Effects and Mechanism of Gaseous Acid on Surface Microtopography of Tight Sandstone Reservoirs

Abstract

Conventional reservoir acidification technology is limited by the rheology of acidic fluid, and it is difficult to penetrate deep into tight reservoirs to achieve effective stimulation. In this study, it is proposed to use gas acid to transform tight sandstone reservoirs. By carrying out the dissolution experiment of gas acid on a tight sandstone surface, the dissolution characteristics of rock surface under the action of gas acid are systematically analyzed, the change of wetting behavior after gas acid dissolution is quantitatively characterized, and the dissolution mechanism of gas acid on tight sandstone is revealed. The experimental results show that under the erosion of gas acid, the cemented minerals on the rock surface gradually dissolve, resulting in the increase of surface roughness. Among them, hydrogen fluoride gas (HF) has the most prominent effect on improving rock surface roughness, and the growth rate of surface area can reach 34%. At the same time, the contact angle of the rock surface decreases by 23.47°. The numerical simulation further shows that the concentration of gas acid, initial injection rate, molecular size, and density all have significant effects on the acidification effect. The above research results provide new ideas and theoretical basis for the efficient development of tight reservoirs.

1. Introduction

Against the backdrop of the global energy transition and the deepening development of unconventional oil and gas resources, tight reservoirs have become a research focus due to their immense resource potential. As a critical method for enhancing reservoir permeability postfracturing, acidification technology improves pore structures and increases recovery factors through acid–rock reactions. However, the complex mineral compositions, microscopic pores, and ultralow permeability of tight reservoirs present significant bottlenecks for conventional acidification. These include excessively high reaction rates, limited penetration distances, secondary precipitation clogging, and performance instability under high-temperature and high-pressure (HTHP) conditions, all of which severely constrain its operational effectiveness.

Reservoir acidification, as an effective stimulation process, has been studied systematically since the 1980s. Shaughnessy first elucidated the mechanism of sandstone acidification, noting that hydrofluoric acid (HF) can rapidly enhance permeability following a hydrochloric acid preflush, though it may also induce damaging silica precipitation. King subsequently confirmed that near-wellbore acidification could achieve order-of-magnitude increases in productivity. Since then, research has gradually shifted toward engineering adaptations. Scholars such as Wang, Abdelmoneim, and Gussenov emphasized that the effectiveness of acidification is highly dependent on the synergistic optimization of formation mineralogy with acid concentration and type. Addressing the specificities of tight and carbonate reservoirs, Guo et al. utilized molecular dynamics (MD) simulations to reveal the nonlinear migration patterns of acid gases in nanoscale fractures. They found that competitive adsorption mechanisms enhance the gas slippage effect and that pore size exerts a more significant influence on diffusion than gas composition. The study suggests that to minimize energy loss, the injection pressure should be maintained above the diffusion transition pressure to ensure that the acidic fluid remains in the molecular diffusion regime.

As oilfield development in China moves toward deeper formations characterized by high temperatures, high pressures, and strong heterogeneity, conventional acidification is evolving toward precision engineering. Extensive work has been conducted by Wang, Liao, Li, and Cao regarding conventional reservoirs and the treatment of acidic reinjection fluids. To address complex challenges, new systems such as organic acids, emulsified acids, and retarded acids have been developed. Shi et al. reduced the acid–rock reaction rate by approximately 70% through the introduction of the SPR-12 retarder, significantly extending the acid penetration distance. Ramy et al. achieved a 10-fold increase in injectivity in ultratight carbonates using a multistage chemical stimulation workflow. Furthermore, Mou et al. and Yan et al. proposed new pathways for process integration and optimization targeting deep formations and shale oil challenges, providing robust support for the adaptive development of tight reservoirs.

