Abstract
The loose sandstone oil reservoirs in the sea mostly adopt gravel packing completion, which leads to a decrease in permeability in the near-well area. Conventional blocking agents were difficult to simultaneously meet the requirements of water cut reduction and oil production increase. Therefore, this paper constructs a temperature-resistant in situ self-growing dispersed gel system and evaluates its displacement performance. The research results show that the formulation of the dispersed gel system is optimized through thermogravimetric analysis, with the monomer concentration determined to be 12 wt % and the cross-linking agent concentration determined to be 0.7 wt %. To improve injectability, the initial water content of the preinjected emulsion is selected as 40%, and its solubilization ability is determined to be 498.8%. During the solubilization process (from 40% to 80% water content), the viscosity increases from 67.89 to 3836 mPa·s. The influence of the change in emulsion water content during the solubilization process on the particle size of the dispersed gel particles is clarified; that is, the particle size first increases and then decreases, reaching the maximum at 70%, and the particle size distribution range becomes wider. In addition, this dispersed gel system shows good strength and long-term stability under different salinities. This study provides a new idea for controlling water and increasing oil production in large well-spacing, loose sandstone oil reservoirs in the sea.
1. Introduction
China is rich in offshore oil and gas resources; according to incomplete statistics, the proven geological reserves of offshore crude oil reached 335 million tonnes in 2022. Water injection development is the main method of secondary development. With water injection progressing into the middle and late stages, the oil wells have generally become flooded, and there is an urgent need for water production reduction and oil production increase management. However, offshore reservoirs are geologically loose, and gravel-filled completions or cuttings-lined completions are often used as sand prevention completion technologies, , which drastically reduce the reservoir permeability in the near-well zone. , Additionally, due to the limitation of development costs and offshore operating platforms, the reservoirs are mostly developed with large well spacing of 350 to 400 m; and these reservoirs are mostly high-porosity, high-permeability, and loose sandstone reservoirs, with an average pore size of about 0.5 mm.
The discontinuous-phase profile control systems, precross-linked gel particles, have a good effect on profile control and water shutoff and are commonly used for deep plugging of offshore reservoirs with large well spacing. − However, due to issues with initial particle size and expansion rate, it is difficult for the system to achieve its effect in offshore loose sandstone reservoirs with large well spacing. An overly small initial particle size restricts the maximum particle radius when the preformed gel particles expand, making it difficult to effectively block large channels. Conversely, an overly large initial particle size leads to difficulties in deep injection in terms of injectability and expansion rate. Despite this, scholars have conducted in-depth research on the expansion rate in an attempt to reduce the rate of deep injection. Liu et al. dispersed acrylamide and acrylic acid in a PVA cross-linked network to form an interpenetrating polymer network structure, which increased the cross-linking density within the three-dimensional network of the gels and delayed the diffusion of water molecules into the gel structure. In addition, “coating” techniques have been proposed to control the swelling rate of the particles. For example, Aghdas Heidari et al. proposed an organic–inorganic gel particle to control the swelling rate of preformed gel particles in simulated formation water by coating acrylamide-based PPG with a layer of tetramethyl orthosilicate. In addition to this, scholars have used microencapsulation techniques, pH-responsive gel particles, and other methods to slow the swelling rate. Although more research progress has been made on the swelling rate and stability of precross-linked gel particles, as well as the compatibility between particle size and pore throat diameter, the particle size of precross-linked gel particles still ranges from several tens of micrometers to several hundreds of micrometers due to limitations in the preparation process. For offshore loose sandstone reservoirs, where gravel-fill completions or cuttings-lined completions are used as sand control completion technologies, the particle size and swelling rate continue to limit the injection of precross-linked gel particles into such reservoirs.
