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Because of the existence of multiscale pores from nano- to macroscale, a multimechanistic shale gas flow process involving the Darcy and Knudsen flows occurs during gas shale well depletion. The respective contribution of the Darcy and Knudsen flows to the permeability is constantly changing with pressure evolution. In this study, laboratory measurements of shale permeability with CO2 injections were carried out under hydrostatic conditions, using the transient pulse-decay method. The "U"-shape permeability curve resulted in both positive and negative effective stress coefficients (Biot's coefficient) χ. A permeability turning point was thus created to partition permeability curves into the Darcy and Knudsen sections. The Knudsen effect was proven to be significant at low pressure/late time in the laboratory. Effective stress and sorption-induced deformation have been found to govern the Darcy permeability evolution under the tested experimental conditions. Thus, negative effective stress coefficients, together with the positive ones, should be applied to a nonmonotonic pressure-permeability evolution to explain the concurrent effect of the Darcy flow and Knudsen flow at different pore pressures. [ABSTRACT FROM AUTHOR]

Fractured sorptive geomaterials (FSG) are ubiquitous in geological systems such as coal, shale and chalk. The solid matrix of FSG can adsorb species in gas or liquid form, the process of which is often accompanied by the deformation and microstructural alternation of the matrix. Such coupling is further obscured by the presence of fracture network, introducing complex fracture–matrix interactions. Predicting the hydromechanical properties of FSG is of particular importance for the production of coalbed methane (CBM) which requires the assessment of coal permeability under varying pressure and stress conditions. This study attempts to investigate the interplay between adsorption, deformation, and permeability evolution of coals. The novel concept of adsorption stress popularized in material science research is adopted here to construct a mechanistic theory describing sorption-induced deformation of coals. The constitutive theory is implemented in a finite element (FE) scheme and then adopted for describing coal matrix in a FE model of coal–fracture system. The model is calibrated for San Juan coals and applied to simulate a typical methane depletion test. It is observed that, depending on the competing effect between desorption-induced fracture opening and poroelastic compaction, the predicted permeability curve may be monotonically increasing (rising type) or decreasing (decline type), or may exhibit reduction first and then increase (rebound type) during gas depletion. Such competition is found to be controlled by the volume ratio, the permeability ratio, and the stiffness ratio between the matrix and the fracture elements. The prediction covers a wide range of permeability data obtained from laboratory tests and field observations. [ABSTRACT FROM AUTHOR]

Understanding the evolution of gob compaction and related gas transport behavior is necessary for the planning and optimization of gas ventilation and control in longwall coal mines. In particular, the detachment of the undermined roof into the gob leaves a loosely compacted perimeter that skirts the longwall panel. This permeable gob perimeter in plan view forms as a result of shear separation from support provided by the solid ribs. This detachment and the resulting rotated and reduced stresses limit compaction, elevate permeability and exert significant control on gas flow during active longwall mining operations. We report gob compaction experiments on in-mine-collected fragmented rock and conduct mechanical compaction on stacked samples that are either uniformly coarsening upwards (case A) or are coarsening upwards, but capped by a segregated upper layer of coarse rock (case B). Observed compaction is linked to a capillary model representing porosity reduction and permeability evolution. As applied uniaxial stress increases from 0 to up to ~ 2000 kPa, the porosity decreases from 0.64 to 0.41(~ 36%) for the uniform stacked material (A) and but only from 0.66 to 0.51 (~ 23%) where the gob is topped with a layer of coarse "roof" rock simulants (B). Particle–particle self-adjustment dominates the compactive behavior at initial low stress and results in significant strain—followed by a linearly elastic region through the remainder of loading. The elastic regime is used to predict the permeability of the loosely compacted gob, considering the redistribution of stresses induced by shear collapse at the rib. Permeability evolution is scaled through the evolving compactive strains and particle size distribution of the fragmented rock, enabling results to be up-scaled to mine scale. These results provide a first rational method for analyzing the interactions between caved gob and the ventilation system towards mitigating gas concentrations and minimizing the hazard. [ABSTRACT FROM AUTHOR]