In recent years, gas acidification technology, represented by CO₂ flooding, has shown great promise for tight sandstones. Because of their low viscosity and small molecular characteristics, gases can effectively penetrate microscopic pore throats. Wang, Zhang, and Liu have preliminarily discussed the modification effects of acidic gases on pore structures and reinjection processes. Although there are approximately 85 acidic gas injection projects globally, and Ozowe confirmed that gas flooding can significantly improve displacement efficiency, Bai et al. pointed out that the dissolution and enhancement mechanisms of multicomponent acidic gases in tight matrices are drastically affected by temperature, pressure, and phase state. Research indicates that under low water saturation conditions, the permeability enhancement capability of acidic gases is superior, echoing the findings of Bai et al. regarding the dominance of pore tortuosity in gas free-movement mechanisms. However, Lu et al. also noted from engineering practices in the Tarim Basin that there remains a lack of systematic understanding regarding the reaction mechanisms of gas acidification in postfracturing tight sandstones.

While pursuing enhanced production, the remodeling of the formation’s physical and mechanical properties by acidic fluids is equally crucial. Gao et al. found through numerical simulations that the distribution of natural fractures (NFs) greatly increases the complexity of acidic fluid behavior, suggesting the alternating injection of cross-linked and micellar acids to balance etching and fluid loss. Lai et al. further clarified that mechanical weakening (such as the decline in Young’s modulus) caused by acid etching is closely related to acid type and concentration. This impact directly affects the safety and efficacy of fracturing operations; for instance, the model established by Wang et al. confirmed that acid-wash pretreatment can significantly alter the breakdown pressure of tight conglomerates. In summary, future research needs to focus further on the nonlinear flow laws of acidic fluids within complex pore throats, combined with the sensitivity assessments proposed by Yang, to achieve precise stimulation for ultralow permeability reservoirs.

To address the “bottleneck” of ultrafine pore throats in tight sandstone reservoirswhere traditional liquid acids are hindered from deep penetration due to rheological constraintsthis study innovatively proposes using carbon dioxide (CO₂) as a carrier for acidic gases. Leveraging the characteristics of ultralow viscosity and superior permeability inherent in gases, this approach penetrates deep into the formation, overcoming the limitations of conventional acidification such as excessively rapid reaction rates and short effective distances. Consequently, it achieves “deep penetration” and effective stimulation of the microscopic pores within tight reservoirs. Through the experimental and simulation framework illustrated in Figure , the mechanisms governing the transformation of gas acidification in tight sandstones are investigated, providing a scientific foundation for enhancing single-well productivity and ultimate recovery factors.

1.

Research technology diagram.

2. Experimental Materials and Methods

2.1. Experimental Materials

Experimental drugs: Hydrochloric acid (HCl, 36%), hydrofluoric acid (HF, 40%), polyphosphoric acid (PPA, 115%), fluoroboric acid (HBF4, 40%), natural sandstone (provided by Daqing No.7 Oil Production Plant of PetroChina, Quartz (46.9%) + clay minerals (35.9%) + plagioclase (14.5%), porosity 7.34%, permeability 1.07 × 10–3 μm²).

The acid solution system used in the experiment was prepared under laboratory conditions (25 °C, 0.1 MPa), with all solutions prepared at a 4% concentration, sealed, and kept ready for use.

Experimental instruments: ISCO pump, water bath heating device, acid atomization device, drying oven, ordinary optical microscope, contact angle tester, laser confocal microscope, and rock cutting machine.

The acidic gas employed in the research facility demonstrates high corrosiveness. In both experimental and engineering applications, strict safety procedures were followed. The entire experimental process was carried out within a fume hood utilizing a sealed circulation system, which was equipped with an alkaline neutralization unit for exhaust gas treatment, thereby ensuring the personnel safety and environmental compliance. On-site operations must fully conform to the SY 6443-2000 safety standards for fracturing and acidizing operations to ensure a safe and stable workflow.

2.2. Experimental Procedure

2.2.1. Experiment on Dissolution of the Rock Surface by Gaseous Acid

This experiment simulates the changes in rock surface dissolution characteristics after injecting gaseous acid into a formation originally containing formation water. The experimental parameters are shown in Table .

