Permeability Evolution Research Articles

Experiment and Model Investigation on the Permeability Evolution of Granite under Triaxial Compressive Damage

Deep granite is the ideal surrounding rock for nuclear waste geological disposal reservoir and its permeability is the key factor for nuclide migration. In order to study the permeability evolution characteristics of surrounding rocks in the near field of the disposal reservoir under different confining pressures during excavation, the mechanical and permeability properties of granite were explored through triaxial compression experiments associated with acoustic emission techniques. Experiment results suggest that the permeability exhibited typical pore percolation and fracture flow during the compressive damage process and it has an obvious relationship with the volumetric strain before and after dilation. According to such a relationship, a full-stage permeability model was proposed which is characterized by only 2 parameters. This permeability model was verified by the experiment data to be of good feasibility and effectiveness, exhibiting well fitness to the experiment under various confining pressures. According to the elastoplastic mechanical model that is available from other researchers, discussion was implemented and which convinced us that it is feasible to reproduce the real conditions of hydromechanical processes by combing the proposed permeability model and damage-based elastoplastic model in numerical modelling.

Deformation behavior and damage-induced permeability evolution of sandy mudstone under triaxial stress

The mechanic and permeability behavior in sandy mudstone is crucial to hazard prevention and safety mining. In this study, to investigate the evolution and characteristic of permeability and mechanical properties of sandy mudstone subjected to loading of in-site stress, a series of triaxial compression–seepage experiments are performed. The increase in permeability and decrease in mechanical strength gradually transformed to the decrease in permeability and increase in mechanical strength responding to the increase in confining stress from 5 to 15 MPa, which corresponds to the transformation from brittleness to ductility in sandy mudstone, and the transformation threshold of 10 MPa confining stress was determined. The penetration shear fracture generated at brittle regime, while plastic flow behavior presented at semibrittle and ductile state. The critical value of the yielding stage in axial strain increases with the increase in confining stress. The relatively higher permeability corresponds to the higher pore pressure during the increase in confining stress. The increase in confining stress promoted the increase in volumetric strain, while increased pore pressure reduced the volumetric strain, and the lower permeability occurred at higher volumetric strain. In addition, an improved permeability model was developed to describe the loading-based permeability behavior considering the Klinkenberg effect.

Poroelastic effects on gas transport mechanisms and influence on apparent permeability in shale

Poroelasticity and gas slippage on the walls of nanopores are two major mechanisms that regulate shale permeability concurrent with hydrocarbons depletion. Nonlinear Klinkenberg plots reported in experiments indicate that gas slippage has a complex relationship with the variation of effective stress; however, a theoretical framework that explains the underlying physics is still missing. In the present contribution, a series of experiments were designed and conducted on intact Utica shale samples to study the simultaneous effects of gas slippage and poroelasticity under constant confining and constant effective stress conditions. In addition, an apparent permeability model was developed considering the effects of rock poroelasticity and a non-constant velocity gradient for gas flow at the pore wall. The model was calibrated and tested against the presented experimental results for Utica shale as well as previously published data. Results show that the concavity in Klinkenberg plots is caused by two separate parameters, each dominant within different ranges of pore pressures: second-order gas slippage and effective stress coefficient. The model demonstrates that gas slippage is inherently coupled with effective stress at low pore pressures. However, the evolution of apparent permeability with gas pressure deviates from the classical Klinkenberg theory only when the pore diameter is sufficiently small. When the effective stress coefficient is smaller than unity, the permeability evolution deviates from the linear Klinkenberg trend only at high pore pressures. In addition to the pore size, gas slippage is found to be closely correlated with the variation of gas viscosity with pressure and temperature.

Salt-induced structure damage and permeability enhancement of Three Gorges Reservoir sandstone under wetting-drying cycles