Gas transport in coal and shale matrices does not always fall into the continuum flow regime described by Darcy’s law. Rather, a considerable portion of this transport is sporadic and irregular when the mean free path of gas becomes comparable to the prevailing pore scale. A nonlinear process influenced by non-Darcy flow components like gas sorption, gas slippage, and diffusion occurs throughout gas recovery. Therefore, a new permeability model with pressure-dependent weighting factors is presented to describe gas flow. This model contains the coupling of matrix flow with explaining the impact of both multiple flow regimes and stress-strain relationship on unconventional gas permeability evolution. The stress-strain relationships were derived from thermal-elastic equations and can be incorporated into the fracture-based flow component, enabling permeability prediction under uniaxial strain and hydrostatic conditions. The “U-shape” permeability trends caused by flow dynamics and geomechanical effects are observed in modeling results, which match experimental data. The agreement between modeling results and experimental data shows that gas permeability can be fully characterized by the presented model. This model has the ability to predict uniaxial strain permeability to hydrostatic permeability in a laboratory scale. [ABSTRACT FROM AUTHOR]

The dynamic changes of strain-induced strains and permeability are two key parameters to determine the production profile of coalbed methane (CBM) wells. Recent field observations from a group of anthracite CBM wells demonstrate that the permeability dramatically changed with depletion. In addition, different drainage strategies will induce different permeability evolution due to different matrix shrinkage behaviors. We carried out a series of experimental measurements on sorption induced strain evolution and its influence on permeability evolution for anthracites using a desorption-seepage testing system. The relationship between sorption pressure and its induced strains was studied. The results show that both axial and radial strains of a coal specimen increase with continuous methane injection pressure. It was found that the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane. Under constant confining and axial stresses, the effective stress decreases with the increase of methane pressure, and permeability initially drops and then rebounds with continuous methane injection. The “check-shape” permeability evolution was observed for all three specimens and the turning points are 4–5 MPa for the tested anthracites. It was also found that sorption swelling strain depends on both petrographical properties and the degree of coalification. The strength of coal increases with the degree of coalification. At a constant confining and axial stresses, both sorption-induced swelling strain and the mechanical-induced strain decline with increase of coal strength. The sorption-induced strain increases with increase of the organic content. During primary CBM production, the permeability of coal initially decreases before reservoir pressure reaches the critical desorption pressure and it starts to increase at low pressure at which the reservoir pressure is below the critical desorption pressure. The results have theoretical and practical significance to guide the reasonable drainage of CBM wells. [ABSTRACT FROM AUTHOR]

This paper proposed a conceptual model of broken rock mass compaction based on elastic theory by simplifying the compression process. This conceptual model assumes that the contact connection between two adjacent rock particles is similar to a cubic mass. With this simplification, the stress-strain constitutive law is established. The change in the secant modulus in the mechanical model derived from the variation of the connection coefficient agrees well with the reported experimental results. The permeability of compacted rock mass evolution was modeled based on the cubic law. The mechanical compression model was coupled with the permeability evolution model. The modeled permeability evolution is consistent with reported simulation and experimental results. The modeled permeability results were validated using Karacan's data with broken shale rock properties. Compared to an intact rock mass, we found that the stress-strain curve of a compacted rock mass takes a longer compression path to reach linearity due to the void space compaction resulting from friction slipping and the re-arrangement of particles. It was also found that the particle elastic modulus does not contribute to the overall bulk compaction and permeability reduction at the initial compaction stage. However, the particle elastic modulus controls the permeability evolution for a fully compacted gob, where the gob can be treated as an intact rock mass. The proposed conceptual models will potentially lay the foundation for future permeability and caving behavior characterizations using numerical simulations for complex gob areas. [ABSTRACT FROM AUTHOR]

