Permeability Damage Rate Research Articles

Experimental study on optimizing tailwater reinjection technology for weakly consolidated sandstone geothermal reservoirs

The difficulty of recharging the weakly consolidated sandstone geothermal reservoirs (WCSGR) is a macroscopic manifestation of the interplay of multiple factors associated with both fluids and formation properties. However, current studies predominantly focus on the influence mechanisms of individual factors on the reinjection of WCSGR. To address this limitation, a high temperature–pressure core flow experimental platform, combined with an orthogonal experimental design, is employed to investigate the dynamic reinjection process of WCSGR under the combined effects of multiple factors. The research indicates that high temperatures significantly impact the movement of 3 μm microspheres, leading to a rapid decrease in core permeability. Additionally, in experiments conducted at flow rates of 2 mL/min, 3 mL/min, and 4 mL/min, the pH stabilization time of the outflow solution decreased, while both the pH value and turbidity of the outflow solution increased, compared to the experiments conducted at flow rate of 0.5 mL/min and 1 mL/min. The permeability damage rate (%) was used as the experimental evaluation index. The primary and secondary order of influence of each factor on the permeability is: microsphere size > flow rate > turbidity > confining pressure > temperature. The optimal levels for each factor are 5 μm, 4 mL/min, 2 mg/L, 1.4 MPa, and 25 °C, respectively. Consequently, the reinjection technology of GT in WCSGR is optimized.

Experimental study of permeability characteristics of fracture-hosted hydrate-bearing clay sediments under triaxial shear

Reservoir permeability during the hydrate exploitation process is closely related to gas production. Fractures influence hydrate distribution and the evolution of reservoir permeability. Moreover, shear during exploitation can disturb the structure of sediments. Therefore, studying permeability and its evolution under shear in fracture-hosted hydrate-bearing sediments is significant for hydrate exploitation projects in the South China Sea. In this paper, standard sand and montmorillonite were mixed, and three different fracture angles were created to simulate sediment fracture. Consolidation, shear, and permeability measurements were employed to investigate mechanical and permeability properties. The results showed that the permeability of fracture-hosted hydrate-bearing sediments decreased and then increased with increasing hydrate saturation, both before and after consolidation. The compressibility of sediments played an important role in the permeability damage rate caused by consolidation and normalized permeability at the breaking point. Furthermore, fractures caused an undesired double peak and the disappearance of the permeability breaking point hysteresis relative to the stress breaking point. However, these effects were mitigated by increasing saturation. Lastly, the analysis of permeability sensitivity coefficients revealed that permeability was most sensitive to strain in the early shear stages. This study can guide methane hydrate exploitation in the South China Sea.

Seepage characteristics and mechanism of double damage basalt fiber reinforced concrete under three-dimensional variable stress states

Concrete experiences a three-dimensional variable stress state during its service life. To explore the permeability evolution and mechanism under such conditions, a permeability test was conducted on double damage basalt fiber concrete subjected to loading and unloading of twice confining pressure. Pore structure and microstructure of specimens were analyzed before and after permeability test. The findings reveal that the reduction in permeability after primary cycle of confining pressure loading and unloading is significantly higher than that after secondary cycle, and residual deformation is also more pronounced after primary cycle. Both high temperature and salt freeze-thaw cycles induce initial damage to specimens, consequently altering internal pore structure and subsequently impacting initial permeability. Notably, initial permeability decreases in the case of 10th salt freeze-thaw cycles, while it increases under other conditions. The greater the initial extent of damage to the specimen, the higher the permeability damage coefficient and the permeability damage rate. As the confining pressure decreases, the attenuated permeability increases, with the permeability decay rate reaching a significant 88.53 %. Plastic deformation of pores during loading and unloading process cannot be fully reversed, leading to the development of macropores and mesopores into smaller pore sizes, resulting in a remarkable 87.17 % increase in the proportion of micropores. Microscopic observations reveal smaller pores within the matrix, shorter crack lengths, and narrower crack widths in the specimen. The application of confining pressure reduces permeability channels within specimens, ultimately causing a decline in permeability.