The salts contained in water can gradually accumulate in pore networks of rocks under wetting-drying cycles, which could cause rock deterioration. The salt-induced damage under this cyclic process can result in dynamically evolving mechanical and hydraulic properties of the rock, which is relevant to many rock-related engineering activities and geological hazards. In this study, the structure damage and permeability evolution of Three Gorges Reservoir (TGR) sandstone subjected to wetting-drying cycles with salt solution were investigated by a series of multiscale experiments. The results showed that the salt solution has a significant impact on the physical properties of TGR sandstone during the wetting-drying process. With increasing wetting-drying cycles, two typical macroscopic damage modes in form of surface peeling and internal cracking were observed. Various physical parameters indicated that the internal structure of the TGR sandstone experienced two stages: 1) Salt accumulation and new crack formation stage and 2) Crack propagation stage. This variation also profoundly affected the permeability of the TGR sandstone. Specifically, the permeability of TGR sandstone decreased slightly at first due to blocked seepage channels by crystals before cycle 5, followed by a slow growth until the 15th cycle and a rapid increase after the 15th cycle. Cubic and exponential functions showed good performances to describe the evolution features of permeability with porosity and wetting-drying cycles, respectively. The deterioration mechanism was further elucidated by an evaporation test. This test indicated that the salt solution was more likely to crystallize in narrow pores and expectedly exert pressure enough to crack the pores, contributing to the structure damage feature and the permeability enhancement. These findings and results in this study deepen our understanding of salt-induced rock deterioration mechanism under the wetting-drying cycles and would be meaningful for rock engineering safety and geological disaster forecast in complex groundwater environments.

On the evolution mechanism of permeability during gas drainage: Insights from deformation field, gas pressure field and temperature field

Permeability is an important factor affecting efficient gas drainage, ensuring coal mine process safety, ecological environment protection and clean energy capture. We develop a gas drainage physical simulation device with multi-physical coupling to study the dynamic response mechanism of permeability during gas drainage and explore the basic relationship between the interaction between boreholes and permeability during parallel borehole drainage. By installing 48 pressure sensors, 14 temperature sensors, and 9 displacement sensors in a test box, gas drainage tests of parallel boreholes with spacings of 250, 504, and 784 mm were carried out. The results show that during gas drainage in parallel boreholes, the dynamic evolution law of permeability in different spatial positions of the coal reservoir has apparent differences. The permeability near the borehole area exhibits a fast decline rate and a large recovery amplitude, and vice versa. The gas pressure, coal deformation, and coal temperature influence the periodical permeability evolution. In the early stage of drainage, gas pressure and coal temperature play a leading role in reducing permeability. In the middle and late stages, coal deformation slowly increases permeability. When the borehole spacing was 250 mm, the permeability decay rate and rebound rate were high with interaction between the boreholes; a drainage superposition area makes the permeability spatial variation difference between the boreholes significant. A correlation is observed between borehole spacing, gas migration rate, and permeability. The study is expected to have a substantial guiding significance for efficient gas drainage, decreasing greenhouse gas emissions, and improving coal mine process safety.

Numerical Simulation Research on the Pressure Relief and Permeability Enhancement Mechanism of Large-Diameter Borehole in Coal Seam

In order to achieve high-efficiency extraction of coal seams with large-diameter borehole and large flow rates, and to rapidly reduce coal seam gas content and pressure, a large-diameter borehole coal seam pressure relief and permeability enhancement technology is proposed. In this paper, numerical simulation is used to study the mechanism of pressure relief and permeability enhancement in coal seams with large-diameter boreholes, and the evolution of stress, cracks, gas, and permeability of coal bodies around boreholes with different diameters is discussed. The research results show that the stress changes in the coal body around the borehole are symmetrically distributed around the borehole. The stress evolution controls the evolution of the coal body’s cracks. Firstly, damage occurs around the borehole and then extends to the upper left and right corners, like a butterfly spreading its wings. The crushing zone increases with the increase of the hole diameter, and the impact radius of the 100-mm, 200-mm, 300-mm, and 350-mm diameter borehole rupture is 0.375 m, 0.65 m, 1.0 m, and 1.25 m, respectively. The rupture radius of the double 350-mm diameter borehole can reach 2.4 m. The larger the diameter of the borehole, the more fissures will be produced, and the air permeability of the coal body in the affected coal seam area will obviously increase. The research results provide theoretical support for gas drainage in soft, low-permeability, high-gas coal seams.

Coal permeability evolution triggered by variable injection parameters during gas mixture enhanced methane recovery

Injection of gas mixture (CO2/N2) into coal seam can both enhance methane recovery and reduce carbon dioxide emission. Coal permeability determines the mass transport speed, which acts an important index in evaluating gas injectability and methane recovery rate. To reveal effects of injected pressure, temperature and CO2 component in mixture on coal permeability evolution, the heat-fluid-solid coupling model in coal seam was proposed and series of simulations on gas mixture enhanced methane recovery were carried out. Results show that the continuous injected gas mixture replaces and drives methane toward the extraction borehole. The coal permeability is related to the heat or gas sorption induced coal swelling/shrinkage, gas pressure and geostress induced deformation, and coal mechanical properties. The coal permeability increases around the extraction borehole as CH4 pressure drops. There appears declining permeability zone as the arrive of CO2, resulting by the greater adsorption affinity than that of N2 and CH4. The higher the injected pressure, the greater increase of permeability. Coal permeability decreases slightly with the injected temperature, however changes dramatically with the varying of injected CO2 component. The effects of injection parameters on permeability evolution are CO2 component, injection pressure, and injection temperature successively.