1. Experimental Parameter Table.

category	parameters	units	values/range
material parameter	initial permeability	μm2	1.07 × 10–3
	porosity	%	7.34
	acidic gas concentration	%	4
test parameter	confining pressure	MPa	10
	temperature	°C	80
	injection rate	mL/min	0.5

Based on the main acid formulation used in the acidizing operations of oil wells in the No.7 Oil Production Plant of Daqing Oilfield and Tadong Oilfield in Xinjiang, the rock surface gaseous acid acidizing simulation experiment was carried out using the experimental device shown in Figure . The specific experimental procedure is as follows.

• (1)A natural core column was selected and cut into thin slices with a thickness of approximately 5 mm using a core cutter. The surface of the core slices was thoroughly cleaned with deionized water to remove debris and impurities and then placed in a constant-temperature drying oven to dry to a constant weight for later use.
• (2)The dried core slices were loaded into a closed reaction chamber in groups of four. One mL of formation water was added to the surface of each rock slice, and then the chamber was sealed to prevent gaseous acid leakage during subsequent experiments.
• (3)The acid solution was converted into micrometer-sized acid mist through an atomization device and stored for later use.
• (4)The CO₂ displacement pump was started and high-purity CO₂ gas was introduced into the reaction system for 5 min to completely replace the air inside the device.
• (5)The acid gas injection pump was turned on and the acid gas injection rate was adjusted.
• (6)The reaction times were set to 5, 15, 30, and 60 min, respectively, to study the dissolution effect of gaseous acid on rock minerals and the improvement degree of permeability under different action durations, providing an experimental basis for the optimization of on-site acidizing process parameters.

2.

Experimental device and process.

2.2.2. Research on the Change Law of Dynamic Wettability of the Rock Surface after Dissolution

As shown in Figure , this is a schematic diagram of the working principle of the self-developed high-temperature and -pressure contact angle measurement apparatus.

3.

High-temperature and high-pressure wetting angle measuring instrument (self-developed).

Prior to wettability testing, all acid-treated rock samples were subjected to standardized pretreatment, with thorough removal of any residual surface liquids to ensure consistency of initial test conditions and comparability of the experimental results.

This study innovatively introduces a high-temperature and high-pressure contact angle measurement system to determine the surface wettability of acid-treated rock samples under simulated actual reservoir temperature and pressure conditions. The apparatus is equipped with a high-temperature/high-pressure chamber and a high-precision imaging system, enabling the capture of the spreading morphology of liquid droplets on rock surfaces and calculation of contact angle values.

The experimental procedure is as follows.

• (1)The contact angle measurement instrument was activated, ensuring normal operation of the imaging system and droplet injection system, and instrument calibration was performed to guarantee the accuracy of the measurement results.
• (2)A clean glass slide was prepared, a droplet injection system was used to generate uniform droplets on the sample surface, and images of the droplets formed on the surface were recorded using the imaging system, ensuring image clarity and accuracy.
• (3)The contact angle formed by the droplet was measured using the instrument software and the measurement results were recorded.
• (4)Repeat the experiments at different locations and on multiple samples to obtain reliable average values.

On the basis of the obtained contact angle data, the influence mechanism of gaseous acid on the chemical properties and wettability behavior of rock surfaces during the acidizing process can be systematically evaluated, providing key experimental evidence for a deeper understanding of the effects of acidizing on reservoir fluid distribution and seepage characteristics.

2.2.3. Research on the Change Law of Rock Surface Roughness after Dissolution

As shown in Figure c, this study employed a laser confocal microscope (LEXT OLS5000 3D Measuring Laser Microscope) to perform topographic scanning of the acidized regions on rock samples. Prior to testing, the sample surfaces were subjected to rigorous cleaning to remove surface impurities and prevent any interference with surface topography measurements.

During the measurement process, the instrument conducted point-by-point scanning over the target area, acquiring real-time three-dimensional height coordinate information for each pixel. Surface profile curves were extracted, and multiple key surface roughness parameters were calculated, including arithmetic mean roughness (Ra), root-mean-square roughness (Rq), and maximum height roughness (Rz).

