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Permeability is a crucial petrophysical rock parameter to assess the reservoir flow ability to capture the reservoir quality and reservoir characterization during petroleum field development. Due to the importance of permeability to minimize the risk of exploration and production forecasting, geoscientists and petroleum engineers should be able to make reliable permeability for sedimentary rock. The study objectives are to examine the effects of petrophysical parameters and parametric sensitivity analysis to develop an improved model while predicting formation permeability (K). The predictive models are constructed by coupling petrophysical log parameters and ensemble machine learning approaches, including random forest and extreme gradient boosting (XGB). To verify the predictive model outcomes for feature ranking, the least square support vector machine with a global approach of coupled simulated annealing is applied, and assessed the accuracy of models using statistical performance indicators. It is found that the XGB-based model achieved better accuracy than other ensemble machine learning approaches. The primary log variables such as bulk density and compressional acoustic travel time are adopted to obtain improved correlation for K estimation. The improved model of K is developed with the least absolute residuals (LAR) technique and it is compared and verified with existing correlation using real field log data which performs a high correlation coefficient (99%) and minimal average error. The improved model of rock K can be adopted to assess the reservoir rock quality in simulation studies for the oil and gas field development stages. The studied model development strategies can be applied for further assessment of reservoir rock and fluid properties in reservoir studies and management with low costs on time. [ABSTRACT FROM AUTHOR]

Abstract With the continuous advance of coal mining, the normal stress in front of coal seams will continue to increase, resulting in changes of internal structure and permeability of coal seams. During the continuous advance of mining face, the increase of vertical stress somewhere in front of coal seams may not be a transient process, but a gradual change process. Therefore, this paper carried out the seepage test of gas-containing coal under discontinuous loading axial stress. The stress, deformation and permeability of coal specimens were measured and recorded during the test, and the influence of discontinuous loading axial stress on the deformation, internal structure and seepage properties of coal specimens were discussed. On this basis, considering the influence of stress on the gas adsorption state, Klinkenberg effect and other factors, combined with Shi-Durucan permeability model, a new permeability model considering the dynamic adsorption state of coal specimens was developed.

Abstract To further raise the gas extraction efficiency of the tight sandstone, the liquid nitrogen (LN2) freezing-thawing cycles method can be employed to improve the permeability of the low-permeability reservoirs. Permeability is generally regarded as a macroscopic description of the pore structure and usually has functional relationship to pore structure properties. The permeability of the rock is closely related to the change of microscopic pore structure. The permeability of rock depends on how the subzero temperatures changed the microscopic pore structure of rock, but it has not yet been confirmed obviously. In this study, the nuclear magnetic resonance (NMR) technique was adopted to investigate the pore structure evolution law and permeability of the tight sandstone with different LN2 freezing-thawing cycles. The results demonstrate that the LN2 freezing-thawing cycles promotes pore development and micro-fracture connection, and enhances the pore connectivity. The proportion of meso-pores, macro-pores and micro-fractures in the sandstone samples increases significantly, which provides a channel for the sandstone gas flow and extraction. Total porosity and effective porosity of the samples present a trend of rapid increase as the number of LN2 freezing-thawing cycles increasing, while the residual porosity decreases as the number of LN2 freezing-thawing cycles increasing. Coates model, SDR model (mean T2 model) and PP model were used to calculate and evaluate the permeability of the samples subjected to different LN2 freezing-thawing cycles. Furthermore, PP model can provide a better permeability estimate than the classical Coates and SDR model.

The study on the evolution characteristics of coal permeability is of great significance for rationally determining gas extraction parameters and increasing gas extraction efficiency. In order to study the effects of different coal stresses and gas pressures on coal permeability, the experiment on the deformation of stress-loaded coal and gas adsorption-diffusion was conducted, the segmented dynamic model of coal permeability was established, and the rationality of the established model was verified by the experimental results. The results shown that the gas adsorption amount and coal deformation both shown a Langmuir-type with the increasing gas pressure, and the dynamic diffusion coefficient of gas decreased exponentially with time. As the gas pressure decreased, the expansion deformation of the stress-loaded coal decreased, and the permeability increased gradually. The permeability and expansion deformation of stress-loaded coal gradually decreased with the increasing stress. The coal permeability shown a “V” shape with continuous stress loading, and it reached the smallest at the stress peak. The coupling between matrix and fracture deformation caused by gas adsorption, the dynamic diffusion of gas in matrix, and the mass exchange between matrix and fracture were all considered in the established permeability model of coal. The rationality of established segmented model of coal permeability was verified by the experimental results. The permeability model of coal based on elastic deformation can reflect the permeability evolution at the stage of elastic deformation. Within the experimental range, the absolute error between the experimental test and numerical simulation results of coal permeability was −0.135×10−15～0.296×10−15 m2, and the absolute error of volumetric strain of the coal due to gas seepage was −0.327×10−5～2.026×10−5. The permeability model considering plastic deformation can also reflect the permeability after stress peak. The error between experimental and numerical results was −0.435×10−15～0.997×10−15 m2.

In deep mining engineering, coal often is subjected to the influence of cyclic loading and water. In order to study the effect of water on gas-containing coal with cyclic loading, the pore evolution, permeability and energy dissipation of gas-containing coal under the effects of water and cyclic axial stress were analyzed in depth. The results showed that with the increase of water content, dissipation energy and permeability of coal samples decreased gradually. The presence of water weakened the increase in micropores and mesopores and promoted the increase in macropores during the cyclic loading process. Compared to dry coal samples, the ratio of micropores and mesopores in saturated coal samples decreased by 0.8872%, while the proportion of macropores increased by 0.0341%. On the basis of the above, a new formula for calculating the dissipation energy of gas-containing coal under the effects of water was proposed. A new damage variable based on energy dissipation was proposed. Finally, a theoretical model was derived to describe the permeability of gas-containing coal under the effects of water and cyclic axial stress. [ABSTRACT FROM AUTHOR]

Abstract Water permeability of reinforced concrete is essential for transportation of ingress ions inside concrete structures. The coupling effect of permeability and loading presents a challenge for the experimental simulation of water-permeate reinforced concrete subjected to tension. This renders the development of the model based on dimensionless analysis, using a series of experimental tests from an innovative experimental system that allows simultaneous measurement of permeability and crack width. The experiments focused on both ordinary concrete and high strength concrete under tension. The relationship between permeability and variables such as deformation, diameter of rebars, tensile load, and crack width under tension was formulated through multiple regression analysis using the testing data. The load to deformation characteristics determines the permeability of the concrete under tension. The proposed model accounts for the influence of continuous loading on permeability, as demonstrated by the robust analysis and proposed yield effective point. The robust analysis demonstrates that the diameter of the rebar, load, and crack width exert minimal influence on the permeability of concrete at lower significance levels. However, permeability variations become pronounced from 0.5 threshold, with significant changes observed between 0.5 and 0.9 thresholds. The findings indicate a differential impact of the variables on the permeability of concrete under tension. The yield-effective points delineate the relationship between the rebar diameter, load, and crack-width on the permeability of concrete with a threshold of 0.5, 0.5, and 0.58, respectively. At a threshold of 0.78, higher permeability will occur in the concrete, attributed to the prevalence of deformation. This deformation highlights the parameters with the most significant influence on the permeability of concrete under tension. The robust analysis and yield effective point derivative are useful parameters to measure concrete permeability and evaluate the behavior of the permeability model under tension.