Sorptive permeability loss determined from strain-based analysis of tightly constrained experiments on shale

We build a model to determine sorptive permeability loss from observed total permeability evolution by considering adsorption and poromechanical expansion as parallel processes. The model is cross verified with a separately derived strain-based model for sorptive permeability evolution. Both models are compared to laboratory data and are shown to have excellent agreement. We isolate the sorptive strain from the total strain measured during serial injection of sorptive then non-sorptive gas species. Our model predicts that injection of a non-sorptive gas into a shale saturated with a sorptive gas causes permeability to approximately double in both the bedding-parallel and bedding-perpendicular directions. We preform nitrogen floods at constant stress, pore pressure, and temperature to observe the sorptive permeability recovery absent other confounding effects. We confirm that in shales the component of permeability evolution due to sorptive swelling can be isolated from effective stress effects. Laboratory results show a 206% and a 234% permeability increase in the bedding-perpendicular and bedding-parallel directions, respectively, as a result of nitrogen flooding. This counters and indeed dominates over any sorptive permeability loss that despite a modest fraction of organic material (∼1–2%) is amplified by the low pore density. We find that sorptive permeability evolution in shales is controlled by the sorptive strain, pore density, and pore geometry.

A fully coupled multidomain and multiphysics model considering stimulation patterns and thermal effects for evaluation of coalbed methane (CBM) extraction

A deep understanding of the process of coalbed methane (CBM) extraction is of significance for both the unconventional energy supply and mine safety. The effects of stimulation patterns and thermal effects (including thermal shrinkage, non-isothermal sorption, and gas density change) significantly affect the CBM extraction process. Although thermal effects on CBM extraction have been investigated in previous studies, these effects have not been coupled with stimulation patterns, leading to an insufficient evaluation accuracy of previous models and a limited understanding of gas extraction in stimulated CBM reservoirs. This study developed a multidomain and multiphysics (thermal-hydraulic-mechanical, THM) model to fully couple different physical processes (including gas flow, coal deformation, gas desorption, and heat transfer) within a CBM reservoir framework considering various stimulation patterns to improve the simulation accuracy and obtain insights into CBM extraction. Three domains with distinct properties, i.e., the stimulated reservoir domain (SRD), non-stimulated reservoir domain (NSRD), and radial primary fracture (PF), were integrated into the CBM reservoir model to characterize the stimulation pattern. A group of partial differential equations (PDEs) was derived to characterize CBM transport within each domain and across different domain boundaries. A stimulated coalbed was defined as an assembly of three interacting porous media: coal matrix, continuous fractures (CF), and PF. The matrix and CF constituted a dual-porosity dual-permeability system, while the PF was simplified as a 1-D fractured medium. The finite element method (FEM) was employed to numerically solve the above PDEs. The proposed model was verified against two sets of field-measured CBM production data and compared to three previously published numerical models to reveal its advantage. The verified model was applied to investigate multidomain effects of stimulation treatment and evaluate the thermal effects of gas depletion on gas extraction in stimulated CBM reservoirs. The simulation results suggested (1) that the distinct properties of the different domains result in different permeability evolutions, which in turn influences CBM production; (2) that increasing the fracture complexity, enlarging the SRD size, and improving the SRD permeability with suitable stimulation techniques constitute effective approaches to enhance the CBM recovery; and (3) that ignoring thermal effects in CBM extraction can either overestimate or underestimate production, which mainly depends on the net effects of the thermal shrinkage strain and non-isothermal sorption during different production periods.