Reservoir damage mechanism and “Double protection” drilling fluid of tight sandstone gas reservoir in central SiChuan Basin

It is very important to clarify the reservoir damage characteristics and damage characteristics and damage mechanism of tight sandstone gas reservoirs in Jinhua-Zhongtai mountain area of central SiChuan Basin, and put forward the technical countermeasures of “Double protection” drilling fluid to protect the reservoir and the environment, which is very important for the efficient well construction in this area. Through X-ray diffraction, scanning electron microscopy, casting thin sections and other testing methods, the mineral composition and microstructure characteristics of the block were analyzed, and the potential damage factors of the reservoir were clarified. Based on the sensitivity evaluation, it was revealed that the overall sensitivity damage of the block was weak. The main damage type was salt-sensitive damage, and the critical salinity was 9472.5 mg/L. On the basis of the environmental protection drilling fluid system used in this block, the surfactant which can effectively prevent gas invasion and reduce surface tension is selected, and the “Double protection” drilling fluid system is constructed. Through comprehensive performance test and reservoir protection performance evaluation, the core permeability damage rate of the optimized drilling fluid system is reduced from 88.77 to 18.66%, and the cuttings recovery rate is increased to more than 66%, and the cuttings expansion rate is reduced to less than 3.2%, which can effectively solve the problem of reservoir damage in drilling in Jinhua-Zhongtai mountain block in central Sichuan.

Experimental Study on The Influence of Effective Stress on Stress Sensitivity of Coal Permeability

In order to study the influence of effective stress on the sensitivity of coal permeability stress, three groups of low permeability dense coal samples were selected in the range of confining pressure of 2 MPa ~ 4 MPa. Under the influence of single factor of effective stress, the cross-contrast seepage experiment of coal samples under different effective stress conditions was carried out by using steady seepage method, and the variation characteristics of effective stress sensitivity of coal samples were systematically analyzed by using permeability damage rate and permeability curvature. The results show that: (1) under various confining pressures, with the increase of effective stress, the damage rate of coal permeability decreases according to the law of negative exponential function, and the greater the effective stress, the greater the damage rate of coal permeability; (2) The effective stress is constant, the smaller the confining pressure is, the greater the curvature of permeability is, the greater the influence of effective stress on coal seam permeability is, and the stronger the sensitivity is.

Experimental study on the impact of effective stress and cyclic loading on permeability of methane hydrate clayey sediments

Permeability evolution directly determine gas production and its efficiency of gas hydrates in the South China Sea. Due to existence of submarine overlying rocks and change of soil internal mechanics during the hydrate exploitation, permeability evolution is significantly affected. Therefore, in this paper, influence of effective stress on the porosity and permeability of sediment with different hydrate saturation was studied, and functional relationship between them was revealed. Effect of cyclic loading and unloading stress on the permeability recovery and damage degree was also discussed. The results show that when hydrate saturation increases, the relation of permeability and effective stress are power function rules, while porosity and effective stress are exponential function rules in clayey sediments. An empirical prediction equation for gas permeability of clay sediments with effective stress was presented, which takes into account hydrate saturation and porosity. In addition, during cyclic loading experiments, the results showed a linear increase in permeability during unloading. The permeability recovery rate and permeability damage rate are completely opposite with change of hydrate saturation. Permeability damage rate of kaolin is different from that of other two kinds of sediment. With the increase of cycle times, the proportion of plastic deformation of sediment decreases gradually.