Therefore, this study proposes the first in situ self-growth dispersed gel particle technology, which is based on stable W/O emulsions to achieve the in situ formation of dispersed gel particles in the subsurface. The emulsion, as the “reaction container” and “transport carrier” of the dispersed gel system during the process of reservoir transport, is the key link to realizing deep transport; the dispersed gel particles, as the target products of the in situ self-growth dispersed gel system, are the decisive factor for the realization of “efficient profile control and plugging water.” As the target product of the in situ self-grown dispersed gel system, the dispersed particles are the decisive factor for achieving “efficient plugging.” In this study, we first constructed an in situ self-growing dispersed gel system, then evaluated the profile control and plugging performance of the in situ self-growing dispersed gel system based on the key factors in the injection, transport, and blocking processes, and clarified the recovery enhancement mechanisms, so as to provide a new idea for the management of water reduction and oil enhancement in offshore loose sandstone reservoirs with a large distance between wells (Figure ).
1.
Profile control mechanism diagram of in situ self-growing dispersed gel system.
2. Experiments
2.1. Materials
Acrylamide (AM), N-vinylpyrrolidone (NVP), and N,N′-methylenebis(acrylamide) (MBA) were obtained from Chengdu Kelong Reagent Factory; 3-methacrylamido dopamine (DA) was sourced from Shanghai McLean Biochemistry Technology Co., Ltd.; and Span80, sodium bisulfite, ammonium peroxysulfate (ASP), anhydrous ethanol, and acetone were obtained from Aladdin Biochemical Technology Co., Ltd. The salt-resistant monomer (PETMEM) and modified silica nanoparticles were manufactured in the laboratory.
The crude oil used in the experiment is from a block of the Bohai oil field. The viscosity of the crude oil is 102 mPa·s, and its density is 0.88 g/cm3. The mineralization of the simulated formation water used in the experiment is 21,567.88 mg/L, and the water type of this simulated formation water is NaCl.
2.2. Preparation of Pre-emulsion System
First, the modified nano-SiO2 and surfactant Span 80 were added into a certain volume of the oil phase according to a specific ratio. The volume ratios of oil to water are 2:8, 3:7, 4:6, 5:5, 6:4, and 7:3. The mixture was placed in a constant-temperature water bath at 45 °C, and a stable dispersed oil phase system was formed after stirring for 30 min with a JJ-1B stirrer. Then, the monomers and cross-linking agent were added to deionized water, placed in an ultrasonic shock apparatus at 45 °C, and ultrasonically dispersed for 10 min to form a stable dispersed aqueous phase system. Finally, the two systems were mixed according to a certain ratio, and the mixed system was placed in a constant-temperature water bath at 45 °C. The pre-emulsion was formed after stirring for 30 min at a shear rate of 600 r/min using a JJ-1B stirrer in a nitrogen environment.
2.3. Preparation of In Situ Self-Growth Dispersed Gel Particles
First, the pre-emulsion system formed above is placed in a nitrogen environment, and the initiator system (ammonium persulfate and sodium bisulfite at a concentration of 0.02%) is added slowly to initiate the polymerization of the monomers in the inner phase of the pre-emulsion. Then, after the pre-emulsion is emulsion-broken, the system is divided into two parts: the oil phase and the dispersed gel particles. After separating the dispersed gel particles from the oil phase, the dispersed gel particles are thoroughly washed with anhydrous ethanol and acetone. Finally, the washed dispersed gel particles are placed into a vacuum drying oven at 100 °C for 24 h. The synthesis process and mechanism of the dispersed gel system are shown in Figure .
2.
Preparation process of dispersed gel particles.
2.4. Characterization of Dispersed Gel Particle Properties
In this study, Fourier transform infrared spectroscopy (FT-IR) was used to characterize the molecular structure of the dispersed gel particles based on the characteristic absorption peaks corresponding to different functional groups and chemical bonds in the dispersed gel; at the same time, scanning electron microscopy was used to characterize the microscopic morphology of the dry powder of the dispersed gel particles; and the thermal stability of the dispersed gel particles was analyzed based on the comparison of thermogravimetric and differential scanning calorimetry (TG/DSC).