Colloid Migration as a Reason for Porous Sandstone Permeability Degradation during Coreflooding

During coreflooding under stationary conditions, permeability is determined by the Darcy formula; thus, the apparent permeability is determined, which characterizes the average hydraulic conductivity of the core, but does not take into account the processes occurring at the microlevel. Transient processes during fluid injection regime change are important for understanding the evolution of permeability and underlie such phenomena as permeability degradation and its hysteresis. Our paper presents the coreflooding methodology and the results showing that during changes in injection conditions, the permeability of samples can significantly decrease due to the migration of colloids. In the developed methodology, coreflooding conditions exclude the influence of other factors on permeability, such as creep, the chemical reactions of rocks and fluids, and chemical mobilization of colloids. It has been established that the decrease in permeability occurs only when fluid is injected through the core, which is indirect evidence of pore blockage by colloids. The article also attempts to evaluate the effect of pore pressure on the sensitivity of the porous medium permeability to the amount of injected fluid. A decrease in pore pressure leads to a more intense decrease in permeability during injection, which most likely indicates additional mobilization of colloids, while the narrowing of pore channels does not affect permeability. Analysis of coreflooding results showed that porous media with lower permeability are more sensitive to colloid migration and pore pressure changes. It has also been found that the sensitivity of porous media to colloid migration is greatly affected by the pore pressure gradient, while media with higher permeability are less sensitive to colloid migration.

Advances in Deformation and Permeability Evolution during Creep of Rocks

The goal of this paper is to review the research advances in deformation and permeability evolution during the creep of rocks in geoengineering problems through aspects of experiments, models, and methods. On the experimental side, we reviewed the reports related to creep-permeability evolution in resolving real geoengineering problems. In the section on the constitutive model, we summarized the equations of the relationship between creep deformation and permeability evolution in reproducing the interaction mechanism of creep-permeability. In addition, in the section on the numerical modeling method, we examined the modelling methods able to apply the mechanism of creep-permeability evolution as a real problem. Our report concludes that it is important to conduct experiments to demonstrate the deformation and permeability evolution during the creep of heterogeneous rocks in multi physics fields (Thermal-Mechanics-Hydraulic-Chemical). Additionally, we confirm that it is necessary to improve the proposed equation of permeability evolution by considering strain and damage. Finally, this paper suggests that the DEM (Discrete Element Method) is available to evaluate the influence of the heterogeneousness of rocks on deformation and permeability evolution.

Modeling and experimental study on methane diffusivity in coal mass under in-situ high stress conditions: A better understanding of gas extraction

Gas desorption and diffusion in pore is not only an important behavior to control the law of gas migration, but also an important theoretical basis for the proposal and optimization of extraction technology. At present, researches on gas diffusion in coal were mainly conducted on granular coal under the stress-free condition. However, on-site gas desorption and diffusion in coal seams occur in coal mass under the in-situ stress condition. In particular, high stress exerts a notable effect on gas diffusion in the deep mining stage. Therefore, experimental study and theoretical model establishment concerning the diffusion characteristics of coal mass under high stress require further investigation. In this work, first, with respect to the desorption and diffusion characteristics of coal mass under high stress, desorption and diffusion experiments were performed on coal mass under different high stress conditions with the aid of a self-developed experimental device, and the corresponding gas desorption laws were obtained. In addition, the law of coal mass permeability evolution under different stresses was obtained through the transient measurement method. Moreover, a coal mass diffusion model which conformed to the characteristics of high stress was established based on reasonable assumptions, a new physical structure model of coal under high stress and the relationship between diffusion length and stress. Moreover, this model, according to the laws of gas desorption under different conditions, it is found that the diffusion coefficient increases with the rise of adsorption equilibrium pressure, and stress is significantly proportional to the diffusion coefficient. Besides, with reference to the relationship between diffusion coefficient and stress value, the fundamental parameter that affects the desorption and diffusion amount, i.e., effective diffusion coefficient, was analyzed. Based on the variations of the effective diffusion coefficient with stress, the mechanism of how the classical pressure relief mining technology to improve gas extraction efficiency was further elaborated. The research results provide a more comprehensive theoretical support for the application of the pressure relief mining technology in deep high-stress coal seams.

Permeability evolution characteristics and microanalysis of reactive powder concrete of drilling shaft lining under stress-seepage coupling

This study aimed to solve the problem of drilling shaft support under deep water-rich and weakly cemented stratum conditions. Thus, ultra-high-performance reactive powder concrete (RPC) is proposed as the wall construction material for the drilling shaft lining. The mechanical behavior, permeability, and microscopic characteristics of RPC were tested and analyzed. First, RPC samples were prepared by standard and autoclave curing with strength grades of C110 and C140, respectively, and their quasi-static mechanical properties were tested and compared. Stress-seepage coupling tests were then conducted under different confining and osmotic pressures to analyze the mechanical deformation, permeability evolution, and strength and deformation characteristics. The results showed that autoclave curing significantly improved the strength grade and early strength growth of RPC. Additionally, RPC generally exhibited low permeability characteristics while its permeability evolution law corresponded to the stress–strain relationship under triaxial loading. The permeability was observed to undergo three stages: a gradual decrease, steady development, and rapid increase. Finally, mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscope microscopy analyses were used to determine the mechanism of improving macro mechanical properties and impermeability of RPC by the autoclave curing method based on pore structure characteristics and microstructure formation aspects.