Impact of hydrate spatial heterogeneity on gas permeability in hydrate-bearing sediments

The permeability characteristic of hydrate-bearing sediments (HBSs) is an essential factor for assessing the exploiting potential of target area. The two trial tests in the South China Sea have demonstrated the production feasibility of clayey hydrate reservoirs, and further clarifying main controlling factors of the permeability characteristic is an essential prerequisite for commercial development. However, the current studies mainly focus on homogeneous HBSs with different saturations and porosities, ignoring the natural spatial heterogeneity of hydrate since the sedimentary and accumulation history. In this study, spatial heterogeneous hydrate (0–23.1%) was generated in montmorillonite specimens, and effective stress (1–5 MPa) was applied. The results show that 1) HBSs permeability with heterogeneity hydrate occurrence presents an exponential-like relationship with saturation. 2) Heterogeneous hydrate occurrence would reduce the permeability sensitivity to saturation. 3) The permeability of heterogeneous HBSs decreases in a power function trend with increasing effective stress. 4) The heterogeneity reduces compression effect and decreases permeability damage rate. 5) Increasing hydrate spatial heterogeneity would decrease HBSs permeability.

Research on the Water Injection Pressure Reducing and Increasing Injection System for Low Permeability Reservoirs in Fushan Oilfield

In response to the problems of increased injection pressure, decreased injection volume, and stopped injection caused by scaling and impurity blockage during the water injection process in the low permeability reservoir of HuaX block in Fushan Oilfield, water quality analysis and scaling experiments were conducted on the injected water in the block to determine the cause of scaling. Secondly, core displacement damage experiments were conducted using natural rock cores and on-site injected water to determine the degree of permeability damage during the water injection process, with a permeability damage rate greater than 70%, Finally, based on the causes of scaling and core displacement experiments, scale inhibitors, anti expansion agents, and surfactants were selected. Through orthogonal experiments, the optimal system formula was determined. The scale inhibition rate was greater than 90%, the anti expansion agent was greater than 90%, and the interfacial tension was less than 0.01mN/m. Core displacement experiments showed that the permeability damage rate of the system to the core was less than 15%, which can achieve the effect of reducing pressure and increasing injection in water injection wells.

Preparation and Performance Evaluation of Quaternary Copolymer Sand Control Agent

Summary Sand production is a common problem in petroleum extraction, and the development of water-soluble polymer sand control agents is currently a research hotspot. However, this research does not provide details of the chemical structure and lacks discussions on adsorption and sand control mechanisms. Herein, a new water-soluble polymer sand control agent (PAADD) is reported on; it is a copolymer of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N-(3-dimethylaminopropyl) methacrylamide (DPM), a functional monomer (DAM) containing ketone groups and self-crosslinking properties, and acrylamide (AM). First, the synthesis conditions of PAADD are optimized, and then its sand control mechanism is explored. Experimental results reveal that PAADD can adsorb on the sand surface through electrostatic attraction and hydrogen bonding and change the surface properties of sand particles, which promotes the aggregation of sand particles and permits good sand control ability. In flow experiments, after the injection of PAADD solution (500 mg/L) into a sandpacked model with a permeability of 500 md, the critical sand production rate of the sandpacked model increased more than 100 times and the permeability damage rate (PDR) was 30–40%.

Micromechanism and mathematical model of stress sensitivity in tight reservoirs of binary granular medium

Research on reservoir rock stress sensitivity has traditionally focused on unary granular structures, neglecting the binary nature of real reservoirs, especially tight reservoirs. Understanding the stress-sensitive behavior and mathematical characterization of binary granular media remains a challenging task. In this study, we conducted online-NMR experiments to investigate the permeability and porosity evolution as well as stress-sensitive control mechanisms in tight sandy conglomerate samples. The results revealed stress sensitivity coefficients between 0.042 and 0.098 and permeability damage rates ranging from 65.6% to 90.9%, with an average pore compression coefficient of 0.0168–0.0208 MPa−1. Pore-scale compression occurred in three stages: filling, compression, and compaction, with matrix pores playing a dominant role in pore compression. The stress sensitivity of binary granular media was found to be influenced by the support structure and particle properties. High stress sensitivity was associated with small fine particle size, high fines content, high uniformity coefficient of particle size, high plastic deformation, and low Young's modulus. Matrix-supported samples exhibited a high irreversible permeability damage rate (average = 74.2%) and stress sensitivity coefficients (average = 0.089), with pore spaces more slit-like. In contrast, grain-supported samples showed low stress sensitivity coefficients (average = 0.021) at high stress stages. Based on the experiments, we developed a mathematical model for stress sensitivity in binary granular media, considering binary granular properties and nested interactions using Hertz contact deformation and Poiseuille theory. By describing the change in activity content of fines under stress, we characterized the non-stationary state of compressive deformation in the binary granular structure and classified the reservoir into three categories. The model was applied for production prediction using actual data from the Mahu reservoir in China, showing that the energy retention rates of support-dominated, fill-dominated, and matrix-controlled reservoirs should be higher than 70.1%, 88%, and 90.2%, respectively.