2.5. Evaluation of In Situ Self-Growth Dispersed Gel System Profile Control and Flooding Performances
Due to the in situ self-growth, dispersed gel technology serves as the internal phase for the dispersed gel system of water-in-oil pre-emulsion injected into the reservoir. In the formation, under the action of shear stress and full contact with the formation water, the dispersed gel particles solubilize and precipitate after breaking the emulsion, and continue to move with the formation water and water expansion, ultimately achieving deep plugging. Therefore, the entire process is divided into three stages to study the in situ self-growing dispersed gel system’s driving performance. The first stage is the injection and solubilization process of the pre-emulsion, in which the injectability of the pre-emulsion is considered first. The initial oil–water volume ratio of the pre-emulsion system, i.e., the initial water content, is optimized using a rotational viscometer with viscosity as the evaluation index. Then, the pre-emulsion is injected into the reservoir and solubilized under the force of stratigraphic shear stress. Consequently, the viscosity variation of the pre-emulsion during solubilization and its solubilization capacity are also investigated in this section. The second stage is the process of precipitating dispersed gel particles after breaking the emulsion. Because the concentration of the dispersed gel system in the inner phase of the pre-emulsion decreases during the solubilization process, whether dispersed gel particles can be formed in this process and the particle size distribution law of the formed particles are of great significance for realizing deep plugging. Therefore, in this section, the particle size distribution of dispersed gel particles formed under different water contents (during the solubilization process) was investigated using laser particle size measurement. The third stage is the plugging stage of the dispersed gel particles after absorbing water and expanding. The particle size growth pattern of the dispersed gel particles after aging for different durations in the formation water was studied. Furthermore, the strength and long-term stability of the dispersed gel particles were evaluated by using the steering pressure method.
3. Results and Discussion
3.1. Optimization of Dispersed Gel System Composition
Thermogravimetric analysis (TG) is a method of evaluating the stability of profiling agents and is often applied to evaluate the stability of microspheres and dispersed gel particles. − It is determined by testing the initial moment when the preformed gel particles begin to lose weight upon heating, which represents the damage to their internal structure. Although TG analysis cannot directly characterize the thermal stability of dispersed gel particles in aqueous solution, it can compare the stability of dispersed gel particles of different compositions under the same conditions, which can lead to the optimization of the system formulation. Therefore, in the present study, dispersed gel particles were prepared under 70% water content, monomer composition (AM:NVP:DA:PETMAM = 9:2:0.5:0.5), and a complex emulsifier system concentration of 5 wt %, respectively, at different monomer concentrations and cross-linker concentrations. The effects of monomer and cross-linker concentrations on the stability of dispersed gel particles were preliminarily characterized based on thermogravimetric analysis (TG). Based on the results, the composition of the dispersed gel system was initially optimized.
Figure shows the TG curves of the dispersed gel particles at different monomer concentrations. When the monomer concentrations are 6, 9, 12, and 14 wt %, the epitaxial onset temperatures of the dispersed gel particles are 200, 220, 300, and 310 °C, respectively, and the weight losses corresponding to the epitaxial onset temperature points are 9%, 10%, 11%, and 13%, respectively. Among these, the temperature resistance increased with the increase of monomer concentration, and when the monomer concentration was higher than 12 wt %, the change of epitaxial onset temperature tended to level off, while the weight loss corresponding to the epitaxial onset temperature decreased with the increase of monomer concentration. Therefore, the monomer concentration of 12 wt % was optimized for the dispersed gel system.
3.
TG curves of dispersed gel particles at different monomer concentrations.
Figure shows the thermogravimetric curves of dispersed gel particles at different cross-linker concentrations. When the cross-linker concentrations are 0.5, 0.6, 0.7, and 0.8 wt %, the epitaxial onset temperatures are 295, 305, 330, and 300 °C, respectively, and the weight losses corresponding to the epitaxial onset temperature points are 12%, 10%, 9%, and 17%, respectively. Among these, when the cross-linker concentration is less than 0.7 wt %, the epitaxial onset temperature (temperature resistance) increases with the increase of cross-linker concentration. Conversely, when the cross-linker concentration exceeds 0.7 wt %, the epitaxial onset temperature decreases with the increase of cross-linker concentration. Additionally, when the cross-linker concentration is less than 0.7 wt %, the weight loss rate corresponding to the epitaxial onset temperature decreases with the increase of cross-linker concentration, whereas when the cross-linker concentration is greater than 0.7 wt %, the weight loss rate corresponding to the epitaxial onset temperature increases with the increase in cross-linker concentration. The epitaxial onset temperature of the dispersed gel particles is highest when the cross-linker concentration is 0.7 wt %. Therefore, it is preferred that the cross-linker concentration of the dispersed gel system be 0.7 wt %.
