Coal Matrix Deformation Research Articles

Two-way coupling dynamics of CH4 adsorption and coal matrix deformation: Insights from hybrid GCMC/MD simulations

CH4 adsorption can deform coal microporous structures, subsequently altering adsorption isotherms. To unravel this intricate interplay at a microscopic level, we use a hybrid Grand Canonical Monte Carlo/Molecular Dynamics (GCMC/MD) simulation at 313.15 K and pressures up to 500 bar on five independent amorphous coal matrix models. Our results reveal that CH4 adsorption increases pore volume and porosity primarily by generating additional pores of similar sizes to those present in coal matrices, thereby maintaining a consistent average pore size across different pressures. The volumetric strain has a linear correlation with CH4 loading, with volumetric swelling amount approximating the expansion of CH4-occupiable pore volume, but less than He-occupiable pore volume which results in increased matrix skeletal density. Local radial density distribution of carbon atoms indicates that the immediate environment around carbon atoms remains unchanged. In a flexible matrix, the energy released during CH4 adsorption is partially absorbed by matrix deformation, resulting in a lower isosteric heat of adsorption compared to a rigid matrix, which suggests easier desorption. This study provides new insights into the mutual relationship between CH4 adsorption and coal matrix deformation, shedding lights on the complex interactions of various hydrocarbons with geomaterials.

Molecular simulation of free CO2 injection on the coal containing CH4 structure and gas replacement

Understanding the replacement process of pre-adsorbed CH4 by injecting CO2 into the coal slit pores is crucial for CO2 storage and coalbed methane (CBM) extraction. We constructed the model of free CO2 injection into coal containing CH4, and used molecular simulation methods to analyze the replacement and diffusion of gas in coal as well as the coal structure deformation at different CO2 injection pressures. Results indicated that the CO2 adsorption following injection aligned with Langmuir law, and CO2 replaced CH4 more efficiently at 6 MPa injection pressure; After CO2 injection, the coal slit pore volume increased, the coal matrix aromatic structure destroyed, the coal matrix micropores volume increased and expanded radially and compressed longitudinally. Among them, the coal matrix deformation facilitated CO2 adsorption whilelittle impacting the CH4 desorption within the micropores. Higher CO2 injection pressure reduced the diffusion coefficients of adsorbed and free CH4 and CO2, but increased the CO2 adsorption depth in the coal matrix and enhanced the CO2 adsorption stability. The results provided valuable insights into the potential mechanisms of CO2-ECBM.

Macromolecular insights into the influence of bituminous coal matrix deformation on CH4-N2 competitive adsorption and diffusion

Competitive adsorption and two-component diffusion of CH4/N2 play a crucial role in the production of N2-enhanced coalbed methane (N2-ECBM). However, the complex production and geological conditions as well as the heterogeneity and anisotropy of the coal can easily result in different degrees of deformation of the coal. Therefore, the competing adsorption and diffusion properties of CH4/N2 in bituminous coal matrix under uniaxial tension and compression strains were studied by macromolecule simulation. The results show that the partial pressure effect of N2 increases the difficulty of CH4 adsorption. Compared to the compression strain, the tension strain has less influence on the gas adsorption potential energy, while the corresponding potential energy distribution has more overlap. Besides, the thermodynamic correction factor (Γ) is positive correlated with the bulk CH4 mole fraction (y(CH4)) and decreasing y(CH4) increases the effect of CH4 displacement by N2. On the other hand, compared to the pores before adsorption, the number of pore throats and smaller pores in the gas-containing coal matrix increases, and the compression strain reduces the flexibility of the coal molecule. More importantly, strain has a dual effect on adsorption-induced coal matrix swelling and gas diffusion, with CH4 diffusion having an inhibitory effect on N2 diffusion. Furthermore, the order of strain influence on the interactions between gases is N2-N2 > CH4-CH4 > CH4-N2 and CH4-N2 tends to cluster compared to pure gases. These results provide important clues for the optimal design of the N2-ECBM technology in deformed bituminous coal reservoirs.