Permeability evolution in coal is critical for the prediction of coalbed methane (CBM) production and CO 2 -enhanced-CBM. The anthracite, as the highest rank coal, has ultra-tight structure and the gas flow dynamics is complicated and influenced by multi-mechanistic flow components. Gas transport in anthracite will be a nonlinear multi-mechanistic process also including non-Darcy components like gas as-/desorption, gas slippage and diffusion flow. In this study, a series of laboratory permeability measurements were conducted on an anthracite sample for helium and CO 2 depletions under both constant stress and uniaxial strain boundary conditions. The different transient pulse-decay methods were utilized to estimate the permeability and Klinkenberg correction accounting for slip effect was also used to calculate the intrinsic permeability. The helium permeability results indicate that the overall permeability under uniaxial strain condition is higher than that under constant stress condition because of larger effective stress reduction during gas depletion. At low pressure under constant stress condition, CO 2 permeability enhancement due to sorption-induced matrix shrinkage effect is significant, which can be either clearly observed from the pulse-decay pressure response curves or the data reduced by Cui et al.'s method. But within the same pressure range, there is almost no difference between Brace's method and Dicker & Smits's method. Gas slippage effect is also significant at low pressure for low permeability coal based on the obtained experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Permeability increase in coalbed methane (CBM) reservoirs during primary depletion, particularly in the San Juan Basin, is a well accepted phenomenon. It is complex since it is influenced by stress conditions and coal matrix shrinkage associated with gas desorption. Understanding the variations in coal permeability is critical in order to reliably project future gas production, or consider other gas migration issues in the reservoir. Since sorption-induced strain plays a critical role in changing the permeability, typically observed, the theoretical strain model should be incorporated into the permeability prediction models. An effort is made in this paper to couple the recently developed Liu and Harpalani sorption-induced strain model with various permeability models. The model first calculates the theoretical coal matrix shrinkage strain and, using the calculated strain, various commonly used permeability models are applied to two sets of field data. The results of the coupled models show that the agreement between the predicted permeability and that observed in the field is very good. The merit of the coupled models is that it can theoretically predict the permeability with less experimental work, making it a more time efficient and economical technique compared to models used in the past. [Copyright &y& Elsevier]

Abstract: In this paper, second of the two-part series, pressure-permeability trend established in the laboratory was compared with modeled results using two analytical models, Shi and Durucan and Palmer et al., developed to predict changes in permeability of coalbed methane reservoirs with continued production. The experimental results did not match the modeled results very well for either of the two models. Also, there appeared to be several uncertainties, primarily the constancy of cleat compressibility and estimation of certain input parameters. A modification of the Shi and Durucan model was then proposed to increase the impact of the shrinkage effect on permeability. With this modification, the measured and modeled results matched perfectly, with the modeled permeability of coal increasing continuously, just as observed in the laboratory and supported by field data, and the rate of increase accelerating at low pressures. The phenomenon was explained by analyzing the permeability as a function of reservoir pressure. [Copyright &y& Elsevier]

• Gas depletion experiments under replicated in situ condition were conducted. • Matrix shrinkage property was coupled into the poroelastic relationships. • Effects of local deviatoric effective stress on local coal failure were analyzed. Matrix shrinkage is a unique property of coal which dictates the sorption induced strain in the constitutive relation. Both permeability and stress profiles are passively modified with continuous depletion and they are intrinsically controlled by both poroelastic and matrix shrinkage properties. The models for quantifying matrix shrinkage related effects including horizontal stress loss, vertical strain variation and permeability evolution were proposed. The matrix shrinkage property was also coupled into the poroelastic relationships to study the potential possibility of local failure. Experimental study was performed to measure the permeability and its dynamic applied horizontal stress under replicated in situ uniaxial strain condition. The effects of sorption induced horizontal stress evolution varies with gas types depending on the sorption intensity. The vertical strain of coal bulk under uniaxial strain condition consists of the sorption induced matrix shrinkage/swelling strain, cleat volume strain and matrix mechanical strain due to changes of pore pressure and external stresses, which was validated against with the experiment data. As gas depletion from 4.14 MPa to 0.55 MPa, matrix shrinkage effects can lead to the permeability ratio nonlinearly increases from 1 to 2.27 (~127%) for methane and from 1 to 4.58 (~358%) for carbon dioxide, respectively. Additionally, the magnitude of the vertical effective stress always increases with continuous gas depletion, but the horizontal effective stress may increase or decrease depending on the intensity of sorption-induced matrix shrinkage. The local deviatoric effective stress tends to increase by considering matrix shrinkage effect, which potentially increase the possibility of local coal failure. [ABSTRACT FROM AUTHOR]