4.
TG curves of dispersed gel particles at different cross-linking agent concentrations.
Since offshore loose sandstone reservoirs are predominantly characterized by high porosity and permeability, large particle sizes of dispersed gel particles formed in situ in the subsurface are required. The concentration of the emulsifier system has been shown to have a significant effect on the emulsion particle size, which indirectly affects the particle size of the dispersed gel formed in situ. Therefore, in this study, under the conditions of 70% water content, 12 wt % monomer concentration (AM:NVP:DA:PETMAM = 9:2:0.5:0.5), 0.7 wt % cross-linking agent (MBA), 0.03 wt % initiator system, and a 3:1 composition of Span 80 and modified nanoparticles in the emulsifier system, the effects of the concentration of emulsifier systems on the particle size of the formed dispersed gel particles were investigated. The effect of emulsifier system concentration on the particle size of the formed dispersed gel was investigated. The particle size and distribution of dispersed gel particles at different emulsifier concentrations are shown in Figure . The median particle sizes of the dispersed particles prepared at emulsifier concentrations of 1, 3, 5, and 7 wt % were 59.146, 131.2, 204, and 280.55 μm, respectively, which showed that, first, the median particle size of dispersed particles increased with the increase of emulsifier concentration. This is because the dispersed gel particles are formed in the internal phase of water-in-oil emulsions, and their shapes and sizes are determined by the shapes and sizes of the emulsion droplets. Therefore, as the concentration of emulsifier increases, the larger the particle size of the emulsion droplets formed by the emulsifier molecules wrapped around them, which leads to the larger particle size of the dispersed gel particles formed in the internal phase of the water-in-oil emulsion. Second, as the concentration of the emulsifier system increases, the distribution of the dispersed gel particles’ particle size becomes more homogeneous. This may be due to the fact that when the concentration of the emulsifier system in the water-in-oil emulsion is low, the emulsion is unstable, resulting in nonuniform droplet sizes, which produces a wider particle size distribution. However, as the concentration of the emulsifier system increases, the stability of the emulsion improves, and the emulsifier molecules cover the interface more uniformly, resulting in a more uniform dispersed gel particle size distribution.
5.
Particle size and distribution of dispersed gel particles at different emulsifier system concentrations.
3.2. Characterization of the Basic Properties of Dispersed Gel Particles
In this study, the molecular structure of dispersed gel particles was characterized by using Fourier infrared spectroscopy, based on the characteristic absorption peaks corresponding to different functional groups and chemical bonds in the dispersed gels. The infrared spectra of the dispersed gel are shown in Figure . The light pink region (3600–3200 cm–1) is the vibrational region of the hydroxyl group, which comes from two adjacent phenolic hydroxyl groups in the structure of catechols, and the peak of the telescopic vibrational absorption of the hydroxyl group in catechols in this region appeared at 3431.81 cm–1.
6.
Fourier infrared spectroscopy of dispersed colloidal particles.
The light blue region (2950–2850 cm–1) is the vibrational region of methylene groups. Among them, the peak of the asymmetric stretching vibration of methylene groups in this region of dispersed gel appeared at 2920.72 cm–1, the peak of the symmetric stretching vibration of methylene groups of dispersed gel molecules appeared at 2850.20 cm–1, and the peak of bending vibration absorption of methylene groups in dispersed gum molecules appeared at 1463 cm–1. The light yellow region (1700–1650 cm–1) is the vibrational region of carbonyl CO. The carbonyl CO in the amide group in acrylamide has the peak of telescopic vibrational absorption in this region at 1653.35 cm–1 at the lower frequency of this vibrational region. The light orange region (1300–1000 cm–1) is the vibrational region of the C–O bond in phenol, and the characteristic absorption peaks of the stretching vibration of the C–O bond in catechols in this region appeared at 1290 cm–1 and 1018.84 cm–1, and both of the absorption peaks were weak. The light blue region (900–650 cm–1) is the vibration region of C–H, and the characteristic absorption peaks of this C–H bond in this region appeared at 879.73 cm–1 and 728.84 cm–1. The characteristic absorption peaks corresponding to hydroxyl, methylene, amino, carbonyl, and phenolic hydroxyl groups in the dispersed gum particles were qualitatively characterized based on FTIR spectroscopy, which identified the dispersed gum as the target product.