An image-based coal network model for simulating hydro-mechanical gas flow in coal: An application to carbon dioxide geo-sequestration

CO2 geo-sequestration is a practical approach to achieve net-zero carbon target. However, one of the main challenges for successful CO2 geo-sequestration is the reduced coal permeability and injectivity that are caused by coal swelling. Coal has complex and heterogenous internal pore and fracture structure. The processes of gases adsorbing, desorbing, and transporting within multiscale heterogeneous coal structures are more complicated compared with conventional rocks. This paper aims to gain insights about the gas transport behaviours in coal by developing a coupled model to simulate gas flow multiphysics as well as dynamic coal deformation. This work develops an image-based 3D fracture network model, called Fracture Box Model (FBNM). In this model, each fracture is described by arrays of box elements such that the regional change of fracture opening widths can be preserved. Compared with other fracture models (e.g. discrete fracture network), FBNM can simulate complicated multiphysical gas transport more efficiently, but also be able to simulate corresponding coal matrix deformation. By comparing permeability results between direct simulation method with FBNM, it is found that FBNM can effectively estimate the permeability of original fracture networks, but requiring significantly less computational cost. To study the implications of gas types, effective stress, gas adsorption, and thermal expansion on coal permeability, gas injection pressures, gas types, coal seam temperatures are varied and investigated in the simulations. In addition to the advantage of computational efficiency, FBNM is more preferable for complicated flow transport simulations where direct simulation methods are still challenging. This work provides a promising framework which could be further developed for multiphase and multicomponent flow simulations for CO2 geo-sequestration projects.

A MULTI-FIELD COUPLED SEEPAGE MODEL FOR COAL SEAM WITH FRACTURES OF POWER LAW LENGTH DISTRIBUTIONS

In the process of gas mining, the fracture distribution with power law length and the pore structure with adsorption effect have an important influence on the coal seam permeability. In recent years, the research on the internal structure of coal seam and the fluid flow mechanism has attracted a large number of researchers. In this paper, by considering the coal matrix deformation caused by adsorption, a pore-fracture model coupled with the multi-field effects and with power law length distribution of fractures in coal seam is established based on the fractal theory for porous media. In this work, we study the influences of the power law exponent [Formula: see text] of fracture length and the ratio [Formula: see text] of the minimum to maximum fracture lengths on the permeability of coal seam and the evolution mechanism of permeability with the structural and mechanical parameters of coal seam. It is found that the permeability of coal seam is inversely proportional to [Formula: see text], directly proportional to [Formula: see text], and to Langmuir volume constant and Langmuir volume strain constant. Compared with other factors, the power law component [Formula: see text] of fractures has the most significant effect on the coal seam permeability.

Analysis of permeability evolution mechanism during CO2 enhanced coalbed methane recovery based on impact factor method

To studying the influence of CO2 injection on CBM recovery and reservoir permeability, a hydraulic-mechanical-thermal coupling numerical model considering gas flow, competitive adsorption, temperature, and coal deformation was established. The model validation was carried out first, then the CO2 enhanced CBM recovery (CO2-ECBM) processes under different injection pressure and temperature were simulated, and the characteristics of CH4 output and reservoir permeability evolution were analyzed. Finally, the impact factor method for quantitative evaluating the contribution of critical parameters was proposed based on the porosity model. The results show that the production increases with the increase of injection pressure. When the injection pressure is 4 MPa, the production increases by 27,860 m3. When the injection pressure was not higher than the initial reservoir pressure, the coal matrix's deformation was dominated by CH4 desorption. When the injection pressure was higher than the initial reservoir pressure, the deformation was dominated by CH4 desorption, and the permeability increased at the preliminary stage. As time increased, the relative impact factor of CH4 desorption (RIFD) decreased, and the relative impact factor of CO2 adsorption (RIFA) increased; the deformation was dominated by CO2 adsorption, so the permeability decreased. The relative impact factor of pore pressure (RIFP) decreases with the increase of injection pressure, and its maximum value is only 0.014. The microscopic mechanism of permeability evolution was analyzed in this paper from the perspective of coal matrix deformation. The results can provide a theoretical reference for the parameter optimization of CO2-ECBM engineering applications.

Stress-Dependent Deformation and Permeability of a Fractured Coal Subject to Excavation-Related Loading Paths