Research on stress sensitivity of fracture-hole carbonate reservoirs under drilling fluid immersion

Fracture-hole carbonate reservoirs feature the development of multi-scale fractures and dissolution holes. Invasion of drilling fluid into the reservoir and its prolonged interaction with the rock can readily induce a variety of damages, including stress-sensitive damage, consequently diminishing the productivity of oil and gas wells. Experiments were conducted on both dry/wet fractured rock samples and dry/wet fractured rock samples containing holes to investigate the mechanism of stress sensitivity in fracture-hole rocks subjected to fluid soaking. The results indicate that: (1) During the loading process, the highest damage rate to the permeability of fractured rock samples can reach up to 95%. The stress sensitivity coefficients for fracture-hole rock samples with half-hole depths of 2.5 mm, 5.0 mm, and 7.5 mm are 0.58, 0.41, and 0.25, respectively. The permeability damage rate of fracture-hole rock samples soaked with fluid is approximately 80%. (2) As the depth of the hole increases, the stress sensitivity coefficient decreases. The hole's depth is inversely proportional to the maximum stress, while being directly proportional to the maximum displacement of the rock sample. (3) Upon fluid soaking, pressure dissolution occurs within the fractured rock samples, shrinking the fracture width by 15.71–26.17%. (4) Following a 48-h immersion of the rock samples in an alkali solution, decalcification occurs and the pore structure is compromised, thus amplifying the degree of stress sensitivity.

The Reservoir Sensitivity of Triassic Baikouquan Formation in Mahu Depression

The Triassic Baikouquan Formation in the slope area of the Mahu Depression is the largest glutenite reservoir in the Junggar Basin, with low porosity and permeability; however, its physical properties are poor, the distribution of oil and gas is quite different, and the output fluctuates greatly. It is of great guiding significance to study the sensitivity characteristics of the reservoir for oil and gas development and productivity design. In this paper, the reservoir of the Triassic Baikouquan Formation in the Mahu Depression of Junggar Basin is taken as the research object, and the geological characteristics, pore structure characteristics and clay mineral characteristics of the reservoir are investigated through the use of X-ray diffraction and scanning electron microscope; moreover, the sensitivity of velocity, water, salt and stress of the reservoir are studied through the use of a sensitive flow test. The research results show that the lithology of the reservoir is mainly glutenite, composed of tuff and metamorphic mudstone, and the minerals are mainly Yimeng mixed-layer clay minerals, with fine particle size, average porosity of 10.5% and an average permeability of 9 × 10−4 μ m2, forming pore structures such as dissolved pores, cemented pores and intergranular gaps, which belong to the poor pore structure reservoir with low porosity and low permeability. The velocity-sensitive damage rate of reservoirs in the study area is between 4 and 46, and the reservoirs are moderately weak and poor in velocity sensitivity. The damage rate of the reservoirs’ water sensitivity in the study area is between 36 and 58, which can be defined as medium–weak and medium–strong water sensitivity. The reservoir in the study area contains clay minerals in a Yimeng mixed layer, which easily hydrate and swell, and the clay minerals in different parts of the Yimeng mixed layer are different, resulting in great differences in salt sensitivity at different depths. The maximum permeability damage rate of the reservoir is 80%, the irreversible permeability damage rate is 20%, and the stress sensitivity is weak. The research results provide theoretical data support for adopting targeted reservoir protection measures in the process of oil and gas exploration, development and construction.