The morphology and particle size of dispersed gel particles are of great significance to achieving efficient plugging. In this paper, the morphology of dispersed gel dry powder was first characterized based on SEM, and the morphology of dispersed gel particles is shown in Figure . First, the dispersed gel particles are in the shape of regular spheres, which is because the emulsion droplets are in the shape of spheres under the action of surface tension, and therefore the dispersed gel particles formed inside the emulsion droplets are in the shape of spheres as well. Second, the particle size of the dispersed gel particles is more uniformly distributed because the emulsion droplets formed under the same conditions are more uniformly distributed in particle size, and therefore the particle size distribution of dispersed gel particles is also more uniformly distributed. Additionally, the particle size distribution of dispersed particles is more uniform because the emulsion formed under the same conditions has a more uniform particle size distribution, so the particle size distribution of dispersed particles is also more uniform.
7.
Morphology of dispersed gel particles.
The stability of the particles is key to determining whether the dispersed gel system can achieve long-lasting plugging; therefore, this paper uses a thermogravimetric analyzer to evaluate the thermal stability of the dispersed gel particles. A trace amount of dispersed gel particles was placed in the sample tray, and the initial temperature was set at 25 °C and the rate of temperature increase set to 1 °C/min on the thermogravimetric analyzer to monitor the weight change of the dispersed gel particles as the temperature increased. The temperature at which significant weight loss occurs can be used as an indicator of the thermal stability of the particles, and the higher the temperature at which weight loss occurs, the better the thermal stability of the dispersed particles. The DSC curves of dispersed gel particles at different temperatures are shown in Figure . In this study, the DSC curves were divided into different temperature intervals according to different weight loss mechanisms, which were the solvent evaporation zone, water loss zone or functional group breakage zone, transition zone, and carbon skeleton breakage zone. From the DSC curves, it can be seen that, first, when the temperature is lower than 100 °C, the DSC curve of the dispersed gel particles has a small, sharp heat absorption peak near 36.3 °C. Second, at temperatures from 100 to 400 °C, the DSC curves of the dispersed gel particles show different shapes of heat absorption peaks near temperatures of 102.3, 318.2, 378.2, and 405.5 °C. The DSC curves of the dispersed gel particles at temperatures from 100 to 400 °C showed different shapes of heat absorption peaks. At the temperature of 102.3 °C, the free water adsorbed on the surface of the dispersed gel particles evaporates, resulting in a small heat absorption peak in the DSC curve, and at the temperature between 300 and 400 °C, multiple heat absorption peaks appear in the DSC curve, which is partly attributed to the strong interaction between the gel and the gel. It is also partly due to the pyrolysis of the functional groups on the side chains in the polymer molecules at 300–400 °C, which leads to the thermogravimetric loss of the dispersed gel particles. Third, when the temperature reaches 525 °C, the DSC curve of the dispersed gel particles shows a very sharp heat absorption peak near 525.6 °C.
8.
DSC curves of dispersed gel particles at different temperatures.