Strain localisation and seepage characteristics of rock under triaxial compression by 3D digital image correlation

The rock mass can be unstable due to the coupling effect of stress and seepage field, adversely affected by the geological field conditions. In this regard, this study implemented a 3D digital image correlation (3D-DIC) technique to conduct triaxial compression tests on sandstone samples under different seepage pressures. We analysed the evolution of strength and permeability during the rock failure process and space-time evolution of the surface radial strain. The results show that the strength and brittleness of the rock decrease with increasing seepage pressure. Further, the mechanical properties of the rock sensitive to were highly dependent on the seepage pressure. We also established a function model of permeability and axial strain consistent with the test results. By analysing the radial-strain-field of the rock surface at specific time points, we observed a gradual process of strain localisation instead of a sudden occurrence before failure. We extracted the spatial coordinates of the first 5% radial strain and subsequently conducted the spatial relation analysis by average-nearest-neighbour-analysis (ANNA). In addition, the approximate failure surface was fitted using the first 5% radial strain points in space, and the distance between the radial strain points and the fitted failure surface in space was analysed. This study shows that points with large strain values gradually converged towards the location of the fracture surface during the gradual failure process of the rock. Finally, this study provides a new method for predicting the failure zones of rock.

Numerical simulation study and application of coal seepage evolution law around water injection borehole in the stope “dynamic-static” pressure zone

A pressure relief anti-reflection model of a borehole along a coal seam goaf was constructed to analyze the stress-plastic spatiotemporal evolution characteristics of a borehole, and the implementation process of pressure relief anti-outbursts from the underground mine of the Huaibei Qingdong Coal Mine was used as the research background. A simulation of the mathematical relationship between the permeability of the coal seam sample and the overburden-gas pressure was performed, incorporating the dynamic balance parameters of the stress after the borehole was constructed along the coal seam; in addition, the water injection pressure relief process and the plastic deformation mechanism of the boreholes in the “dynamic-static” pressure zone were simulated. These simulations were performed to explore the evolution of stress permeability of the coal body before and after water injection. The results showed that after water injection, the stress unloading in the stress relief region was 12.8 ∼ 31.1%, and the permeability was increased by 23.5 ∼ 42.9%; the stress unloading in the stress concentration region was 26.7 ∼ 58.4%, and the permeability was increased by 263.7 ∼ 563.5%. With the application of water injection technology in the drilling zone of a bedding layer at the 726 working face of the Qingdong Coal Mine, the average cutting volume in the water injection area decreased by 17.19%, the K1 and Δh2 values decreased by 40.3% and 12.2%, respectively, and the return gas concentration decreased by 31%. Thus, pressure relief had an outstanding anti-outburst effect.

Mechanical characteristics and permeability evolution of salt rock under thermal-hydro-mechanical (THM) coupling condition

The mechanical characteristics and gas tightness of salt rock play a decisive role in the safety evaluation of salt caverns. Triaxial compression coupled permeability tests were performed on Chinese impure salt rocks under thermal-hydro-mechanical (THM) coupling condition. The results revealed that the peak strength decreased with the decrease of confining pressure and the increase of temperature. The permeability of salt rock decreased with the increase of confining pressure, while the increase of temperature led to an increase in permeability under lower effective confining pressure, and a decrease in permeability under higher effective confining pressure. Moreover, there is a critical effective confining pressure, which controls the evolution trend of salt rock permeability with temperature. The dilatancy boundary and failure strength envelope of impure salt rock at different temperatures were given, which can be expressed by exponential and hyperbolic functions respectively. An empirical formula for the evolution of the final permeability of the salt rock with the effective confining pressure at different temperatures was established. Considering the effects of effective confining pressure and inelastic volume strain, a new model was proposed to predict the permeability evolution of salt rock under THM coupling condition at different temperatures. The results of this paper can help understand the mechanical characteristics and permeability evolution of salt rock under THM coupling condition, and provide the necessary basis for the safety evaluation of salt caverns.