Abstract Coal bed methane (CBM), the high-quality and efficient fuel, has caught the interest of many nations as they strive for environmentally friendly development. Therefore, the efficient exploitation and utilization of CBM has become one of the international focal research problems. A significant factor affecting the mining of CBM is coal permeability. To better capture the changes that occur during the extraction of CBM, the internal swelling coefficient of matrix (ISCM) has been gradually in permeability introduced into the permeability models, and such models have become an important type of the development of permeability models. The goal is to find out more precisely the evolution mechanism of the ISCM and its influence on the permeability models. In this paper, the selection of coal structure, determination of boundary conditions and influencing factors of permeability for were first analyzed. Then, according to the research process of ISCM, the permeability models including the ISCM were reviewed and divided into four phases: proposal phase, development phase, evaluation phase and display of internal structure phase. On the basis of the ISCM values in the current coal permeability models, the primary influencing factors and evolutionary laws of the ISCM are explored. The results obtained provide guidance for future theoretical refinement of permeability models with the ISCM.

Water permeability of reinforced concrete is essential for transportation of ingress ions inside concrete structures. The coupling effect of permeability and loading presents a challenge for the experimental simulation of water-permeate reinforced concrete subjected to tension. This renders the development of the model based on dimensionless analysis, using a series of experimental tests from an innovative experimental system that allows simultaneous measurement of permeability and crack width. The experiments focused on both ordinary concrete and high strength concrete under tension. The relationship between permeability and variables such as deformation, diameter of rebars, tensile load, and crack width under tension was formulated through multiple regression analysis using the testing data. The load to deformation characteristics determines the permeability of the concrete under tension. The proposed model accounts for the influence of continuous loading on permeability, as demonstrated by the robust analysis and proposed yield effective point. The robust analysis demonstrates that the diameter of the rebar, load, and crack width exert minimal influence on the permeability of concrete at lower significance levels. However, permeability variations become pronounced from 0.5 threshold, with significant changes observed between 0.5 and 0.9 thresholds. The findings indicate a differential impact of the variables on the permeability of concrete under tension. The yield-effective points delineate the relationship between the rebar diameter, load, and crack-width on the permeability of concrete with a threshold of 0.5, 0.5, and 0.58, respectively. At a threshold of 0.78, higher permeability will occur in the concrete, attributed to the prevalence of deformation. This deformation highlights the parameters with the most significant influence on the permeability of concrete under tension. The robust analysis and yield effective point derivative are useful parameters to measure concrete permeability and evaluate the behavior of the permeability model under tension. [ABSTRACT FROM AUTHOR]

Coal bed methane (CBM), the high-quality and efficient fuel, has caught the interest of many nations as they strive for environmentally friendly development. Therefore, the efficient exploitation and utilization of CBM has become one of the international focal research problems. A significant factor affecting the mining of CBM is coal permeability. To better capture the changes that occur during the extraction of CBM, the internal swelling coefficient of matrix (ISCM) has been gradually in permeability introduced into the permeability models, and such models have become an important type of the development of permeability models. The goal is to find out more precisely the evolution mechanism of the ISCM and its influence on the permeability models. In this paper, the selection of coal structure, determination of boundary conditions and influencing factors of permeability for were first analyzed. Then, according to the research process of ISCM, the permeability models including the ISCM were reviewed and divided into four phases: proposal phase, development phase, evaluation phase and display of internal structure phase. On the basis of the ISCM values in the current coal permeability models, the primary influencing factors and evolutionary laws of the ISCM are explored. The results obtained provide guidance for future theoretical refinement of permeability models with the ISCM. [ABSTRACT FROM AUTHOR]

The extraction of coalbed methane can ensure the safe production of coal mines and the efficient use of resources. According to the anisotropy of coal seam, CT spiral scanning and triaxial gas seepage tests are carried out, the seepage characteristics of orthotropic coal are analyzed, and a coal permeability model considering orthotropic structure is proposed. Results show that: The permeability of parallel bedding direction changes obviously with axial pressure. The confining pressure is 5 MPa, the permeability in the parallel bedding direction is about 1.6 times that of the vertical bedding, and the confining pressure is 13 MPa, the permeability in the parallel bedding direction is about 1.01 times that of the vertical bedding. The coal permeability model with anisotropic structure is verified by the experiment data in both parallel and vertical bedding directions, and shows great applicability. Research results provide references for the efficient development of coalbed methane. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing causes the fracture zone phenomenon in a coal seam. The effectiveness of coalbed methane (CBM) extraction is determined by the seepage characteristics of various fracture zones under gas pressure attenuation by gas extraction in coal seams. Based on this, this paper developed a zonal seepage test device for hydraulic fracturing coal, designed a series and parallel test method for intact coal sample, microfracture coal sample and penetration fracture coal sample. This method can simulate the migration process of CBM in different disturbance areas near the wellbore in the initial stage of hydraulic fracturing (transverse seepage) and the migration process of CBM from the matrix to the wellbore in the later stage (longitudinal seepage). On this basis, the seepage characteristics of the complex zonal phenomenon produced by hydraulic fracturing under the condition of gas pressure attenuation were studied. A series and parallel permeability model combining Klinkenberg effect, expansion effect and effective stress was established. This model can well describe the series and parallel permeability variation law with gas pressure. It indicated that the adsorption expansion effect and effective stress have an impact on the coal matrix during the longitudinal seepage process, which restricts the seepage of both natural and artificial fractures. Artificial fractures are less impacted by the expansion effect and effective stress during the transverse seepage process, which makes them the primary seepage channel. Combined with the extraction data and the permeability model, the gas production trend in the Qinshui Basin is divided into two stages: transverse seepage dominant stage and longitudinal seepage dominant stage. [ABSTRACT FROM AUTHOR]