3.3. Evaluation of Dissection Performance of the Dispersed Gel System
3.3.1. Pre-emulsion System Viscosity Change Law
The in situ self-grown dispersed gel system is transported in the reservoir as a pre-emulsion before forming dispersed gel particles. Therefore, in order to optimize the injectivity of the system, the viscosity of the system at different oil/water volume ratios was first investigated in this paper, and then a suitable oil/water volume ratio was preferred. Figure shows the viscosity of the pre-emulsion system prepared under different oil–water volume ratios. As can be seen from the figure, the viscosity of the pre-emulsion system prepared under different oil–water volume ratios increases with the increase of water content. The viscosity is lower when the water content is 20% to 50%, and the viscosity of the pre-emulsion starts to increase sharply when the water content is greater than 50%. At the same time, considering that the solubilization of the pre-emulsion system in the reservoir will also lead to a decrease in the concentration of the internal-phase dispersed gel system, to ensure the good injectivity of the pre-emulsion system and the formation of dispersed gel particles, the water content of the pre-emulsion is preferred to be 40% in this study.
9.
Changes in the viscosity of pre-emulsion at different water contents.
As the pre-emulsion system is injected into the reservoir, it comes fully into contact with the formation water under formation shear, causing the system to continue solubilizing. This leads to changes in the viscosity of the pre-emulsion, and the viscosity change rule has an important impact on its ability to transport within the reservoir. Therefore, this section investigates the viscosity change rule and solubilization ability of the pre-emulsion system during the solubilization process. Table shows the viscosity change rule of the pre-emulsion during the solubilization process. As can be seen from the table, during the solubilization process of the pre-emulsion from 40% to 80% water content, the viscosity of the pre-emulsion increased from 67.89 to 3836 mPa·s. After thepre-emulsion reached 50% water content, its viscosity began to increase sharply, with the maximum solubilization capacity of the pre-emulsion reaching 498.8%. It shows that the prepared pre-emulsion of the in situ dispersed gel system has excellent injectivity. As time increases, the pre-emulsion continues to solubilize, causing its viscosity to gradually increase, which helps inhibit the water–oil fluidity ratio and regulate the drive. When the water content reaches 80%, the viscosity of the corresponding pre-emulsion can reach 3636 mPa·s, and it can also play a certain regulatory role. During the solubilization of the system underground, the formation water further acts as the raw material for the system, which not only saves costs but also reduces the burden on the ground preparation process.
1. Viscosity Variation Rule of Pre-emulsion at Different Water Contents.
| Water content (%) | Emulsion type | Viscosity (mPa·s) | Shear stress (dyn/cm2) | Solubility (%) |
|---|---|---|---|---|
| 40 | W/O | 67.89 | 4.15 | 83.3 |
| 50 | W/O | 273.9 | 19.24 | 166.7 |
| 60 | W/O | 1244 | 96.37 | 125 |
| 70 | W/O | 1982.5 | 162.1 | 291.7 |
| 80 | W/O | 3636 | 267.2 | 498.8 |
Meanwhile, in this study, the changes in emulsion micromorphology during pre-emulsion solubilization were observed using a Leica microscope, as shown in Figure . During the pre-emulsion solubilization process, the emulsion particle size increased with the increase of the phase volume within the emulsion, and the stability of the emulsion further decreased. Dispersed gel particles were formed, and the dispersed particles precipitated when the emulsion was demulsified.
10.
Microscopic morphology of emulsion droplets at different emulsifier concentrations.
3.3.2. Particle Size Distribution Pattern of Dispersed Gel Particles
After the pre-emulsion system is injected into the reservoir, full contact with formation water under shear causes the pre-emulsion system to solubilize continuously, leading to a gradual decrease in the concentration of the internal-phase dispersed gel system during solubilization. In order to simulate this process, a pre-emulsion system with a water content of 50% was prepared in this section, with a monomer concentration of 12 wt % and a cross-linker concentration of 0.7 wt %. Pure water was then continuously added to the pre-emulsion system under shear to increase the water content to 60%, 70%, and 80%. Accordingly, the concentration of the dispersed gel system in the inner phase of the pre-emulsion was diluted to 0.4 to 0.7 times the original concentration, but it could still form dispersed gel particles. Figure shows the variation of the particle size of the in situ dispersed gel particles at different water contents. It can be seen that the median particle sizes were 115.72, 144.93, 249.51, 167.89, and 121.28 μm at water contents of 50%, 60%, 70%, 80%, and 85%, respectively. The distribution of dispersed gel particles becomes wider with increasing water content, and when the water content is below 70%, the particle size of the dispersed gel particles increases with increasing water content. However, when the water content exceeds 70%, the particle size decreases rapidly. This is because when the water content is below a certain value, the size of the water-in-oil emulsion increases with the increase of water content, which leads to an increase in the particle size of the dispersed gel particles formed in the inner phase of the water-in-oil emulsion. When the water content reaches 80%, the particle size of the dispersed gel particles decreases due to the dispersion of the emulsion droplets into smaller droplets. It is shown that the largest particle size of dispersed gel particles can be formed when the water content is 70% under the experimental conditions, as shown in Figure . In actual reservoir injection, the in situ self-growing system is injected in the form of a pre-emulsion. During shear transport in the reservoir, it easily solubilizes with the formation water, thus increasing the volume of the internal phase. However, due to the influence of formation water content and the distribution of dispersed gel particles, the corresponding water content at the time of formation is difficult to calculate accurately. Therefore, it is necessary to study this in combination with the specific reservoir aperture distribution and the distribution of oil and water. In this section, the rule of change of particle size of dispersed gel particles formed under different water contents is provided, offering a theoretical basis for its prediction.