• Evaluation of fracture permeability damage induced by water-based fracturing fluid. • Evaluation of volume of residue of water-based fracturing fluid in hydraulic and natural fracture. • Effect of brine flushing with different pressure on permeability recovery. • Mechanisms of residue on permeability of artificially fracture and natural fracture. • Impact of proppant size on permeability damage of artificially propped fracture. The water-based fracturing fluid is commonly used in coalbed methane (CBM) industry. The fracturing fluid invades into both stimulated and natural fractures. The fracture permeability are pivot parameter for fracturing stimulation and production planning. In this study, laboratory studies have been conducted to measure the fracture permeability evolutions for both artistically propped and natural fractures of anthracite. Three commonly used water-based fracturing fluids, including the slickwater, the guar gel and the viscoelastic surfactant (VES) fracturing fluids, were used for the experiments. The anthracite core specimens were prepared with either artistically propped (simulating hydraulic fracture) or natural fractures. The damage of the water-based fracturing fluid to the permeability of anthracite was comprehensively analyzed. Our experimental results show that both the slickwater and the guar gel can induce apparent damage to the permeability of the artificially propped and natural fractures. The VES fracturing fluid has less damage than the slickwater and the guar gel. The degree of permeability damage to the artificially propped fracture generally increases with an increase of concentration or the viscosity of the fracturing fluid. The fracturing fluid gives higher permeability damage to the artificially propped fracture supported by smaller proppant than that for large proppants. For each type of fracturing fluid, the size and density of the residue mass increased with the increase of the concentration of the fracturing fluid. For the permeability damage induced by the invasion of the fracturing fluid, the intrinsic reason is the blockage and occupation of the residue to the internal space of the fracture. [ABSTRACT FROM AUTHOR]

Although many permeability models have been proposed in last few decades to characterize the permeability evolution in poroelastic coalbed methane (CBM) reservoirs, none of the existing model is capable of defining the failure-involved coal permeability behavior which is the common situation for deep coal in-seam boreholes and over matured CBM reservoirs. In this study, a conceptual model named equivalent fractured coal (EFC) was initially proposed to characterize the structure of plastic deformation with new fracture generation. Based on the EFC model, the fracture dilation with deviatoric stress evolution within coal bulk is quantified by an exponential function, and a plastic deformation-based coal permeability (PDP) model was then proposed by incorporating the quantified fracture generation and dilation into a poroelasticity-based permeability model we built previously. Then the PDP model was validated by matching it with the tested permeability data in the unloading-flow experiments. The experiment results show that at the initial stage, the permeability increases slowly with a decrease of the confining pressure, which is mainly caused by the increase of fracture aperture. Later, with the further decrease of the confining pressure, the permeability increases by orders of magnitude, this is dominantly resulted from the generation of large amounts of new cracks. In addition, the results show that the PDP modeled permeability evolution well agreed with the experimental measured results for the whole process of unloading. This suggests that the PDP model cannot only predict the permeability variation under fracture generation and dilation process during plastic deformation, but also can describe that in the case of elastic deformation. The primary implications for this study is to provide a new mechanism-based approach to predict the permeability behavior in deep CBM reservoir during primary depletion or coal seams disturbed by underground coal mining. • EFC model is developed to characterize the structure of plastically deformed coal. • PDP model is developed to predict the permeability evolution of coal seam with mining disturbance. • Unloading-flow experiments are conducted to validate the PDP model. [ABSTRACT FROM AUTHOR]