Construction waste with more fine particle occupies the amount of land and the utilization rate is low. Using fine-particle construction waste to fill mining gob is an effective way to release land resources and to slow down surface subsidence in mine area. The permeability of filling body is the key factor to affect the geological environment and regional hydrological cycle of filling gob. The paper analyzed respectively the components of fine particle and large-size particle of construction waste by the diffraction analysis, explored the formation mechanism of seepage pore in construction waste samples containing more fine particle, characterized quantitatively the porosity and seepage path of the samples by Matlab binaryzation and mass fractal dimension respectively, established the pore-fractal-permeability model of construction waste filling samples containing more fine particle, and verified the model by experiment. The study showed that the permeability weakened with the increase of < 2 mm particle (fine-particle) content. Permeability coefficient of the samples containing 15% cement was less than that of non-cemented one when fine-particle content was less than 26.09%, vice versa in fine-particle content more than 26.09% because ineffective pores among the fine-particle samples gathered into regular oval effective pores under cement bonding. Both cementation and permeation reduced the effective porosity of the samples, while their influence was different when they worked together. The influence of permeation on the effective porosity of the samples was greater than that of cementation when fine content was 21.38–46.51%, vice versa in fine content less than 21.38% or more than 46.51%. The effective porosity of the permeable cemented samples was less than the one of the impermeable non-cemented samples in fine content 26.09–46.51%, vice versa in fine content 21.38–26.09%. The Matlab binary of sample images could well characterize the porosity, and the mass fractal dimension could describe the permeation path characteristics in the samples. And the theoretical calculation of the model was in accord with the experimental results. [ABSTRACT FROM AUTHOR]

Copyright of Iraqi Journal of Chemical & Petroleum Engineering is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The fracture network structure of coal is very complex, and it has always been a hot issue to characterize the fracture network structure of coal by using a tree-like branching network. In this paper, a new rough fracture permeability model of water injection coal based on a damaged tree-like branching network is proposed. In this model, fractal theory and sine wave model are used to characterize the rough characteristics of fracture structure. In addition, the applicability of the model is verified by the self-developed experimental system, and the sensitivity analysis and weight analysis of the influencing factors in the model are carried out. The results show that the factors affecting the permeability of the established permeability mathematical model are mainly the amplitude factors of sinusoidal fluctuation, length ratio, maximum branch series, number of damaged fractures, opening ratio, opening fractal dimension and initial fracture length. Among them, the sinusoidal fluctuation amplitude factor has the greatest influence on the permeability and the other two are inversely proportional. Therefore, with pulsating water injection, the frequency of changes in the water pressure was altered to improve the water injection efficiency in coal fractures with sinusoidal distribution to increase permeability. The research results provide a theoretical basis for further improving the theoretical models for seepage in porous media and fluid flow analysis of damaged tree-like branching networks. [ABSTRACT FROM AUTHOR]

More and more gas extraction in deep coal seam is conducted under extreme geological conditions. The coal permeability determines the efficiency of gas extraction. To research the evolution of permeability and gas seepage in deep coal seam under three-dimensional (3D) stress condition, a permeability model (DCTDS model) was established by introducing the ratio of internal swelling and the elastic modulus reduction ratio under 3D stress. The influence of adsorption–desorption and effective stress on fracture aperture was considered in the DCTDS model. Subsequently, the DCTDS model was validated. Sensitivity analysis of parameters was conducted based on the laboratory test data under various boundary conditions. The results demonstrate that the change of coal permeability is effectively presented by the DCTDS model. The coal permeability decreased more with increasing pressure when the porosity and Biot's coefficient were smaller in the dominant range of adsorption swelling. The coal permeability increased more when the porosity, Poisson's ratio, and ratio of internal swelling were smaller in the dominant range of effective stress. Finally, based on the constructed DCTDS model, gas extraction in deep coal seams was simulated. The extraction effect was improved with smaller borehole spacing, and elastic modulus, Poisson's ratio of coal. The extraction effect became more notable with larger borehole aperture, Biot's coefficient, and initial permeability. The gas extraction efficiency decreased gradually with extension of extraction time. [ABSTRACT FROM AUTHOR]

Purpose: The macropore structure and seepage characteristics profoundly influence the stability of granite residual soil (GRS) slopes. However, accurately predicting the permeability of undisturbed GRS (U-GRS) is challenging owing to its complex and susceptible pore structure. Aims and methods: Employing X-ray computed tomography (CT) technologies, a three-dimensional (3D) pore structure of U-GRS, was established. Permeability prediction for U-GRS samples was conducted using three simulation methods, namely, the pore network model (PNM), finite element method (FEM), and the lattice Boltzmann method (LBM), along with two empirical models (EMs)—specifically, Kozeny–Carman (K–C) and Katz–Thompson (K–T) models. Subsequently, the methods were comparatively analyzed for calculating efficiency and accuracy. Finally, a piecewise permeability prediction model (PPPM) for U-GRS based on the CT-LBM was proposed. Results: The ranking of permeability estimation methods in terms of accuracy was as follows: LBM > PNM > FEM > EMs. Substantial disparity was observed in the permeabilities obtained using both FEM and EMs compared to other methods, which exhibited a deviation of up to six orders of magnitude. The PPPM demonstrated smaller prediction deviations than the EMs, with its accuracy influenced by the strategy for selecting calculation parameters. Conclusion: The CT-LBM, which uses real pore structures, was employed to estimate the permeability of U-GRS. The PPPM, established based on this method, was found to be applicable for estimating U-GRS permeability. [ABSTRACT FROM AUTHOR]

Water seepage in rocks, in geotechnical engineering such as the hydrofracturing of hard rocks, excavation of underground chambers, and prevention of mine water disasters, is a common problem. According to rock mechanics theory, the deformation and stress of rocks influence seepage behavior. In this study, a modified permeability model of argillaceous sandstone under coupled hydromechanical conditions was established to reveal the relationship between permeability and effective stress, including external stress and internal water pressure. The modeling results indicate a negative exponential relationship between the argillaceous sandstone permeability and the effective stress. The proposed effective stress-sensitive permeability model was validated by conducting two sets of seepage experiments based on controlling the water pressure and external stress, with the results obtained considered satisfactory. Based on the proposed permeability model, a fully coupled multifield model of the water seepage and rock deformation was developed. Fully coupled scenario-based numerical simulations were conducted in a finite element environment to investigate water seepage evolution and rock deformation. The experimental and numerical results show that the trends in the evolution of the entire compressive stress–strain and permeability curves are reversed, and the maximum value of the permeability was not consistent with the failure of argillaceous sandstone. This model and corresponding numerical simulations can provide insights for water seepage research and serve as a reliable theoretical basis for evaluating roof water injection and hydraulic fracturing in rock and mining engineering. Article Highlights: Effective stress-sensitive permeability model was developed and validated experimentally Established fully coupled model of water seepage and rock deformation Numerical simulation revealed evolutions of the rock permeability and deformation [ABSTRACT FROM AUTHOR]