11.
Particle size and distribution of dispersed gel particles at different water contents.
12.
Particle size of dispersed gel particles at different water contents.
When the pre-emulsion system is transported to the deep part of the reservoir to break the emulsion, the dispersed gel particles precipitate out and continue to be transported with the formation water in the deep part, absorb water, and expand gradually to regulate the high-permeability layer. Therefore, this section focuses on the growth pattern of dispersed gel particles in the process of water absorption and expansion in the formation water after precipitation. First of all, under the conditions of 70% water content, 5 wt % emulsifier system concentration, 12 wt % monomer concentration, and 0.7 wt % cross-linking agent concentration for the preparation of dispersed gel particles, the particles were dispersed in pure water and aged in an oven at 100 °C for 5, 10, and 20 days. A laser particle sizer was used to test the size of dispersed gel particles at different aging times. Figure shows the variation rules of dispersed gel particles under different aging times. As can be seen from the figure, the median particle size of the dispersed gel particles in the initial state at day 0 is 166.9 μm. After aging for 5 days, the median particle size increases to 327.3 μm. After 10 days of aging, the median particle size reaches 476.2 μm, and after 20 days of aging, the median particle size reaches 489.1 μm. The particle size of the dispersed gel particles increased with aging time and tended to stabilize after 10 days of aging.
13.
Particle size of dispersed gel particles at different aging times.
3.3.3. Evaluation of Strength and Long-Term Stability of Dispersed Adhesive Systems
The strengths of the dispersed gel system at different aging times are shown in Figure . As can be seen from the figure, the strength of the dispersed gel system in the initial state and after aging for 5, 10, and 20 days were 102.37, 117.67, 121.03, and 122.24 kPa, respectively. With the increase of aging time, the strength of the dispersed gel system showed a trend of increasing and then stabilizing. This is because, with the increase of aging time, the dispersed gel particles gradually absorb water and expand, causing the particle size of the dispersed gel particles to gradually increase, which results in the steering pressure of the dispersed gel system required to break through the sieve mesh becoming larger. After 10 days, the particle size of the dispersed gel particles gradually reaches a stable level, which causes the steering pressure of the dispersed gel system required to break through the sieve mesh to stabilize. The strength was 122.24 kPa after 20 days of aging, indicating that the dispersed gel particles in pure aqueous solution still exhibited good strength and stability after 20 days of aging.
14.
Change in the strength of dispersed gel particles in pure water with aging time.