The deformation and permeability of coal are largely affected by the presence and distribution of natural fractures such as cleats and bedding planes with orthogonal and abutting characteristics, resulting in distinct hydromechanical responses to stress loading during coal mining processes. In this research, a two-dimensional (2D) fracture network is constructed based on a real coal cleat trace data collected from the Fukang mine area, China. Realistic multi-stage stress loading is designed to sequentially mimic an initial equilibrium phase and a mining-induced perturbation phase involving an increase of axial stress and a decrease of confining stress. The geomechanical and hydrological behaviour of the fractured coal under various stress loading conditions is modelled using a finite element model, which can simulate the deformation of coal matrix, the shearing and dilatancy of coal cleats, the variation of cleat aperture induced by combined effects of closure/opening, and shear and tensile-induced damage. The influence of different excavation stress paths and directions of mining is further investigated. The simulation results illustrate correlated variations among the shear-induced cleat dilation, damage in coal matrix, and equivalent permeability of the fractured coal. Model results are compared with results of previous work based on conventional approaches in which natural fracture networks are not explicitly represented. In particular, the numerical model reproduces the evolution of equivalent permeability under the competing influence of the effective stress perpendicular to cleats and shear-induced cleat dilation and associated damage. Model results also indicate that coal mining at low stress rates is conducive to the stability of surrounding coal seams, and that coal mining in parallel to cleat directions is desirable. The research findings of this paper have important implications for efficient and safe exploitation of coal and coalbed methane resources.

Understanding competing effect between sorption swelling and mechanical compression on coal matrix deformation and its permeability

Coal deformation during gas depletion primarily involves the effective stress controlled bulk compression and gas sorption/desorption-induced coal matrix swelling/shrinkage. The two concurrent deformation behaviors can significantly impact coal permeability. Majority coal permeability models are defined as a cubic function of coal porosity, in which the porosity is a linear superposition of bulk strain and sorption strain. Nevertheless, laboratory measurements have observed that besides the sorption-induced swelling the injected gas pressure can also hydrostatically compress coal matrix. The observations reflect the complexity of fracture-matrix interaction on the coal porosity, which brings significant challenges to the reliable prediction of the evolution of coal permeability. In this study, we developed an experimental approach to separating the sorption-induced swelling strain of coal matrices and hydrostatic compression. We also modified a coal-gas interaction model by introducing an internal swelling coefficient and implemented into a coupled numerical model. Our experimental results based on nitrogen injection show that gas pressure rise can increase both the swelling strain and hydrostatic compressive strain of the coal specimen. If the measured coal strain is uncorrected, the corresponding isothermal sorption-strain parameters will be underestimated, resulting in the overestimation of the coal permeability. Numerical modeling results show that when the confining stress remains unchanged, the matrix swelling contribution to narrow the fracture width can attenuate with the increasing pore pressure. When the effective stress remains unchanged, the matrix swelling contribution can increase. Comparison of the published permeability data and these modeling results suggests that the actual contribution of the matrix deformation to fracture is significantly related to geomechanical state of coal reservoir, indicating that the internal swelling coefficient should be considered as a key variable when selecting a coal permeability model.

Experimental and theoretical investigation on sorption kinetics and hysteresis of nitrogen, methane, and carbon dioxide in coals

Gas Sorption kinetics and hysteresis of coals is crucial for coalbed methane (CBM) resources assessment as well as carbon dioxide (CO2) sequestration potential evaluation in US coal formations. We experimentally studied the sorption kinetics and hysteresis of three different sorbing gases including nitrogen (N2), methane (CH4), and CO2 for Appalachian basin coals. Both experimental data and theoretical results showed that all tested coals have highest CO2 affinity compared to N2 and CH4. It was also found that the diffusion coefficient and equilibrium time are the two most important sorption kinetics parameters for defining gas sorption behaviors. Both diffusion coefficient and parameter k related to equilibrium time are highly gas type and pressure dependent. Sorption hysteresis in the tested coal samples not only gas type dependent but also controlled by pore structure and irreversible deformation of coal matrix due to sorption. We proposed a new theoretical approach to quantify the potential of gas sorption hysteresis by coupling the effects of diffusion coefficient and equilibrium time. Based on the proposed approach, the results demonstrate CO2 has highest hysteresis which well agreed with the experimental results. High irreversibility at high pressure enhances the significant sorption hysteresis in CO2 sorption.

Fully Coupled Multi-Scale Model for Gas Extraction from Coal Seam Stimulated by Directional Hydraulic Fracturing

Although numerous studies have tried to explain the mechanism of directional hydraulic fracturing in a coal seam, few of them have been conducted on gas migration stimulated by directional hydraulic fracturing during coal mine methane extraction. In this study, a fully coupled multi-scale model to stimulate gas extraction from a coal seam stimulated by directional hydraulic fracturing was developed and calculated by a finite element approach. The model considers gas flow and heat transfer within the hydraulic fractures, the coal matrix, and cleat system, and it accounts for coal deformation. The model was verified using gas amount data from the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China. Model simulation results show that slots and hydraulic fracture can expand the area of gas pressure drop and decrease the time needed to complete the extraction. The evolution of hydraulic fracture apertures and permeability in coal seams is greatly influenced by the effective stress and coal matrix deformation. A series of sensitivity analyses were performed to investigate the impacts of key factors on gas extraction time of completion. The study shows that hydraulic fracture aperture and the cleat permeability of coal seams play crucial roles in gas extraction from a coal seam stimulated by directional hydraulic fracturing. In addition, the reasonable arrangement of directional boreholes could improve the gas extraction efficiency. A large coal seam dip angle and high temperature help to enhance coal mine methane extraction from the coal seam.