This study aims to conduct a comprehensive formation evaluation of a pilot area within the Upper Shale Member of the Rumaila Oil Field. This evaluation is an essential step in the full development of the field. The application of well-log data and core analyses can help in obtaining the desired information about the geological characteristics of the formation. The process begins with measuring the formation temperature and water resistance utilizing Schlumberger’s charts and equations. The volume of shale was determined by two different methods, which were then used together to obtain the final shale volume. The porosity was determined using the conventional porosity equations from the porosity logs and the saturation was estimated based on Archie's equation. In core data analysis, an unconventional technique was utilized to determine rock type and permeability. The core porosity and the permeability were classified into four groups mainly using a self-organizing map and an unsupervised machine-learning method, and selected regression equations of each group were applied to estimate permeability in the core. The method depicted a good agreement between the core and estimated permeabilities, proving it as an effective tool. A complicated training data set was constructed based on the use of a multilayer perceptron neural network on coreless wells to identify rock types and permeability. Analyzing the petrophysical properties of the study area showed evidence that this area is characterized by heterogeneity. The heterogeneity of this formation is due to the presence of a considerable amount of shale, in addition to the significant characteristic differences in the layers and the same layer in different locations. The abundance of shale rock poses challenges during drilling operations, particularly due to shale washout which can lead to mechanical issues with the drilling string. Therefore, caution is advised when drilling new wells in the area to mitigate shale washout risks. Furthermore, the analysis identified layers with high hydrocarbon saturation that are viable for production. Conversely, some layers, characterized by shale presence and poor rock quality, are deemed unsuitable for production, and should not be considered as reservoir rock.

Sequestration of CO2 in unworkable coal seams is one of the most promising carbon reduction strategies. The mobility of CO2 in coal reservoirs is closely related to permeability, especially in low-permeability reservoirs. To this end, a method of reconstructing 3D pore-fracture networks by combining nuclear magnetic resonance and CT was proposed; and the relationship between the structural characteristics of 3D micron pore-fracture networks and permeability was studied. It was found that the average pore–throat diameter was positively correlated with permeability (R2 = 0.84) among the parameters reflecting the complexity of the pore-fracture networks, which plays a vital role in the permeability of micron pore-fractures. Subsequently, skeleton deformation and permeability evolution of 3D micron pore-fracture networks under high-pressure CO2 was studied using COMSOL software, and a permeability evolution model was established based on the structural characteristics. To evaluate the reliability of the model, it was verified to experimental results, resulting in an excellent linear positive correlation between theoretical and experimental permeability (R2 = 0.999). This showed that the permeability evolution model is instructive for engineering. In addition, the adsorption-induced strain swelling, slippage effects and fractures have a crucial effect on the permeability of coal, which is necessary for predicting reservoir permeability. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Northeast Petroleum University is the property of Journal of Northeast Petroleum University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Studying the seepage process in fracture channels (where coal particles are deposited) is of great significance for improving the performance of both on-site coal seam water injection and dust reduction technology. Through a self-developed simulation experiment of water-borne coal particle migration and accumulation and computer graphics, we investigated the influencing factors of particle accumulation in water injection and their influence law on seepage, discussed the interaction relationship between the fractal structure of coal and the characteristics of accumulated coal particles, and established a new fractal model of fracture permeability based on different particle accumulation states. The results show that the seepage velocity and the particle size jointly affect the migration and accumulation process of water-borne coal particles. When the coal particle size is constant and the seepage velocity increases, then the output of the coal powder increases, the deposition decreases, and the structure fractal dimension D3 of fractures decreases. At the same seepage velocity, with the increase of the coal particle size, the output of coal powder decreases, the deposition increases, and the structure fractal dimension D3 of fractures increases. In addition, the amount of coal powder produced in the intermittent water injection process is smaller than that produced in the continuous water injection process, more easily leading to accumulation. The variation law of the theoretical permeability with porosity remains consistent for different particle accumulation states: with the increase of porosity, the structure fractal dimension D3 of fractures decreases, while the theoretical permeability increases. The above research results can provide a theoretical basis for reducing the seepage damage of coal under the particle blocking effect. [ABSTRACT FROM AUTHOR]

To explore coal permeability evolution mechanism under the comprehensive action of multiple factors including stress - adsorption - water and slippage effect, considering the coal deformation induced by stress - adsorption, the expression of water film thickness was corrected to quantitatively characterize the effective pore size, and based on this, the intensity of gas slippage effect of water-bearing coal was further quantified and the permeability model was established under the comprehensive action of multiple factors. Combined with experimental research to verify the reliability of permeability model, and then the evolution mechanism of coal permeability, water film and slippage factors under the comprehensive action of multiple factors was further revealed. The results show that under different water saturation conditions, the permeability decreases sharply first and then tends to flat with the increase of effective stress; under the same effective stress condition, the permeability decreases with the increase of water saturation. The water film thickness changes dynamically under the action of stress - adsorption – water, the water film thickness has a negative correlation with stress and adsorption, but a positive correlation with water saturation; the slippage factor increases gradually with the increase of water saturation, but the increase trend is gentle under low stress condition, and more sharply under high stress condition. In addition, based on the disjoining pressure of gas-liquid-solid surface, the expressions of dynamic water film in square and equilateral triangle under the effect of stress-adsorption were deduced, and the evolution mechanisms of gas permeability, water film and slip coefficient of pores with different geometric shapes are compared and analyzed. Due to the presence of corner holes, the order of water film thickness in pores of different geometric forms is circle > square > equilateral triangle from large to small, the order of permeability is opposite; the slippage factor in circular is larger than that in angular pore, while the slippage factor in square and equilateral triangle pore has little difference.