Continuous measurement of gas permeability in non-homogeneous hydrate reservoirs under effective pressure via a novel apparatus

Gas permeability is a key physical parameter to determine the exploitation efficiency of natural gas hydrate (NGH). However, the relevant research mainly focuses on the permeability of homogeneous hydrate reservoirs, there have been no studies of non-homogeneous hydrate reservoirs. Therefore, a novel permeability measurement apparatus was designed in this study, which used two core holders in parallel to achieve the spatio-temporal continuous measurements of gas permeability for non-homogeneous reservoirs. Marine soils obtained from the South China Sea were used to remold the non-homogeneous hydrate reservoirs, including two homogeneous hydrate reservoirs with water saturation of 40% (reservoir A) and 80% (reservoir B), respectively. The constant gas flow of 0.5 ml/s and constant back pressure of 3 MPa was carried out to measure the gas permeability. The experimental results showed that the gas permeability measured in this work had similar regularities with other literature data. When the effective pressure increased from 1 MPa to 6 MPa, the permeability of reservoir A, reservoir B, and non-homogeneous hydrate reservoir decreased from 15.2 mD to 10.9 mD, 10.5 mD to 4.6 mD, and 12.5 mD to 8.5 mD, respectively. In addition, the variation in the non-homogeneous coefficient showed three stages: 1–2 MPa stage, 2–3 MPa stage, and 3–6 MPa stage. What's more, the permeability damage rate of non-homogeneous reservoirs increased by nearly 35% with the increase of effective pressure. These findings obtained by this apparatus will enhance the understanding of gas permeability in the non-homogeneous hydrate reservoirs, and guide further field exploitation of hydrate reservoirs.

Multi-Sized Granular Suspension Transport Modeling for the Control of Lost Circulation and Formation Damage in Fractured Oil and Gas Reservoirs

Transport and retention of multi-sized suspended granules are common phenomena in fracture media of oil, gas and geothermal reservoirs. It can lead to severe permeability damage and productivity decline, which has a significant impact on the efficient development of underground resources. However, the granule transport and retention behaviors remain not well understood and quantified. The novel stochastic model is proposed for the multi-sized suspended granule transport in naturally fractured reservoirs accounting for granule retention and fracture clogging kinetics. A percolation fracture network is proposed considering fracture connectivity evolution during suspended granule transport. Granule retention and fracture clogging dynamics equations are proposed to account for incomplete fracture clogging by retained granules. The microscale stochastic model is allowed for upscaling to predict the multi-sized granule transport behavior in naturally fractured reservoirs. The model solution exhibits preferential plugging of fractures with sizes equal to or below the granule size. Multi-sized suspended granule shows great advantages over mono-sized suspended granule in the control of permeability damage induced by granule retention and fracture clogging. The retained granule concentration and permeability damage rate decrease with fracture network connectivity improvement. The experimental investigation on size-exclusion suspended granule flow has been performed. The model-based prediction of the retained granule concentration and permeability variation history shows good agreement with the experimental data, which verifies the developed model.

Evaluation method for reservoir damage of cementing slurry

The damage of cement slurry to the reservoir is directly related to the production of oil and gas wells, and is a problem that must be solved for the efficient and economic development of oil and gas reservoirs. According to the characteristics and field conditions of reservoir damage caused by cement slurry, this paper analyzes the main factors affecting the degree of reservoir damage, puts forward the technical index requirements of the evaluation device, and establishes the evaluation method of reservoir damage caused by cement slurry in case of loss of injection. The results show that the influence of temperature on the damage of the cement slurry layer is small, and the influence of pressure difference on the damage to the cement slurry layer is large; When the pressure difference is 22 Mpa, the penetration damage rate caused by cement slurry to the pore center without mud cake attachment is about 43%. After a day of core maintenance, the permeability damage rate will reach 61.2%; The damage of cement slurry to the pore micro-fractured core is very serious. After 1 day of maintenance, the damage to the core permeability after cement slurry solidification is as high as 83%, with an average of 76.5%; When the risk of leakage is low, the cement slurry system with low density and low water loss can be used for cementing operation. The filtrate of cement slurry should be better matched with the formation clay minerals, formation water, and drilling fluid filtrate. When the risk of leakage is high, high-density cement slurry, balanced pressure cementing, pressure control cementing, and other technologies to avoid leakage can be used for cementing operations. When the cementing conditions are more difficult, the cementing slurry system can be used to dissolve the cementing slurry system at a later stage. The research results provide theoretical data support for the study of reducing the damage of cement slurry to the reservoir.