Flow field evolution during gas depletion considering creep deformation

Unsteady creep of coal seams and the resultant abnormal flow of coal seam gas are the main reasons for dynamic disasters in deep mines. In this study, for investigating the evolution of gas depletion in deep mines, the flow field evolutions during gas adsorption and desorption were simulated first based a multi-field (coal deformation field, diffusion field and seepage field) coupling model considering the creep deformation. Firstly, a laboratory experiment was performed to validate the mathematical model. Then, spatial and temporal evolutions of matrix and fracture gas pressures, coal permeability and equivalent stress during gas adsorption and desorption were analyzed. The results show that in the initial stage of adsorption, gas pressure in fractures increases rapidly under the action of pressure gradient. As the gas gradually diffuses into matrix pores, gas pressure in the matrix goes up, which results in swelling deformation of coal matrix and alters the fracture aperture. Accordingly, coal permeability and equivalent stress were affected. During the desorption process, gas pressure in fractures drops rapidly at first, and then it falls at a lower rate. The diffusion of gas into fractures gradually leads to matrix shrinkage and enhances coal permeability. With a decrease in fracture gas pressure, the equivalent stress of coal declines rapidly first, followed by a slow rise induced by the diffusion of gas in the matrix. Furthermore, changes in the amount of gas desorption was analyzed by fitting the experimental test data in a specific time, which verified rationality of the mathematical model. The study can provide theoretical guidance for prevention and control of dynamic disasters in coalmines and the gas pre-drainage through borehole in deep mines.

Effect of temperature on gas seepage characteristic based on coal-gas interaction model

The temperature has a significant impact on the coal seam gas extraction. A fully coupled model is established in this study, which takes into account the coal-gas interaction characteristic. The numerical result shows that the coalbed CH4 migratio and transport evolution coal bed CH4 reservoir is not only dependent on the coal matrix deformation, gas pressure and gas adsorption, but also closely related to temperature.

Permeability characteristics of coal containing gangue under the effect of adsorption

In order to find out the influence of adsorption on gas flow characteristics in coal seam, we analyzed the influence of gangue on permeability characteristics of coal, and then deduced the calculation method of the equivalent permeability of the specimen containing gangue. The gas flow characteristics under different conditions of pulverized coal sample, sand sample and coal containing gangue (simulated coal seam containing gangue) were studied using self-developed device “a test device for cracked coal-rock mass of gas migration characteristics”, and then the experimental results were analyzed. The results show that: the coal seam adsorption causes the swelling deformation of coal matrix, reduces the coal seam porosity and the coal seam permeability, and the swelling deformation of coal matrix and gas pressure conform to the rule of quadratic function. The two gangue layers with different porosity both can promote the equivalent permeability. However, the promotion effect is different with the increase of the thickness of the gangue layer. The existence of the gangue layer does not change the relation between permeability and gas pressure, and the both still conform to the quadratic function. The Klinkenberg effect of the gangue coal is weakened, and the slip factor is linearly negative to the thickness of the gangue. With the increase of the gas pressure, the extrusion coefficient of the two phases of coal and gangue first decreases and then increases.

Coal Matrix Deformation and Pore Structure Change in High-Pressure Nitrogen Replacement of Methane

Coal matrix deformation is one of the main controlling factors for coal reservoir permeability changes in nitrogen foam fracturing. The characteristics and mechanism of coal matrix deformation during the process of adsorption/desorption were studied by isothermal adsorption/desorption experiments with methane and nitrogen. Based on the free-energy theories, the Langmuir equation, and elastic mechanics, mathematical models of coal matrix deformation were developed and the deformation characteristics in adsorption/desorption processes were examined. From the study, we deduced that the coal matrix swelling, caused by methane adsorption, was a Langmuir-type relationship with the gas pressure, and exponentially increased as the adsorption quantity increased. Then, the deformation rate and amplitude of the coal matrix decreased gradually with the increase of the pressure. At the following stage, where nitrogen replaces methane, the coal matrix swelling continued but the deformation amplitude decreased, which was only 19.60% of the methane adsorption stage. At the mixed gas desorption stage, the coal matrix shrank with the reduction of pressure and the shrinkage amount changed logarithmically with the pressure, which had the hysteresis effect when compared with the swelling in adsorption. The mechanism of coal matrix deformation was discussed through a comparison of the change of micropores, mesopores, and also part macropores in the adsorption process.