It is of practical significance to investigate the dilatancy and seepage characteristics of tight sandstone gas under different confining pressures for its efficient development. Therefore, fluid–solid coupling triaxial loading experiments with gas-bearing sandstone were conducted. The results showed that the gas-bearing sandstone exhibited brittle characteristics with tensile–shear composite failure. The dual logarithmic model can better characterize the sandstone strength (R2 = 0.9952), whereas the fitting effect of the linear Mohr–Coulomb criterion is poor (R2 = 0.9294). The dilatancy capacity of sandstone was negatively correlated with confining pressure, and the dilatancy index decreased by 38.4% in the form of its convex power function with the increasing confining pressure. The sandstone underwent significant damage dilatancy during the yielding stage, resulting in a significant permeability recovery, with an increase of 67.0%~70.4%, which was greater than the decrease of 9.6%~12.6% in the elastic stage. In view of the different dominant factors of permeability reduction induced by pore compaction and recovery induced by crack development, the permeability model was established with volumetric strain and radial strain as independent variables, which could better reflect the whole process of permeability evolution. [ABSTRACT FROM AUTHOR]

Coal mining is gradually moving towards a deep area of more than 1000 m. At this depth, coal seams are under extremely high stress, leading to the formation of macroscopic fractures. The effects of cleats and macroscopic fractures on permeability evolution and comparative analysis based on established permeability models between intact and fractured coal are still limited. In this study, the permeability of intact and fractured coal specimens was tested by a multi-functional test system. The permeability data were fitted and analyzed based on the established permeability models. Our results show that the permeability curve of fractured coal has an L-shaped trend different from the S-shaped trend of intact coal permeability. The macroscopic fractures increased the permeability of coal samples by 1–3 orders of magnitude. The exponential model had a better fit for the permeability of intact and fractured coal specimens than the cubic model. The mean fitting degrees based on Chen's and Yu's models were 0.973 and 0.948, respectively. The slip of the fracture surface resulted in permeability fluctuations during the progressive deformation of fractured coal. The macroscopic fractures increased the slope of permeability in the post-peak stage and reduced coal compressibility and strength. [ABSTRACT FROM AUTHOR]

The current paper presents case study of the Budenovskoe sandstone-hosted uranium deposit, the host sequence of which has been modelled geostatistically, using Plurigaussian conditional simulation (PGS) and Sequential Gaussian conditional simulation (SGS). The PGS method minimises subjectivism of the geological interpretations, by constraining the lithological models to geostatistical and lithostratigraphic characteristics of the host sequence, and accurately reproduces litho-stratigraphy of the sedimentary sequence, realistically conveying its complexity. This was integrated with the rock permeability generated using SGS. Integrating two models has significantly enhanced definition of the fluid infiltration passages in the host sequence. The model was built using stochastic algorithms of PGS and SGS, and therefore can be used for estimation the project risk. In practice, the errors, initially estimated to the blocks used for the conditional simulation, are grouped to the volumes of a practical interest, including the resource and reserve domains, production blocks and the leach cells. [ABSTRACT FROM AUTHOR]

The release rate of liquid core material from microcapsules is crucial for the surface properties of self-protective metal/liquid microcapsule composite plating coating. However, there is no method to accurately predict the release rate of microcapsules. In this paper, the permeability experiments of different shell membranes and core materials were carried out using the weight loss method, and the permeability model of liquid microcapsules was studied based on a multiple linear regression method. The results show that three-variable mathematical model C, including membrane porosity, the viscosity of core material, and membrane thickness is suitable to describe permeability, and the membrane thickness is the most significant influence factor. Additionally, the accuracy of model C was experimentally verified, and the error of the permeation rate is about 2.06% between predictive and experimental values. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Porous asphalt concrete (PAC) has been widely used. However, clogging is the major limiting factor in PAC's use. In this work, the permeability model and characteristics of PAC under the condition of cyclic clogging and cleaning were studied. Firstly, three clogging materials were selected for cyclic clogging-cleaning tests to conduct research on PAC's permeability with different cleaning methods. Further, the relation between mixture parameters and the permeability was analysed, and the permeability coefficient model was established. Last, the clogging factor β was introduced and the relation among clogging factors and the permeability coefficient was expressed by the regression equation. The results revealed there was an obvious exponential relation between the permeability coefficient and clogging times. The V ′ C C mixture parameters were correlated highly with the permeability coefficient in the model. Correlation analysis was applied to determine the relations between β and the mixture parameters. Models of β c l o g g i n g and β c l e a n i n g were established with different cleaning methods. [ABSTRACT FROM AUTHOR]

In order to investigate the dynamics and vortex shedding of flexible supersonic canopies, a compressible permeability model combined with fabric structure parameters is proposed, and the periodic oscillation of the supersonic parachute which is referred to as breathing phenomenon is simulated based on the Arbitrary Langrangian Eulerian (ALE) method. The calculated results by new permeability model are consistent with the experimental results. The underlying mechanism of canopy breathing motion is then investigated. Moreover, the influence of canopy permeability on breathing phenomenon of supersonic parachute is analyzed. The results indicate that the periodic growth and shedding of the canopy vortex causes the variation of the pressure differential, which finally lead to the periodic oscillation of the canopy. With the increase of fabric permeability, the vortex rolled up from the canopy skirt move backward and become more slender. The influence of vortex shedding on canopy breathing motion weakened. Those lead to the decrease of the average value of canopy projected area and parachute dynamic load. So are the oscillation amplitude and frequency. The parachute deceleration performance decreases while the parachute swing angle decreases as the canopy permeability increases. [ABSTRACT FROM AUTHOR]

The permeability of concrete pavement is the primary property in a durability system, and it is an important characteristic of concrete to prevent the invasion of water, gas and ions from the external environment and to resist the occurrence of various distresses, such as freeze–thaw damage and steel corrosion. To systematically investigate the effects of traffic load, ambient temperature and their coupling effects on the permeability of pavement concrete, the rapid chloride migration (RCM) method was used to determine the chloride diffusion coefficient, and then permeability models under a single field and coupling field were established. Finally, the microstructures were examined by scanning electron microscopy, and the deterioration mechanism for chloride permeability was analysed. The results show that under a single field with load or temperature, the chloride diffusion coefficient increases with the extension of the action time. As the load level increases, the compression process shortens, and the deterioration time advances. Load and freeze–thaw cycle coupling conditions have the most significant effect on concrete permeability. The dynamic fatigue load factor Kσ, the temperature factor KT, and the coupling field factor Kσ+T were introduced to correct the permeability models. High load stress and temperature expansion and contraction stress generated inside the concrete cause the energy to accumulate rapidly at the crack tip, causing the cracks to rapidly expand and interconnect with each other, which accelerates the deterioration of concrete durability. [ABSTRACT FROM AUTHOR]