Permeability variation and sensitivity of CO2 injection into coals under the control of effective stress and temperature

ABSTRACT The injection of CO2 into coal reservoirs offers a promising approach for reducing CO2 emissions and enhancing coalbed methane production. An inadequate understanding of the CO2 permeability variation in coals under the coupled constraints of temperature and effective stress limits the accurate evaluation of the CO2 injection potential. To address this issue, coal samples of different ranks were collected from the Sihe, Zhangjiamao, and Dahebian coal mines in China (designated as SH-1, ZJM-1, and DHB-1, respectively). These samples were used to investigate the characteristics of permeability variation and sensitivity to coal rank, temperature, and effective stress. The coal samples ZJM-1, DHB-1, and SH-1 had absolute permeability of 0.01355, 0.03953, and 0.00064mD, respectively. The SH-1 sample had the lowest absolute permeability due to its high coal rank, and the DHB-1 sample had the highest absolute permeability due to its tectonically deformed and fractured coal structure. The association of permeability to effective stress can be more accurately described by the permeability model with variable pore compression coefficient than that with constant pore compression coefficient. The CO2 permeability of the coal samples decreased exponentially with increasing effective stress, while the permeability damage rate increased exponentially and the permeability curvature decreased exponentially. Based on the fitted curve of the permeability damage rate to effective stress, three stages of stress sensitivity were identified: rapidly increasing stage, slowly increasing stage, and approximately stable stage. The permeability and temperature sensitivity coefficient exhibited a decreasing trend with increasing temperature, following a power function relationship. Among the undisturbed coal samples, the SH-1 sample showed higher sensitivity to stress and temperature compared to the ZJM-1 sample. The DHB-1 sample, characterized by a fractured coal structure, exhibited the lowest temperature sensitivity and the highest stress sensitivity. The temperature sensitivity of different coal samples tended to be consistent at temperatures above 70°C. Predictive models for CO2 permeability under the dual constraints of temperature and effective stress were constructed for the three samples using the surface fitting method. These models can assist in evaluating the feasibility of CO2 injection into coal reservoirs at various depths in associated areas.

Performance evaluation of oil-displacement viscoelastic zwitterionic surfactant fracturing fluid

Viscoelastic surfactant (VES) fracturing fluid has advantages in sand-carrying and micelle-breaking without residues, while its practical applications are always limited due to the poor temperature and shearing resistance. Accordingly, a kind of stearicamide propyl hydroxylsulfobetaine (SPHB) was self-synthesized by using stearic acid, N, N-dimethyl-1,3-propane diamine, and sodium 3-shloro-2-hydroxypropane sulfonate as raw materials, and a novel VES fracturing fluid (HCF) was prepared by optimizing the ratio of SPHB and counterion. The composition of the novel fracturing fluid system was determined according to the viscosity varies with temperature. Moreover, the temperature and shearing resistance, viscoelastic, sand-carrying, drag reduction, micelle-breaking, and oil displacement properties of the system were systematically studied. The results showed that the optimum molar ratio of SPHB to salicylate counterion in the fracturing fluid system was 1:1. Surface tension test showed the critical micelle concentration (CMC) of SPHB surfactant solution is 6.27 × 10-4 mol /L and the surface tension γcmc is 40 mN/m. The apparent viscosity of the novel fluid was kept above 50 mPa·s after continuous sheared for 60 min under the condition of 170 s−1 at 130 ℃. Besides, it has good viscoelastic property and sand-carrying performance, and the experimental results showed that the drag reduction rate of the novel fluid reaches more than 60% at different temperatures. The micelle-breaking performance of the novel system was perfect. There is basically no solid residue in the micelle-breaking solution, and the permeability damage rate of the micelle-breaking liquid to the natural core was only 9.41%, which showed the novel fluid has the characteristics of low damage. Finally, low-permeability natural cores were used to carry out micelle-breaking fluid oil displacement experiments, including single core flooding and double-tube parallel core flooding, and the mechanism of micelle-breaking fluid improving oil recovery was revealed.

