contributing to air, water, and soil pollution. These processes also tend to consume large amounts of energy, increase carbon emissions, and exacerbate climate change. Therefore, from a life cycle perspective, the environmental impact of such additives spans their entire production, use, and disposal phases.

To address these environmental challenges, the development and application of biodegradable antisagging additives is essential. Natural polymer materials such as cellulose and chitin-based nanomaterials offer excellent biocompatibility and biodegradability. These materials can be broken down by microorganisms into harmless substances in the natural environment, thereby reducing long-term ecological impacts. In addition to their environmental advantages, these biodegradable materials also demonstrate good performance in preventing sedimentation of heavy materials, reducing solid content and friction, and protecting the formation. Furthermore, their production often involves green chemistry techniques, which help minimize harmful byproducts and energy consumption, promoting environmental sustainability throughout the life cycle.

However, the practical application of biodegradable additives still faces several challenges. Natural polymer materials often exhibit relatively weak thermal stability in drilling fluids, which need to be enhanced through chemical or physical modifications. Moreover, their production cost remains relatively high, which may limit widespread industrial adoption. Therefore, the promotion and application of these additives require a careful balance between performance, cost, and environmental benefits to ensure their feasibility and economic viability in drilling operations.

5. Conclusions

As traditional oil and gas resources decline, high-temperature and high-pressure formations are becoming key targets. Deeper drilling requires high-density fluids to maintain bottom hole pressure and prevent wellbore collapse. Common weighting agents, including Barite, ilmenite, hematite, magnetite, Micromax, and manganese dioxide, help meet these pressure demands but often exhibit sagging, especially under high-temperature and high-pressure conditions. However, their sagging issue compromises the uniformity of fluid density and may lead to wellbore instability and inefficient drilling performance. The sagging rate is influenced by factors such as density differences, particle size, fluid viscosity, flow velocity, temperature and pressure conditions, and wellbore inclination angle. Therefore, effective antisagging measures are essential. Four commonly used strategies to improve the stability of weighting materials in drilling fluids include: mixing different weighting agents, reducing particle size, surface modification of weighting agents, and adding antisagging additives. Among these, the addition of antisagging additives is the most effective. By enhancing the rheological properties of drilling fluids, these additives can significantly reduce sedimentation and improve suspension stability.

This review offers valuable insights for researchers and field engineers in the drilling fluid industry. For researchers, it serves as a comprehensive reference, consolidating recent advances and laying a strong foundation for future innovation. It identifies current knowledge gaps and highlights promising directions for further investigation, such as developing environmentally friendly weighting materials and antisagging additives, and optimizing existing formulations and processes. However, the present study is limited by the scope of experimental conditions and data availability. The findings are based on specific materials and test conditions, and may not be directly applicable to all drilling scenarios. Future research should aim to broaden the range of tested conditions and materials, and conduct extensive field trials to validate laboratory results. These efforts are critical to addressing increasingly complex drilling environments and stricter environmental regulations, ultimately supporting the safe, efficient, and sustainable development of drilling engineering.

6. Challenges and Future Directions

The prevention of sagging of weighting materials is a challenging task in drilling engineering, which involves many key areas, and requires comprehensive consideration of weighting material selection and optimization, rheological property control, environmentally friendly antisagging additive development, and monitoring and early warning system establishment. In response to these challenges, future research and development should advance in multiple areas, providing more possibilities and solutions for the efficiency, safety and environmental protection of drilling operations.

• (1)The selection and optimization of weighting materials in drilling fluid is one of the key issues. At present, there are many kinds of weighting materials used in drilling fluids, including Barite, hematite, Micromax, ilmenite, magnetite and manganese tetroxide. However, their deposition in drilling fluids is still very prominent. In order to solve this problem, it is necessary to deeply study the characteristics of different weighting materials and their sagging mechanisms in drilling fluids under severely conditions. Subsequently, optimizing their selection and concentration becomes crucial to reduce the deposition tendency and improve the stability and controllability of drilling fluids.
• (2)The control of rheological property of drilling fluid is very important to solve the sedimentation problem. Rheological properties not only determine the suspension ability of drilling fluids, but also directly affect the transport and circulation process of drilling fluid. Therefore, it is an important task to optimize the rheological properties of drilling fluid by regulating the amount and type of rheological modifiers as needed. This requires in-depth study of rheological properties and the development of more efficient formulations to ensure superior fluid flow stability and resistance to sedimentation under complex operating conditions.
• (3)With the improvement of environmental awareness, the research and development of environmentally friendly weighting materials and antisagging additives become an important direction. Current drilling fluid formulations commonly used in drilling operations often contain environmentally harmful chemicals, which pose potential risks to groundwater and soil environments. Therefore, future research should focus on developing more environmentally friendly alternatives to weighting materials and antisagging additives, as well as exploring green, biodegradable drilling fluid formulations to reduce adverse environmental impacts. For example, weighting agents derived from natural minerals or biobased materials can degrade naturally after drilling, minimizing environmental pollution. Antisagging additives based on nanotechnology can significantly enhance suspension performance, and reduce required dosages. However, the relatively high production costs of these new eco-friendly nanomaterials currently limit their widespread application. Thus, it is essential to reduce costs through technological innovation and process optimization, optimize drilling fluid formulations to minimize additive usage, and lower overall operational costs. At the same time, evaluating the economic feasibility and environmental benefits of new technologies is crucial to ensure long-term sustainability. By doing so can drilling fluid performance be maintained while achieving a win–win outcome for both the economy and the environment.
• (4)Establishing effective monitoring and early warning systems is also a key approach to addressing sedimentation problems. By integrating sensor technology with data analytics, real-time monitoring and analysis of drilling fluid sedimentation can be achieved, allowing early detection and timely mitigation of sedimentation issues, thereby ensuring uninterrupted drilling operations. This requires the development of efficient and reliable monitoring systems, along with early warning mechanisms and responsive measures to minimize the impact of sedimentation on drilling efficiency.

In conclusion, the above efforts are essential to meet the challenges of increasingly complex drilling environments and stricter environmental regulations, and to provide robust technical support for efficient, safe, and sustainable drilling operations.

The authors declare no competing financial interest.