• Stress change and permeability evolution of coal under uniaxial strain condition was replicated in the laboratory. • Both the changes of effective horizontal stress and permeability were modelled. • An index was proposed to evaluate the stability of the CBM reservoir during depletion. Coalbed methane (CBM) reservoir is generally believed to be under the uniaxial strain condition. Because of this stress/strain controlled boundary, the horizontal stress is passively changed with gas depletion. This dynamic stress evolution has great impact on permeability and stability of the CBM reservoir. Even though it is generally known that the stress depletion is both poromechanics and desorption controlled process, however, how the gas pressure and the gas type affect the horizontal stress profile and its induced coal failure is still unclear. In this work, we experimentally simulated the uniaxial strain condition in the lab. Both the horizontal stress and permeability were continuously monitored during of different gases (He, N 2 , CH 4 and CO 2) depletion. The results show that regardless of gas type, the horizontal stress decreases with the reduction of gas pressure. And gas with a higher adsorption capacity corresponds to a greater horizontal stress loss. During the depletion, the effective vertical stress rises, while effective horizontal stress show various trends for different gases. The effective horizontal stress increases during helium depletion, which leads to a reduction of the permeability. And it decreases during methane and carbon dioxide depletion, which corresponds to a significant increase of the permeability. While for nitrogen, the permeability decreases first and followed by a slight uptick. Both the lab test and modeling results show that a significant non-linearity exists in effective horizontal stress change during adsorptive gas depletion, which is expected to affect the reservoir stability. To evaluate the reservoir stability during depletion, a stability factor (SF) was put forward, and the influencing factors on this parameter were analyzed. With this parameter, the critical reservoir pressure indicating reservoir failure can be determined with appropriate geomechanical parameters input. This research will advance the understanding of the geomechanics change during CBM production. [ABSTRACT FROM AUTHOR]

Permeability and its evolution directly control the flow behaviors of the subsurface energy fluid extractions. Coalbed is known as a fractured, dual-porous media, and whose permeability is directly related to effective stress and the manner of stress loaded and unloaded. Coal has been historically treated as a transverse isotropic media and the permeability can also be influenced by the sorption induced deformations. As a cleat-rich porous medium, coal is potentially bearing various loading paths, such as fracturing, mining and natural geological tectonics activities. The permeability variation under cyclic loading and different stress paths is crucial for the engineering design for both mining and gas extractions from coal seams. The pulse-decay method was employed to estimate the permeability of cylinder coal specimen. An average permeability damage concept with a complete loading-unloading cycle is introduced to quantify the effects of maximum stress, loading path, holding time period, and gas type on permeability evolution. It was demonstrated that the permeability hysteresis occurred during the cyclic loading-unloading, and that the average permeability damage was subject to the loading-unloading paths. The average permeability damage for the coal specimen is more sensitive to maximum loading stress and the loading-unloading path compared to gas injection pore pressure and the holding period. Gas adsorption induced swelling dramatically reduced the initial permeability and average permeability damage during cyclic loading. The permeability evolution was analyzed through poroelastic theory coupled with the matrix shrinkage concept. When applied to permeability and stress relationship, Biot's coefficient should be cautiously applied for coal reservoir because it could be larger than unity when the adsorption induced non-elastic swelling is considered, and it even could be negative when the irreversible cleats closure is considered. • Estimation of permeability damage under various loading conditions. • Sorption induced matrix shrinkage/swelling and its impact on permeability damage. • Evaluation of permeability damage due to various maximum stresses and holding durations. • Estimation of Biot coefficients for sorptive coal. [ABSTRACT FROM AUTHOR]