Compared with conventional mechanical contact profilometers, the laser confocal microscope offers the advantages of noncontact and nondestructive measurement, effectively avoiding scratches or deformation of fragile rock surfaces that could be caused by a probe tip. This provides reliable technical support for evaluating the impact of acidizing on the evolution of the rock surface topography.

2.2.4. Model Simulation Study on Acid Gas Erosion on the Rock Surface

To gain deeper insights into the microscopic mechanisms of gas acidizing, a microscopic reaction model coupling dissolution and etching was developed with its basic principle illustrated in Figure . This model comprehensively accounts for the entire process: diffusion of gaseous acid within rock pores, its dissolution in pore water to form active acidic liquid, and subsequent chemical reactions with mineral components, leading to etching.

4.

Acid gas dissolution and corrosion-model principle.

It should be noted that the numerical simulations focus primarily on investigating the relative trends of gaseous acid diffusion and interaction at the molecular scale, whereas the laboratory experiments address the macroscopic optimization of fluid injection. Consequently, no direct quantitative correlation or numerical comparison is intended between the two distinct scales.

By integrating multiphase flow, mass transfer, and reaction kinetics, the model aims to elucidate, at the microscopic scale, the evolution patterns of rock surface properties under the action of gaseous acids and to investigate the key factors influencing the effects of gaseous acids on rock surfaces.

To facilitate a deeper understanding of the microscopic mechanisms of gas acid dissolution and etching, the microscopic acid gas dissolution–etching coupled reaction model developed in this study was appropriately simplified with the following assumptions.

• (1)It is assumed that the etching process of acid gas molecules on the rock surface exhibits a complete one-to-one correspondence.
• (2)It is assumed that the motion of acid gas molecules within microfractures follows superimposed Brownian motion with an initial velocity and gravitational effects.
• (3)It is assumed that acid gas molecules undergo random rebound upon contact with nontest surfaces.
• (4)It is assumed that the temperature and pressure remain constant throughout the reaction process.

By defining the formation geometry, permeability, diffusion coefficient, and reaction parameters and by setting the injection source and boundary conditions, the model can dynamically output key parameters such as gas concentration distribution, pressure field, velocity field, and diffusion coefficient.

Through the imposition of these simplifying assumptions, the model defines the formation geometry, permeability, diffusion coefficient, and reaction parameters and configures the injection source or boundary conditions, thereby enabling dynamic output of critical parameters including gas concentration distribution, pressure field, velocity field, and diffusion coefficient. Acid gas enters the system with an initial velocity from the inlet, is subjected to gravitational effects, and diffuses within the pores, satisfying the following governing equation of motion:

$$m\frac{\mathrm{d}v}{\mathrm{d}t}=\mathrm{m}\mathrm{g}-\gamma v+F_{\mathrm{b}\mathrm{r}\mathrm{o}\mathrm{w}\mathrm{n}\mathrm{i}\mathrm{a}\mathrm{n}}$$ 1

wherem: mass of acid gas molecules.v: speed of acid gas molecules.γ: friction coefficient.F _Brownian: the force of Brownian motion.

In this study, we mainly consider the reaction between HF gas molecules and tight rock (SiO₂), when the acid gas molecules touch the boundary, a part of them will participate in the acid rock reaction, and the change of porosity satisfies the Kozeny–Carman model:

$$k=k_0{\left(\frac{\varphi }{{\varphi }_0}\right)}^3{\left(\frac{1-{\varphi }_0}{1-\varphi }\right)}^2$$ 2

wherek _0, ϕ₀: initial permeability (m²) and porosity.μ: acid viscosity (Pa·s), HF solution viscosity is 1.0 × 10^–3 Pa·s.

SiO₂ dissolution and porosity evolution meet:

$$\frac{\partial \varphi }{\partial t}=\frac{R_{\mathrm{H}\mathrm{F}}·M_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}}{{\rho }_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}}$$ 3

where $M_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}$ : molar mass of SiO₂, 60 × 10^–3 kg/mol. ${\rho }_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}$ : density of SiO₂, 2650 kg/m³.