Part 2: Oil displacement by foam injection in Hele-Shaw cell with a pattern of impermeable regions

Foam flooding is an enhanced oil recovery (EOR) technique that is used to control gas mobility, particularly in heterogeneous and fractured reservoirs. This paper aims to numerically investigate the effect of foam flooding on oil displacement efficiency at the fracture scale. Also, an empirical model for relative permeability, which remains difficult and troublesome to measure, is proposed and implemented in the numerical model. The experimental data used in this study include five injection methods that are water, gas, tertiary gas, foam, and tertiary foam. Fluid injection was performed through 2D-printed models of an actual rock cross-section, which was obtained from high-resolution micro-computed tomography. In the numerical model used, pressure and saturation equations of injected fluids were solved using the COMSOL software.Based on the findings, both experimental data and numerical simulation results of the five injection methods were in good agreement, which shows the robustness of the proposed relative permeability model with a single fitting parameter. The oil recovery factor predicted using the proposed model outperformed that of the Buckley-Leverett and Brooks-Corey models. Also, the relative permeability model was further tested against coreflooding experimental data from the literature. This study provides more insight into numerical modeling of foam flooding at the fracture scale. Moreover, we believe that the proposed relative permeability model is robust and capable of capturing the evolution of relative permeability with saturation along the injection process, which encourages further investigation and improvement by the subsurface community. Further field-scale studies, including sensitivity and optimization, can be performed based on this work.

STUDY OF THE INTERACTION BETWEEN MICROSTRUCTURE AND MACROSCOPIC BEHAVIOR FOR HYDRAULIC FRACTURING AREAS DURING SHALE GAS EXTRACTION

Horizontal drilling is the major method of shale gas extraction, and hydraulic fracturing improves the gas production effectively. However, as the main gas migration channel, shale microstructures, such as the natural fractures and pores, are extremely complicated, and it is difficult to characterize them quantitatively by conventional approaches. Besides, the microstructural evolution of the hydraulic fracture (HF) area is more intricate. In order to quantitatively characterize the evolution of shale microstructure after hydraulic fracturing, and to investigate the contribution to the gas extraction process, a fully coupled fractal thermal–hydrological–mechanical permeability model is proposed. And the interaction of shale microstructure, thermal–hydrological–mechanical effects, and gas permeability in the HF area is also quantitatively investigated. Shale thermal conduction, gas pressure, and adsorption–desorption effect are defined as functions of the effective stress responsible for shale matrix deformation, and operate directly on shale porosity, which induces changes in shale microstructure, including the shale fractal dimension (for characterizing the density of natural fractures in shale) and the maximum fracture length. Multiphysical effects have a major impact on gas seepage, and directly affecting the permeability. Furthermore, compared to the widely used cubic permeability model, this model is significantly superior in analyzing the permeability evolution in the hydraulically fractured area, and the reliability is verified by the production data from the Marcellus Shale.

Numerical Evaluation on Stress and Permeability Evolution of Overlying Coal Seams for Gas Drainage and Gas Disaster Elimination in Protective Layer Mining

Numerical Evaluation on Stress and Permeability Evolution of Overlying Coal Seams for Gas Drainage and Gas Disaster Elimination in Protective Layer Mining

Investigation on gas drainage effect under different borehole layout via 3D monitoring of gas pressure

Gas drainage using underground boreholes is an important method for the prevention of coal gas mine accidents. To study the influence of the increase in the number of boreholes on the gas drainage effect using a self-developed device for gas drainage, pressure sensors and branch boreholes were installed in a coal body. Four groups of physical simulation experiments of gas drainage were conducted. The results revealed that with an increase in the number of boreholes, the attenuation rate of the gas pressure is accelerated; the decrease in the gas pressure accelerates in the form of a natural logarithmic function. The effective drainage area, with characteristics of dynamic expansion and variations, has an approximately circular expansion with the borehole as the center. It gradually increases with the advancement of drainage. The relationship between the effective drainage area and the extraction time is in the form of a power function. With an increase in the number of boreholes, the effective drainage area increases more rapidly, and the regional outburst elimination occurs earlier. The effective stress and matrix shrinkage effect exhibit a coupled influence on the permeability, which causes the permeability evolution to exhibit periodic variations. At the early stage of drainage, the effective stress plays a major role, which reduces the permeability. At the middle and late stages of drainage, the matrix shrinkage effect plays a major role, which increases the permeability. With an increase in the number of boreholes, the attenuation rate and recovery amplitude of the permeability increase.