Extraction of uranium from low-permeability sandstone is a long-standing challenge in mining. The improvement of sandstone permeability has therefore become a key research focus to improve the uranium leaching effect. To address the low-permeability problem and corresponding leaching limits, leaching experiments are performed using newly developed equipment that could apply low-frequency vibration to the sandstone samples. The test results indicate that low-frequency vibration significantly improves the uranium leaching performance and permeability of the sandstone samples. The leaching effect of low-frequency vibration treatment is approximately nine times more effective than ultrasonic vibration treatment, whereas the concentration of uranium ions generated without vibration treatment is not detectable. Mathematical model that considers the combined action of physico-mechanical vibration and chemical erosion is established to describe the effect of low-frequency vibration on the permeability. The calculated results are in good agreement with the tested permeability values. This study thus offers a new method to effectively leach more uranium from low-permeability sandstone using CO2+O2 and provides an insight into the impact of low-frequency vibration on the uranium leaching process.

Studying the anisotropic seepage characteristics of coal is of great significance to the safe and efficient extraction of coalbed methane (CBM). The effective stress law (σe = σ – αp) is often used to solve the fluid–solid coupling problem of rocks. However, the Biot's coefficient α = 1 is often assumed in the current study, and the remaining researches on the α evolution of coal are mainly focused on the conventional triaxial stress state and the elastic range, which cannot reflect the anisotropic evolution of α of damaged coal under true triaxial stress state in actual formation. In the current study, an experimental study on the effect of induced damage on α and permeability of coal subjected to different true triaxial stress states was carried out, and the anisotropy of coal structure was also considered. The experimental results show that αij and permeability are closely related to the fracture evolution, and both increase with the increase in fracture density and opening. Influenced by the induced damage, αij exhibits a significant anisotropy. α1 achieves larger than α2 and α3, and α3 > α2 occurs in the horizontal direction, and the difference between α3 and α2 increases with the increase in the horizontal stress difference. Under different initial stress states, αij for high horizontal stress is lower than αij in the low due to the restraining effect of horizontal stress on fracture propagation. Under different flow directions, αij increases the most with the increase in axial strain under the gas flow perpendicular to bedding, indicating that the natural fractures distributed along the bedding face are more likely to propagate under the external stress drive compared with the face and butt cleats. At the initial plastic stage, αij exhibits αface cleat > αbutt cleat > αbedding, and the permeability also shows kface cleat > kbutt cleat > kbedding, indicating that the natural fractures distributed along the cleats are more than those near the bedding. In this study, a new coal permeability model considering the damage effect and anisotropy of Biot's coefficient is established, which shows good agreement with the experimental permeability of simulating mining stress disturbance, and considering anisotropic αij in the model is closer to the measured permeability than assuming αij = 1. Highlights: Biot's coefficient αij and permeability of damaged coal increase with fracture density and opening increase, and always show α1 > α3 > α2. Considering anisotropic coal structure, αface cleat > αbutt cleat > αbedding, and αij increases the most under gas flow perpendicular to bedding. New permeability model considering damage and αij is established, considering αij in the model is closer to the permeability than assuming αij = 1. [ABSTRACT FROM AUTHOR]

In order to investigate the dynamics and vortex shedding of flexible supersonic canopies, a compressible permeability model combined with fabric structure parameters is proposed, and the periodic oscillation of the supersonic parachute which is referred to as breathing phenomenon is simulated based on the Arbitrary Langrangian Eulerian (ALE) method. The calculated results by new permeability model are consistent with the experimental results. The underlying mechanism of canopy breathing motion is then investigated. Moreover, the influence of canopy permeability on breathing phenomenon of supersonic parachute is analyzed. The results indicate that the periodic growth and shedding of the canopy vortex causes the variation of the pressure differential, which finally lead to the periodic oscillation of the canopy. With the increase of fabric permeability, the vortex rolled up from the canopy skirt move backward and become more slender. The influence of vortex shedding on canopy breathing motion weakened. Those lead to the decrease of the average value of canopy projected area and parachute dynamic load. So are the oscillation amplitude and frequency. The parachute deceleration performance decreases while the parachute swing angle decreases as the canopy permeability increases. [ABSTRACT FROM AUTHOR]

Reasonable use of different types of low-frequency vibration can better improve the production of coalbed methane (CBM). To understand the gas transport properties under low-frequency vibration in coal, a new permeability model of CBM reservoirs was proposed in this study. This model takes into account the deformation of coal matrix and fractures caused by vibration stress, gas adsorption and effective stress. In addition, an experimental system of low-frequency vibration was built to explore the influences of low-frequency vibration on permeability of coal samples. The experimental results were compared with theoretical permeability values to verify the reliability of proposed permeability model. Finally, a series of sensitivity analyses were conducted to identify key vibration parameters to the permeability of CBM reservoir. The results showed that the vibration time, vibration force and vibration frequency have significant effects on permeability of CBM reservoirs. The longer vibration time and the higher vibration force caused the greater damage of coal, resulting in the increase of permeability. However, with the increase of vibration frequency, coal permeability growth rate of coal first increases and then decreases, and a better permeability enhancement is observed near the resonant frequency. The experimental results indicate that the permeability growth rate of bituminous coal and anthracite coal is the highest at 25 Hz, reaching 371.8% and 268.5%, respectively. These results are expected to provide theoretical guidance for employing the low-frequency vibration to improve the gas extraction rate. [ABSTRACT FROM AUTHOR]

The purpose of the project is to establish a standardized operation method of the in vitro permeability model to maximize mucosal integrity and viability. The model drug lidocaine permeability, 20 kDa fluorescein isothiocyanate-dextran, H&E staining, and mucosal viability were used as evaluation indicators. Firstly, the buccal mucosae of rats, rabbits, dogs, porcine, and humans were analyzed by H&E staining and morphometric analysis to compare the differences. Then, we studied a series of operation methods of isolated mucosa. The buccal mucosae were found to retain their integrity in Kreb's bicarbonate ringer solution at 4 °C for 36 h. Under the long-term storage method with program cooling, freezing at −80 °C, thawing at 37 °C, and using cryoprotectants of 20% glycerol and 20% trehalose, mucosal integrity and biological viability can be maintained for 21 days. The heat separation method was used to prepare a permeability model with a mucosal thickness of 500 μm, which was considered to be the optimal operation. In summary, this study provided an experimental basis for the selection and operation of in vitro penetration models, standardized the research process of isolated mucosa, and improved the accuracy of permeability studies. [ABSTRACT FROM AUTHOR]