Dynamic grids and level sets:

$$\frac{\partial h}{\partial t}=\frac{R_{\mathrm{H}\mathrm{F}}·M_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}}{{\rho }_{\mathrm{S}\mathrm{i}{\mathrm{O}}_2}·A_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}}}$$ 4

whereh: depth of erosion pit, m. $A_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{c}\mathrm{t}}$ : droplet–rock contact area (m²) with wettability dynamics.

The other part will bounce off the wall and burst toward the far end, satisfying the velocity reversal equation:

$$v_{\mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r}}=-e·v_{\mathrm{b}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{e}}$$ 5

wheree is the coefficient of recovery (0 ≤ e ≤ 1, e = 1 for perfectly elastic collisions).

3. Results and Discussion

3.1. Analysis of Acidification and Dissolution Characteristics

According to the experimental procedure described in Section , gas acid etching experiments were conducted on the surfaces of tight rock samples. Upon completion of the experiments, the acquired surface topography images were uniformly preprocessed, followed by binarization using the threshold segmentation method to enhance the contrast between the etched regions and the unreacted matrix, as shown in Figure .

5.

Comparison of the threshold segmentation method before and after processing.

The processed binary image is shown in Figure c, where the white regions represent dissolution pores or grooves formed by the action of acid gas, while the black regions correspond to rock surfaces that have not undergone significant chemical erosion.

6.

Analysis of dissolution characteristics of the rock surface. The scale of Figure c is identical to that of Figure .

Figure a illustrates the primary action zone of gaseous acid. Following hydraulic fracturing operations, the gaseous acid primarily acts in the surrounding fracture zones. Due to its ultralow viscosity, the gas exhibits exceptional permeability, enabling it to penetrate deeper into formation pores. Consequently, its effective action range is significantly broader than that of the liquid acid, resulting in superior acidification outcomes.

Figure b demonstrates the dissolution effects on rock surfaces induced by acid gas dissolving in formation water. Analysis of Figure c reveals that HF and HBF₄ exert pronounced etching effects on tight sandstone. In all cases, longer acid gas etching durations resulted in more prominent etching features. In contrast, HCl exhibits negligible etching action on quartz, feldspar, and other siliceous minerals, while PPA fails to react effectively with siliceous minerals, rendering changes to the rock surface to be negligible.

As a retarded acid, HBF₄ shows limited effects over short durations; however, with a prolonged time, HBF₄ undergoes slow hydrolysis to generate hydrofluoric acid, progressively intensifying the reaction. It can also be observed that, during the initial stage of reaction, the acidizing effect appears as scattered pointlike features, gradually becoming more uniform over time, indicating uniform etching of rock surfaces near the fractures.

The production enhancement mechanism of gaseous acid involves the penetration of gaseous acid into pore channels, where it dissolves in the formation water to form localized acidic solutions. These solutions subsequently corroded the rock downward. However, the resulting acid concentration remains relatively low with limited H⁺ supply. As the acid–rock reaction proceeds, the H⁺ concentration in the solution decreases, leading to a reduction in the reaction rate and intensity.

3.2. Evolution Law of Dynamic Wettability during Acid Etching of the Rock Surface

To further investigate the regulatory mechanism of gaseous acid acidification on rock surface wettability and thereby evaluate the evolution patterns of fluid distribution characteristics and seepage efficiency within reservoir pores, this study employed a high-temperature and high-pressure contact angle goniometer to measure contact angles under simulated formation conditions. The variation pattern of the water-phase contact angle on tight sandstone surfaces induced by the same acid system is shown in Figure .

7.

Change law of the contact angle after the same acid action.

Figure presents statistical data on the changes in the contact angle of the original rock surface induced by various acids after different durations of acidizing treatment.

8.

Change of the water contact angle on the rock surface by gas acid.

Figure illustrates the changes in the water-phase contact angle on tight sandstone surfaces after treatment with gaseous acids for 5, 15, 30, and 60 min. The results indicate that, at all time points, HF gas consistently exhibits the most pronounced improvement in the water-phase contact angle of the rock, confirming its excellent compatibility with tight sandstone. Furthermore, the magnitude of contact angle reduction increases with prolonged treatment time, reaching a decrease of 23.47° at 60 min.