Figure shows the strength change rule of the dispersed gel system under 2000 mg/L mineralization at different aging times. Since the formation water has a certain degree of mineralization, the formulated dispersed gel particles were placed in water with different degrees of mineralization (2000 and 4000 mg/L, with the water type being NaCl type) and aged at an ambient temperature of 100 °C, after which their strengths were tested. The aging time was still kept as the initial state: 5, 10, and 20 days. Figure shows the change rule of the strength of the dispersed adhesive system under 2000 mg/L mineralization at different aging times. As can be seen from the figure, the strength of the dispersed gel system was 98.21, 107.92, 117.65, and 119.43 kPa in the initial state and after aging for 5, 10, and 20 days, respectively. It can be found that the strength of the dispersed gel system showed a tendency to increase initially and then stabilize with the increase in aging time. This is because, within 10 days of aging, the dispersed gel particles absorb water and expand faster, causing the particle size of the dispersed gel particles to gradually increase. This results in the steering pressure of the dispersed gel system to break through the sieve mesh becoming larger. After 10 days, the particle size of the dispersed gel particles gradually reaches stability, causing the steering pressure of the dispersed gel system to break through the sieve mesh to stabilize. In addition, the strength of the dispersed particles was 119.43 kPa after 20 days of aging, indicating that the dispersed particles still had a certain strength and stability in the aqueous solution with a mineralization level of 2000 mg/L. The strength of the dispersed particles was 119.43 kPa after 20 days of aging.
15.
Dispersed gel particle strength with aging time in aqueous solution (2000 mg/L).
16.
Dispersed gel particle strength with aging time in aqueous solution (4000 mg/L).
Figure shows the strength change rule of the dispersed gel system under 4000 mg/L mineralization at different aging times. From the figure, it can be seen that the strengths of the dispersed gel system at the initial state and after aging for 5, 10, and 20 days are 89.14, 102.56, 112.47, and 113.43 kPa, respectively. With the increase of aging time, the strength of the dispersed gel system shows a trend of increasing and then flattening. This is because, within 10 days of aging, the dispersed gel particles gradually absorb water and swell, causing the particle size of the dispersed gel particles to increase gradually. As a result, the steering pressure required for the dispersed gel system to break through the sieve screen becomes larger. After 10 days, the expansion rate of the dispersed particles decreases, and the particle size tends to stabilize, resulting in the breakthrough pressure of the dispersed system through the sieve mesh also tending to stabilize. In addition, the strength of the dispersed gel particles in aqueous solution with a mineralization level of 4000 mg/L was 113.43 kPa. Compared with the strength of the dispersed gel in pure water, the increase in mineralization degree has a slight effect on the strength of the dispersed gel particles, but they still exhibit good strength.
4. Conclusions
This paper constructs an in situ self-growth dispersion gel system and evaluates the shutoff and oil-displacement performance of the pre-emulsion and dispersion gel particles within this system. The specific conclusions are as follows:
Based on the stability of the dispersion gel particles, the composition of the dispersion gel system was optimized. The experiment shows that when the monomer concentration is 12 wt % and the cross-linker concentration is 0.7 wt %, the dispersion gel system has good thermal stability.
To improve injectability, the initial water content of the pre-emulsion was determined to be 40%. The solubilization ability of the pre-emulsion (498.8%) and the viscosity change rule during the solubilization process were clarified; that is, when the water content increases from 40% to 80%, the viscosity increases from 67.89 to 3836 mPa·s.
The influence of the change in emulsion water content during the pre-emulsion solubilization process on the particle size of the formed dispersion gel particles was investigated. The results show that during the process of increasing the water content from 40% to 80%, the particle size of the formed dispersion gel particles first increases and then decreases. When the water content is 70%, the particle size reaches a maximum value of 249.51 μm, and the particle size distribution range gradually widens.
The strength and long-term stability of the dispersion gel particles under different salinities (pure water, 2000 and 4000 mg/L) were studied using the turning pressure method. The results show that after aging for 20 days, the dispersion gel particles still have good strength (122.24, 119.43, and 113.43 kPa respectively), indicating good strength and long-term stability.
Acknowledgments
The authors would like to express their gratitude to the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Engineering at Southwest Petroleum University for its technical support and experimental assistance. These contributions have played a crucial role in our research work, providing us with the necessary resources and opportunities to complete this study. They sincerely appreciate the professional technical support and equipment resources provided by the laboratory. Lastly, the authors acknowledge the National Natural Science Foundation of China (Grant No. U22B6005) for providing financial support for this study. They also thank the reviewers for their constructive comments.
The authors declare no competing financial interest.
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