The development of a cost-effective and accurate permeability characterization method for large-scale mining overburden conditions is topical for engineering assessment of mine water and gas migration characteristics. Based on laboratory test results, a new permeability characterization method considering loading–unloading times and elastic–plastic stress evolution is proposed, which is realized by fish language embedded in FLAC 3D software. The proposed method is first compared with other available models via the inversed experimental results and then applied to evaluate the protected seam permeability-enhanced range. The COMSOL Multiphysics and FLAC data interaction is used to optimize the protected seam extraction borehole layout. It is shown that the proposed model can characterize the permeability evolution more accurately than other (elastic, damage strain) models. The protected seam stress paths have non-equal loading and unloading amplitudes in vertical and horizontal directions, while its permeability is divided into six areas and the enhanced area exceeds that of the gob. Based on the permeability zoning characteristics, the non-uniform layout parameters of extraction boreholes are quantified to achieve the identical-size layout of working faces of the protected and protective seams and reduced workload of extraction boreholes.

The geometric structure and distribution of the fracture network significantly impact the coalbed methane flow characteristics. The indoor optical microscope test is utilized to analyze the distribution and structural characteristics of natural fractures in coal. The results indicate that the fracture network in coal consists primarily of irregular bifurcated fractures, but the influence of the bifurcation fracture network’s structural characteristics on permeability remains unclear. Therefore, the fracture network geometric structure characteristic parameters are considered in accordance with the fractal theory, and the analytical formula of the bifurcation fracture network permeability is established. Meanwhile, the bifurcation fracture network geometric model with varied structural parameters is reconstructed using the pixel probability decomposition algorithm. Finally, the influence of the key parameters of the reconstructed bifurcation fracture network on the coal seam permeability is analyzed through numerical simulation. The results indicate that the permeability of the bifurcated fracture network increases with the increase of fracture porosity φf, aperture ratio χ, and proportionality coefficient η, and decreases with the increase of tortuosity fractal dimension DT, bifurcation angle θ, fractal dimension Df, and bifurcation level n. Among them, fracture porosity and proportionality coefficient have the greatest influence on permeability, followed by tortuosity fractal dimension, aperture ratio.

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. [ABSTRACT FROM AUTHOR]

Gas seepage and progressive failure of coal are common energy-driven mining phenomena. A comprehensive understanding of the energy-driving mechanism behind the catastrophic behavior of mining-induced coal is fundamental to innovating the technology of coal and gas co-mining. Thus, this study simulated three typical mining stress evolution process in protective coal-seam mining (PCM), top-coal caving mining (TCM), and non-pillar mining (NM) to investigate the energy evolution and distribution patterns of coal. The results indicate a strong correlation between energy dissipation and gas seepage. By transitioning from PCM and TCM to NM, the peak elastic strain energy of gas-bearing coal increased by 155.92 %, and the ratio of peak dissipative energy decreased from 51 % to 41 %. Under the PCM stress path, gas seepage decreased the energy storage by 13.52 %, whereas the pre-mining pressure relief and enhanced permeability simulation increased in peak dissipation energy by 49.66 %. Using the cumulative dissipative energy as a damage variable reveals that the degree of coal damage evolution under PCM is higher than other mining methods. Based on the energy-driven damage mechanism, a new coal permeability model was established, and its comparison with classical permeability model demonstrated its excellent fitting effectiveness. [ABSTRACT FROM AUTHOR]

Studying the roles of different migration mechanisms in shale matrix gas transport is beneficial for shale gas extraction and CO 2 sequestration. The roles of different transport mechanisms in shale H 2 O–CH 4 gas mixture transport were investigated considering their contributions to fluid flux. A permeability model coupling multiple gas migration mechanisms was developed by combining the pore network-related effective medium approximation, which was validated using process data from simultaneous H 2 O–CH 4 transport in shales. The results showed that the gas transport mechanisms dominating methane and water vapor migration in shale differed. Slip flow and Knudsen diffusion contributed approximately 99% to methane migration. Water vapor surface diffusion as well as slip flow and Knudsen diffusion primarily contributed to the water vapor permeability before and after a time point, respectively. Water vapor and methane permeability generally decreased with time. The equilibrium water vapor and methane permeabilities ranged from 2.7699×10−22–12.312×10−22 m2 and 0.2640×10−21–4.9036×10−21 m2, respectively. The methane surface diffusion permeability increased slowly with time before approaching methane monolayer adsorption (before ∼100 s). An increase in relative humidity (approximately 300–3600 s) can promote the transport of water vapor. Water vapor and methane adsorption, especially the condensation of adsorbed water, could largely explain the decrease in shale gas permeability. • Characterization of H 2 O–CH 4 gas mixture migration in shale nanopores is investigated. • Roles of different migration mechanisms in water vapor and methane transport differ. • A permeability model coupling multiple gas transport mechanisms is developed. • Contributions of different mechanisms to H 2 O–CH 4 gas mixture transport are revealed. [ABSTRACT FROM AUTHOR]

The poor barrier of cellulosic paper against water vapor and oil limits its wide application as a packaging material. Lamination, extrusion, and dispersion coating are applied on paper surface to improve barrier properties. Dispersion coating can be applied to paper surface on line during paper preparation using rod coater. In the present study, an attempt has been made to increase the process speed of coating preparation by single-step process at varying pigment concentration. Coating dispersion was applied on kraft paper using laboratory rod coated and dried in hot air oven at 80°C for 5 min. Furthermore, thermal stability, water vapor barrier properties, and grease resistance of PVOH/kaolin and PVOH/talc-coated paper were studied. Additionally, the effect of single layer and bilayer coating on paper properties was also studied. Permeability model was used to predict the orientation of pigments to the surface of paper. Viscosity of both kaolin and talc-based dispersion increased with the increase in pigment concentration. Thermal studies showed that at 600°C residual mass (%) of PVOH film increased from 0.6 to 9.89% and 15% with the addition of 25% (by weight) kaolin and talc pigment. At high pigment concentration (40–50%, by weight), the highest reduction in WVTR was observed for both the pigments. Excellent coverage of paper surface and high grease resistance was observed for all coating formulations. Our study showed that talc provides better thermal and barrier properties to coated paper than kaolin. [ABSTRACT FROM AUTHOR]