In contrast, HCl primarily dissolves carbonate cements and shows a relatively stable improvement in the water-phase contact angle of the rock surface within 60 min, mainly by reducing the adsorption of hydrophobic ions and thereby optimizing hydrophilic conditions on the rock surface. HBF4, as a delayed acid, demonstrates time-dependent effects on the rock surface. During the early stage of the reaction, its improvement in the contact angle is limited due to the slow hydrolysis rate and insufficient release of effective hydrofluoric acid. As the reaction time extends, HBF4 gradually hydrolyzes and continuously releases hydrofluoric acid, leading to a rapid enhancement in both the etching and wettability alteration effects. By 60 min, the magnitude of the contact angle change surpasses that of HCl. PPA consistently yields the lowest degree of improvement in the contact angle on tight sandstone surfaces throughout the entire experimental period. This is attributed to its inability to react effectively with siliceous minerals or carbonate cements in sandstone, resulting in only a minor wettability regulation through the weak acidity of the environment.

The overall wettability results of the rock surfaces obtained throughout the process are presented in Figure .

9.

Results of the effect of acidification on the wettability of demonstrated surfaces.

In the oil displacement process, when the direction of the capillary force aligns with the direction of oil displacement, the flow resistance of crude oil can be reduced. The aforementioned research findings demonstrate that gaseous acid treatment can effectively alter the wettability of rock surfaces by reducing the water-phase contact angle on the rock surface, thereby enhancing the oil displacement efficiency and improving crude oil recovery. This represents one of the primary mechanisms by which acidization increases oil recovery.

The improvement in rock surface wettability induced by acid primarily manifests as the acid’s ability to etch mineral components, such as quartz and feldspar in the rock, exposing fresh rock surfaces rich in hydroxyl groups and forming hydrophilic structures. This transformation directly leads to a significant reduction in the contact angle, resulting in a marked decrease in the water-wet tendency of the reservoir. The enhancement of hydrophilicity facilitates the formation of a continuous water-wet film on the rock surface, driving the oil phase toward the center of the pores and promoting the formation of discrete oil droplets. Consequently, this improves oil displacement efficiency and effectively increases crude oil recovery.

3.3. Roughness Analysis of a Laser Confocal Microscope

To more precisely analyze the acidification effects of gaseous acid on rock surfaces and thereby elucidate the modification mechanisms of acidizing, the surface roughness and topographic undulations of acidized rock samples were compared and the etching efficacy of gaseous acid was examined. The etching characteristics induced by different acid types on rock surfaces were observed by using a laser confocal microscope, as shown in Figure .

10.

Laser confocal image of an acidified rock surface.

We measured the following surface roughness parameters after gaseous acid treatment: arithmetic mean roughness (Sa), root-mean-square roughness (Sq), and maximum distance of wave peaks and peaks to valleys (Sz). In rock surface roughness analysis, the selection of parameters must be closely integrated with geological features and research objectives, while in chemical dissolution studies, Sa (arithmetic mean height) is commonly used to quantify the increase in the reaction surface area and acid diffusion efficiency. The measurement results of Sa on the rock surface after the acid action of four gases are shown in Figure .

11.

Variation law of roughness after acid dissolution of different gases.

From the root-mean-square roughness (Sa) curve, it is observed that the roughness values of all experimental groups show an increasing trend with the passage of time, and the roughness of three groups except HBF4 has a decreasing trend after 30 min, especially in the dissolution process of HF. This is due to the gradual communication of rock pores in the process of continuous acid reaction, especially the local collapse in the contact process, which leads to the decrease of roughness; in the whole process, HCl showed a slow increase trend, while PPA almost did not change. Because of its sustained release characteristics, HBF₄ was still in the erosion burst stage at about 60 min. The concentration of HCl showed a slow increase trend, but HBF₄ was still in the erosion burst stage of HBF₄ due to its sustained release characteristic, and HBF₄ was still in the erosion-burst stage.