Experimental study on water saturation effect on coal sample permeability under different effective stresses

During the drainage and production of coalbed methane (CBM) wells, the constant changes in stress and water saturation of reservoir restrict the dynamic change of the reservoir permeability. By carrying out stress sensitivity experiments with different water saturations in coal, the correlation between permeability and the coupling of effective stress and water saturation was analyzed. The water saturation sensitivity and stress sensitivity of reservoir were evaluated by the stress sensitivity index (S), permeability damage rate (PDR), and stress sensitivity coefficient (αk), and the change law of coal permeability under different stresses with different water saturations was revealed. The results showed that the coal reservoir permeability decreased with increasing stress following a negative exponential function and decreased nearly linearly with increasing water saturation. In addition, the coal water saturation sensitivity was positively correlated with effective stress, and the coal stress sensitivity was positively correlated with water saturation. Finally, a mathematical model for predicting coal permeability that considered the impacts of water saturation and effective stress was established, revealing the controlled mechanism affecting the permeability change.

Experimental Study on the Evolution of Compressibility and Gas Permeability of Sediments after Hydrate Decomposition under Effective Stress

The effective stress and types of particles significantly affect the permeability of hydrate sediments. In this study, effects of hydrate saturation, hydrate decomposition, effective stress, and particle type on gas permeability and sample deformation of montmorillonite samples and quartz-montmorillonite mixtures were investigated. The results showed that the gas phase permeability increased with hydrate formation. In addition, the decline of sample porosity is the fundamental factor determining the permeability failure rate under effective stress. During methane hydrate decomposition by depressurization, the compression coefficient of samples increases with decreasing pore pressure, which means that decomposing natural gas hydrate makes experimental samples more sensitive to stress. Furthermore, the addition of sand particles significantly affects the compressibility of sediments. In addition, because of the decline of sample porosity and the swelling of montmorillonite, the results of gas permeability show a downward trend when the pore pressure decreases during the decomposition of methane hydrate. Gas permeability of the sample ranges from a few millidarcies to several hundred millidarcies. Furthermore, the content of clay and the particle size in the sample are the key factors in determining the permeability damage rate when the stress increases during decomposition of methane hydrate. Therefore, the addition of quartz sand can reduce damage of gas permeability of sediments caused by effective stress and hydrate decomposition.

Experimental study on stress-dependent permeability of coal samples with different salinities of water in coal

The stress sensitivity and water sensitivity of coals are key factors restricting the productivity of coalbed methane (CBM) wells. In this study, stress sensitivity experiments with different salinities and X-ray diffraction (XRD) tests were conducted with coal samples from the Zhaozhuang (ZZ), Sihe (SH), and Xishan coalmine (XS) in the Qinshui Basin to analyse the correlation between permeability and the coupling of salinity and effective stress. To reveal the revolution of coal permeability under the coupling of salinity and stress, the permeability damage rate (PDR), the stress sensitivity coefficient (αk), and the water sensitivity index (IW) were introduced to evaluate the stress sensitivity and water sensitivity. The results showed that the permeability varied with the effective stress following a negative exponential function and was positively correlated with salinity within a certain range. When the salinity increased from 0 ppm to 5000 ppm, the stress sensitivity decreased; when the salinity increased from 5000 ppm to 10,000 ppm, the stress sensitivity remained constant, and the water sensitivity increased with increasing effective stress. Finally, a mathematical model for predicting coal permeability that considered the impacts of stress and salinity was established, and the control mechanism of stress and salinity on permeability was revealed.