The prediction of permeability and the evaluation of tight oil reservoirs are very important to extract tight oil resources. Tight oil reservoirs contain enormous micro/nanopores, in which the fluid flow exhibits micro/nanoscale flow and has a slip length. Furthermore, the porous size distribution (PSD), stress sensitivity, irreducible water, and pore wall effect must also be taken into consideration when conducting the prediction and evaluation of tight oil permeability. Currently, few studies on the permeability model of tight oil reservoirs have simultaneously taken the above factors into consideration, resulting in low reliability of the published models. To fill this gap, a fractal permeability model of tight oil reservoirs based on fractal geometry theory, the Hagen–Poiseuille equation (H–P equation), and Darcy's formula is proposed. Many factors, including the slip length, PSD, stress sensitivity, irreducible water, and pore wall effect, were coupled into the proposed model, which was verified through comparison with published experiments and models, and a sensitivity analysis is presented. From the work, it can be concluded that a decrease in the porous fractal dimension indicates an increase in the number of small pores, thus decreasing the permeability. Similarly, a large tortuous fractal dimension represents a complex flow channel, which results in a decrease in permeability. A decrease in irreducible water or an increase in slip length results in an increase in flow space, which increases permeability. The permeability decreases with an increase in effective stress; moreover, when the mechanical properties of rock (elastic modulus and Poisson's ratio) increase, the decreasing rate of permeability with effective stress is reduced. [ABSTRACT FROM AUTHOR]

Abstract Owing to the characteristics of coal reservoirs, the gas flow capacity and permeability exhibit strong anisotropy. The anisotropy in terms of the magnitude, which corresponds to the permeability in the horizontal direction being several orders of magnitude larger than that in the vertical direction, has been investigated. However, the anisotropy in terms of the mechanical boundaries, specifically, the presence of constant volume and stress boundaries in the horizontal and vertical directions, respectively, has not been examined. Therefore, a coupling model of the gas flow and coal seam deformation was developed, and a model to reflect the permeabilities in the horizontal and vertical directions was established considering the internal swelling factor. The models were verified considering field production data, and a numerical analysis was performed. In the early and later stages of gas production, the gas appeared from the fracture and matrix systems, and it was extracted in different regions in the timescale and synchronously, respectively. Owing to the constant stress boundary condition in the vertical direction of the coal seam, the reduction in the gas pressure in the fractures decreased the horizontal fracture opening, thereby decreasing the permeability in the horizontal direction. Because the horizontal direction exhibited a constant volume boundary condition, the desorption of the adsorbed gas resulted in volumetric shrinkage of the matrix, thereby increasing the permeability in the vertical direction. Non‐Darcy effects reduced the gas flow rate and exerted considerable influence in the early stage of the extraction. Moreover, this effect exhibited anisotropy, which was more pronounced in the horizontal direction. The surface diffusion coefficient continuously increased, which promoted the flow of the adsorbed gas in the matrix. The proposed model can be used to estimate the impact of the permeability anisotropy on the coalbed methane and underground gas extraction.

Abstract The evolution of rock permeability has been studied exhaustively, and a broad array of permeability models has been proposed. These models are normally derived under the assumption of elastic deformation when subjected to external stress. Under this assumption, these models define fracture permeability as a function of either gas pressure or effective stress. However, experimental observations indicate that rock fracture may experience unrecoverable deformation during the loading process. The goal of this study is to resolve this contradiction. In this study, derivation of fracture permeability correlation for elastoplastic contact of rough surfaces is presented. The proposed method for describing permeability evolution not only considers the topography of fracture surfaces but also and, more importantly, integrates the plastic deformation of rock fracture. Subsequently, the deformation and permeability change of shale sample containing a single rough‐walled fracture is experimentally investigated. The results show that the permeabilities obtained during the loading process are larger than the permeabilities obtained during the unloading process under the same stress conditions and that the fracture deformation cannot be fully recovered during the unloading process. At last, the proposed permeability model is applied to the experimental results and it is shown that the proposed model can predict the laboratory permeability data.

In the process of coalbed methane (CBM) exploitation, permeability is a key controlling parameter for gas transport in the CBM reservoirs. The CBM reservoir contains a large number of micropores and microfractures with complex structures. In order to accurately predict the gas permeability of the micropores and microfractures in CBM reservoirs, a fractal permeability model was developed in this work. This model considers the comprehensive effects of real gas, stress dependence, multiple gas flow mechanisms (e.g., slip flow, Knudsen diffusion and surface diffusion) and fractal characteristics (e.g., pore size distribution and flow path tortuosity) of micropores and microfractures on the gas permeability. Then, this fractal permeability model was verified by the reliable experimental data and other theoretical models. Finally, the sensitivity analysis is conducted to identify key factors to the permeability of CBM reservoir. The results showed that the fractal characteristics of the micropores and microfractures have significant effects on the permeability of CBM reservoir. Higher fractal dimension of micropores diameter and microfractures aperture represents the larger number of micropores and microfractures, resulting in a higher permeability. Higher tortuosity fractal dimension of micropores and microfractures means higher gas flow resistance, leading to the lower permeability. The multiple gas transport mechanisms coexist in micropores and microfractures of CBM reservoir. The permeability of slip flow and Knudsen diffusion both increases with the decrease in pore pressure. Surface diffusion is an important gas transport mechanisms in micropores, but it can be ignored in the microfractures. Knudsen diffusion plays a more obvious role in the lower pore pressure, which controls the gas transport in microfractures. And microfractures are beneficial to improve gas transport capacity of coalbed methane reservoir. [ABSTRACT FROM AUTHOR]

A sound understanding of the water permeability evolution in fractured shale is essential to the optimal hydraulic fracturing (reservoir stimulation) strategies. We have measured the water permeability of six fractured shale samples from Qiongzhusi Formation in southwest China at various pressure and stress conditions. Results showed that the average uniaxial compressive strength (UCS) and average tensile strength of the Qiongzhusi shale samples were 106.3 and 10.131 MPa, respectively. The nanometre-sized (tiny) pore structure is the dominant characteristic of the Qiongzhusi shale. Following this, we proposed a pre-stressing strategy for creating fractures in shale for permeability measurement and its validity was evaluated by CT scanning. Shale water permeability increased with pressure differential. While shale water permeability declined with increasing effective stress, such effect dropped significantly as the effective stress continues to increase. Interestingly, shale permeability increased with pressure when the pressure is relatively low (less than 4 MPa), which is inconsistent with the classic Darcy's theory. This is caused by the Bingham flow that often occurs in tiny pores. Most importantly, the proposed permeability model would fully capture the experimental data with reasonable accuracy in a wide range of stresses.