Regarding the decrease in roughness values observed after 30 min of HF treatment, this phenomenon is attributed to the physical evolution of tight rock microstructures from “differentiated corrosion” to “structural collapse”. Experimental results indicate that the specific surface area growth rate under HF treatment peaks at 30 min (34%) before declininga trend that closely correlates with changes in roughness parameters. Mechanistically, HF directly erodes silicate framework minerals such as quartz and feldspar within the rock, rather than merely acting on cementing materials. During the initial reaction phase, heterogeneous erosion generates numerous pits of varying depths, significantly increasing surface roughness. However, beyond 30 min, excessive erosion of framework minerals weakens the microscopic support structures, leading to “local collapse”. This collapse causes sharp erosion features to fragment or smooth out, while fine silicate precipitates formed during the reaction fill smaller pores, resulting in a smoother surface morphology at the microscopic scale and consequently reduced macroscopic roughness values in later experimental stages.

In practical applications, it is advisable to control the duration of acidification and strive to improve the continuous injection and replenishment ability of gas acid to achieve the ideal effect.

At the same time, the change trend of the surface area growth rate is shown in Figure .

12.

Trend chart of the surface area growth rate.

It can be seen from the experimental data in Figure that all acid treatments can increase the specific surface area of rocks but there are obvious differences in their growth patterns and efficiencies.

According to the experimental data, HF shows the strongest dissolution ability, reaching a maximum surface area growth rate of about 34% at 30 min, which indicates that HF reacts very violently in the initial stage and can rapidly expand the rock pores and increase the surface roughness. However, with the prolongation of the reaction time, the surface area growth rate of HF-treated samples decreased slightly, which may be caused by the precipitate formed during the reaction or collapse during contact. Nevertheless, HF is still an ideal choice for rapid retrofitting of reservoir structures, especially where the rapid formation of high permeability pathways is required.

In contrast, HBF4, as a retarded acid, did not react violently in the early stage, gradually increasing throughout the test period and reaching levels close to the peak HF at 60 min. For HCl, it showed a relatively steady growth trend, gradually increasing from approximately 11% in the initial stage to 24%. The growth rate of HCl is slow and ultimately limited without obvious fluctuations. This shows that HCl mainly depends on dissolving cement and does not dissolve the main components of rock in a large area. The performance of PPA is the least significant, and its surface area growth rate only slowly increased from 11% to 14%, with little obvious change. As a weak acid, PPA has an extremely limited role in dissolving rocks, and it is difficult to effectively change the rock-face structure.

According to past experience and acid gas acidification experiment results, there is a significant positive correlation between the dissolution degree of the rock surface and the dissolution time. As shown in the figure, with the increase of acid gas injection time, the roughening characteristics of the rock surface become more and more obvious, and the evolution of its micromorphology can be divided into five stages, as shown in Figure .

13.

Evolution stages of micromorphology.

During the whole process, the degree of rock dissolution gradually increases with time, and the change of microscopic morphology is highly compatible with the dynamic evolution of the acid–rock reaction, which provides an intuitive microscopic basis for the selection of acid type and action duration in gas acidification transformation.

3.4. Simulation Analysis of Acid Gas Dissolution and Corrosion

According to the established model, the process of gas acid forming an acidic liquid with water droplets and dissolving pores after entering the aqueous channel is simulated.

The simulation was conducted under conditions of 300 K and 1 MPa, with an acid gas release rate of 0.1 m/s, acid gas particle size of 2 μm, gas density of 1000 kg/m³, and a release frequency of 10 events per second. Nontest boundaries were set to diffuse reflection and stationary conditions (50% probability), while test boundaries included freezing and mass deposition (10% rebound probability). The release location was randomly determined at the inlet, and the outlet condition was set to complete disappearance (100% probability). The results are shown in Figure .

14.

Dissolution characteristics of the formation water wall.