During coalbed methane (CBM) formation stimulation, large volumes of fracking fluids are injected, and these fluids include water, hot steam, CO 2 , N 2 , foams, and their mixtures. These injected fluids can potentially generate new fractures and/or reactivate pre-existing faults/cleats. Coal, unlike other subsurface rocks, has systematic cleats that cause deterioration of the coal's mechanical strength. Unique to coal, fluid sorption-induced swelling and softening are commonly observed. With these unique properties, friction-based fracture sliding in stressed coal exhibits differing mechanisms. To better understand the frictional sliding behavior and permeability evolution in well-cleated coal, sliding friction measurements were conducted using artificially generated coal fractures. Various fluids (helium, carbon dioxide, water, and moisturized methane) were injected into coal to investigate the dynamic characteristics of friction coefficients and permeability evolution during faulting. Unlike the traditional view of stress-based fault reactivation, our results demonstrated that the initial sliding reactivation stress on the coal was not only pressure dependent but also fluid-type dependent. The friction strength of the coal fracture significantly dropped after injecting sorbing fluids, which could lead to fault reactivation at a low pore pressure since the friction failure curve can approach Mohr's circle envelope. Fracture permeability during sliding is sensitive to the softening effect of injected fluids; and water-involved injections of fluids may lead to an abrupt increase of friction due to the dilation caused by interactions between surface teeth and gouge particles. Both the velocity increase and decrease steps showed an increase in velocity strengthening when investigating the rate-state-friction parameters, indicating a continuous friction increase with sliding distance. • Various fluids were injected to investigate the dynamic friction during faulting. • The reactivation stress for initial sliding was not only pressure dependent but also fluid-type dependent for coal. • The significant drop of friction strength after fluids injection could lead to fault reactivation at a low pore pressure. • Fracture permeability during sliding was sensitive to the softening effect of injected fluids. • Both velocity increase and decrease steps showed velocity strengthening by investigating rate-state-friction parameters. [ABSTRACT FROM AUTHOR]

For the significant impact of pore structure on gas storage and transport in coal seams, research on coal pore structure characterization has been a hotspot. Benefited from the high spatial resolution synchrotron-based nano-CT instrument, pore structure characterization of coal is investigated in nano scale. Image alignment and 3D reconstruction were completed at the platform designed by National Synchrotron Radiation Laboratory and Chinese Academy of Sciences. The segmentation of the unimodal grey-scale value histograms is solved by Between-class Variance Maximisation (BCVM) algorithm and the nano-CT images are segmented into three components, pore, organic components and mineral components. Based on the voxel number, components fraction is computed. Pore size distribution (PSD) presents bimodality. Pores with equivalent radius less than 60 nm account for 84% of the total pore number. Throats with equivalent radius less than 60 nm account for 89% of the total throat number. Throats with length less than 100 nm account for 58% of the total throat number and throats with length less than 400 nm account for 84% of the total throat number. Pore number decreases with the increase of coordination number. There are over 50% of pores without coordination pore and pore connectivity was analysed. Nanopore structure-based computational fluid dynamics (CFD) simulation was explored. The permeability in three coordinate axes directions presents anisotropy. [ABSTRACT FROM AUTHOR]

Enhanced coalbed methane (ECBM) achieved by injecting liquid carbon dioxide (LCO 2 ) has been proposed and applied in industrial production for decades and has been demonstrated to be an applicable method to boost CBM production. Most of the studies have concentrated on the gas bursting and flooding effect and have rarely focused on the accompanying “freeze–thaw” phenomenon, and the temperature effect of cyclic LCO 2 injection on the pore variation of different coals has been partly investigated. In this paper, the influence of cycle parameters, such as cycle number and cycle time, on the pore variation was studied. Infrared thermal imagery (ITI) and low-field nuclear magnetic resonance (NMR) were used to measure the temperature and pore size distribution (PSD) change, respectively. The results show the following: (1) The gas pressure displayed square cyclicity with different cycle time, the temperature of gasified CO 2 was almost 248.15  K , and the end and lateral surface temperatures of a core were in the range from 259.35 to 261.85  K , which could cause the water within the pores to freeze with a 9% volume increase, and the fracturing formula was deduced; (2) The relaxation time spectra obtained by different cycle parameters expressed changeable PSD of cores with increasing cycle parameters, and the magnified proportion of bulk water and capillary water, as well the diminished proportion of adsorbed water, all indicated that the increased number of macropores and mesopores formed a larger free volume; (3) The increased total porosity φ t and the decreased T 2cutoff of six cores with the increasing cycle parameters meant that the larger cycle number could enhance the porosity due to amount of damage accumulation, and the larger cycle time might make the water freeze completely with larger ice swelling stress; (4) There is a polynomial fitting between relative increase ratio Rφ and cycle time, and the fitting coefficients were all higher than 0.99, and the larger the cycle time was, the greater the Rφ (e/t) increment and Rφ (r/t) decrement were. The interval increase ratio Iφ e was positively correlated to cycle time without obvious increase behavior; however, the Iφ r variation expressed that the greater the cycle number was, the lesser the Iφ r with the increasing cycle time was, which indicates that the increasing cycle parameters might help the proportion of connected pores to increase and provide more pathways for permeable fluid; (5) The NMR permeability k SDR of a core increased as the cycle number increased, and the longer cycle time was superior in terms of permeability enhancement. [ABSTRACT FROM AUTHOR]