Impact of Effective Stress and Matrix Deformation on the Coal Fracture Permeability

The permeability of coal is an important parameter in mine methane control and coal bed methane exploitation because it determines the practicability of methane extraction. We developed a new coal permeability model under tri-axial stress conditions. In our model, the coal matrix is compressible and Biot’s coefficient, which is considered to be 1 in existing models, varies between 0 and 1. Only a portion of the matrix deformation, which is represented by the effective coal matrix deformation factor \(f_\mathrm{m}\), contributes to fracture deformation. The factor \(f_\mathrm{m}\) is a parameter of the coal structure and is a constant between 0 and 1 for a specific coal. Laboratory tests indicate that the Sulcis coal sample has an \(f_\mathrm{m}\) value of 0.1794 for \(\hbox {N}_{2}\) and \(\hbox {CO}_{2}\). The proposed permeability model was evaluated using published data for the Sulcis coal sample and is compared to three popular permeability models. The proposed model agrees well with the observed permeability changes and can predict the permeability of coal better than the other models. The sensitivity of the new model to changes in the physical, mechanical and adsorption deformation parameters of the coal was investigated. Biot’s coefficient and the bulk modulus mainly affect the effective stress term in the proposed model. The sorption deformation parameters and the factor \(f_\mathrm{m}\) affect the coal matrix deformation term.

Coal matrix deformation characteristics in the process of carbon dioxide displacing different gas saturation coal-bed methane

It is fundamental that changes in coal reservoir permeability are researched, in particular, the accurate determination of variations in the coal matrix caused by CO2 replacing CH4 at different gas saturation conditions. Based on the surface free energy, the extended Langmuir isothermal adsorption model, combined with CO2 replacing CH4 in experimental trials, and calling on the more general principles and characteristics of the field, mathematical models describing the coal matrix as it undergoes different processes such as CO2 injection and desorption were established. Combined with laboratory data about CO2 replacement under different methane saturation conditions, a law governing the variations in coal matrix CO2 replacement under different methane gas saturation conditions was obtained. The results showed that: in the injection process, the coal matrix expansion rate caused by CO2 or CH4 was exponentially increased with the CO2 pressure increase, the expansion caused by CO2 was far greater than the expansion caused by CH4 in the desorption process, the coal matrix shrinkage caused by CO2 or CH4 was exponentially increased with the pressure decrease, the shrinkage caused by CO2 was larger than the shrinkage caused by CH4 under the same pressure and different gas saturation, the total shrinkage in the desorption process in the coal matrix was greater than the total expansion in the injection process. At higher gas saturations, the total coal matrix shrinkage volume exceeded the total expansion corresponding to pressure points higher in the desorption process.

How sorption-induced matrix deformation affects gas flow in coal seams: A new FE model

The influence of sorption-induced coal matrix deformation on the evolution of porosity and permeability of fractured coal seams is evaluated, together with its influence on gas recovery rates. The porosity-based model considers factors such as the volume occupied by the free-phase gas, the volume occupied by the adsorbed phase gas, the deformation-induced pore volume change, and the sorption-induced coal pore volume change. More importantly, these factors are quantified under in situ stress conditions. A cubic relation between coal porosity and permeability is introduced to relate the coal storage capability (changing porosity) to the coal transport property (changing permeability). A general porosity and permeability model is then implemented into a coupled gas flow and coal deformation finite element model. The new FE model was used to compare the performance of the new model with that of the Palmer–Mansoori model. It is found that the Palmer–Mansoori model may produce significant errors if loading conditions deviate from the assumptions of the uniaxial strain condition and infinite bulk modulus of the grains. The FE model was also applied to quantify the net change in permeability, the gas flow, and the resultant deformation in a coal seam. Model results demonstrate that the evolution of porosity and of permeability is controlled by the competing influences of effective stresses and sorption-based volume changes. The resulting sense of permeability change is controlled by the dominant mechanism.