As shown in the above figure, Figure shows the corrosion of the formation water walls. It can be seen that the longer the acidification time, the more obvious the acid etching effect, which is manifested by the downward corrosion of the rock surface as a whole, including many pits with high and low undulations. From the perspective of time evolution, in the initial stage of 0–1 s, the measured values of both figures are close to the baseline, and the acid etching has not been fully activated; the reaction intensifies at 2–5 s, and the dissolution depth fluctuates violently, which reflects the competition between local dissolution and accumulation of corrosion products in the nanoscale region. There is another obvious feature at the nearest entrance. The corrosion at the slightly distal end of this area is not obvious. The reason may be that it is close to the acid gas inlet, has high acid gas concentration, less loss, and can be fully dissolved and corroded; by the late stage of 7–10 s, the reaction was still intensifying and the depth was further increased.

Considering the influencing factors of dissolution characteristics, this paper mainly considers four factors: gas acid concentration, initial flow rate of gas acid, gas acid size, and acidification reaction time. The final effect of each factor obtained by changing different conditions is shown in Figure .

Figure compares the effects of gas acid release density, initial acid gas flow rate, acid gas size, and acidification reaction time on the acidification effect. It can be seen from the figure that the higher the acid gas concentration, the slower the initial flow rate, the larger the size and the greater the density, and the greater the corresponding acid etching depth. It can be seen from Figure a that with the increase of acid gas release density, the corrosion depth deepens at the same time; however, when the release density reaches a certain degree, the change of corrosion pits tends to be limited. It can be seen from the simulation diagram that when the release density is 20 s^–1, the change range reaches the maximum value; after that, the release density is increased, and the variation range of pits is extremely limited.

15.

Comparative analysis of influencing factors of the dissolution effect.

In contrast, the influence of acidic gas particle size is more pronounced. As illustrated in Figure c, larger particle sizes correlate with enhanced dissolution capacity, as increased dimensions significantly promote the diffusion rates and reaction kinetics of the acidic gas within the pore water. While a moderate increase in atomized particle size improves the dissolution efficiency per unit volume, excessively large particles are not recommended. Due to the ultralow permeability inherent in tight sandstones, oversized particles tend to bridge or clog the microscopic pore throats, thereby impeding deep penetration. Since this study specifically targets tight reservoirs, excessive particle size would impair injectivity and negate the unique advantages of acidic gases. Consequently, in practical operations, the particle size must be carefully calibrated to ensure compatibility with the formation’s pore structure, thereby optimizing the stimulation performance.

From the density of acid gas (Figure d), the greater the density of acid gas, the stronger the acid etching effect on the inlet area of acid gas, but the overall difference is not large, so the density of acid gas is not considered to be the main factor affecting the acidification effect.

4. Conclusions

This study employs a combined experimental and numerical simulation approach to systematically elucidate the mechanism by which gaseous acids modify the surface microstructure and wettability of tight sandstone reservoirs. It clarifies the dissolution characteristics of different gaseous acid systems and identifies the key regulatory principles for achieving a balance between deep penetration and reservoir integrity through optimized injection parameters, yielding the following conclusions:

• (1)The modification of the rock surface wettability and roughness by gas acid is also one of the main mechanisms of acidification modification to increase production. At the same time, gas acid can effectively connect pores and improve the permeability of tight sandstone reservoirs.
• (2)Among the four gas acids HCl, HF, PPA, and HBF₄, HF shows the strongest dissolution ability, which can significantly change the micromorphology and wettability of rocks, while HBF4 has slow-release characteristics, and its final effect is no less than HF.
• (3)Gas acidification treatment can increase the specific surface area of the rock by about 34% at the maximum, and the surface roughness is significantly positively correlated with the acidification time. However, because HF can dissolve siliceous minerals, it may cause the collapse of the rock skeleton and silicate precipitation, and the roughness decreases in the later stage of the experiment.
• (4)The release density, molecular size, and initial injection rate of gas acid are the key parameters to regulate the acidification effect. Moderately increasing the release density and molecular size can enhance the corrosion depth, and a higher injection rate is conducive to deep penetration; therefore, comprehensive optimization is needed to balance efficiency and reservoir integrity.

The authors declare no competing financial interest.