The aims of this research are to quantitatively evaluate the complexity of the pore structure in coal frozen with liquid nitrogen (LN 2 ) and then study the influence of the modified pore system on coalbed methane (CBM) extraction. To do this, nuclear magnetic resonance (NMR) and fractal dimension theory were used to determine the properties of the coal's pore system after samples of low rank coal had been frozen and then thawed. The fractal dimensions of pores in frozen-thawed coal samples were divided into five types according to pore size and the state of the fluid in the coal pores. The results showed that the fractal dimension D A of adsorption pores was less than two, indicating that these pores did not exhibit fractal characteristics. The fractal dimensions D ir and D T representing closed pores and total pores presented low fitting precision, so the closed pores showed insignificant fractal characteristics. However, the fractal dimensions D F and D S representing open pores and seepage pores had high fitting precision, suggesting that open and gas seepage pores exhibited a favorable fractal characteristic. Correlation analysis revealed that D F and Ds were negatively correlated with LN 2 freezing time and the number of freeze-thaw cycles. After being frozen and thawed, coal porosity and permeability showed a strong negative correlation with fractal dimension and this relationship allowed predictive models for permeability and fractal dimensions (D F and D S ) to be constructed. The models showed that the smaller the fractal dimension, the more uniformly the pores were distributed and the higher their degree of connection. These properties favor the production of CBM. This study also showed that compared with single LN 2 freezing events, repeated cyclic freezing with LN 2 followed by thawing is more favorable for CBM production. [ABSTRACT FROM AUTHOR]

Liquid carbon dioxide (LCO 2 ) fracturing technology has been applied in the enhanced coalbed methane recovery (ECBM), and it has made some progress in the physical experiments and field applications. However, the freeze phenomenon during the injection process might induce coal matrix shrinkage, hindering the fracturing efficiency. A multiple cycle LCO 2 fracturing technology is proposed, and the feasibility of the cryogenic effect from LCO 2 on the crack evolution of five different coal cores under the loading state was investigated by using an innovative cryogenic loading experimental system. Nuclear magnetic resonance (NMR) and infrared thermal imaging (ITI) were used to measure the pore changes and temperature distribution, respectively. After 25 injection cycles, some cracks on the side and lateral surfaces of five cores were generated, and a low temperature distribution was formed. The temperature values were almost less than −18 °C, which could cause the saturated water to freeze into ice with a 9% volume increase; thus, the stress analysis diagram during one cycle injection was analyzed, and the initiation criterion was deduced. The T 2 spectra variation showed that the various pore sizes changed with the increased number of cycles. The peaks increased in amplitude and shifted to the right under saturated conditions, while they decreased and shifted to the left under centrifuged conditions, causing the amplitude increment ΔA in the post-test stage to be greater than that in the pre-test stage, which indicated that the cryogenic effect of LCO 2 could significantly improve the connectivity of pores. The total porosity φ t and effective porosity φ e of all five cores increased with the number of cycles. A quadratic function described the relationship between incremental ratio of φ e ( D c ) and cycle number, the fitting coefficients for which all exceeded 0.99, which indicated that the cryogenic effect of LCO 2 could improve the permeability observably. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN 2 ), a non-aqueous medium, has attracted attention in recent years as a fluid for fracturing in the petroleum/energy industry. This study proposes a freeze-thaw method using LN 2 to improve coal permeability for the production of coal bed methane. Experiments were conducted using nuclear magnetic resonance (NMR) to explore the physical properties of frozen-thawed coal. The coal samples were subjected to different LN 2 freezing times and to freeze-thaw cycles and coals of different rank and with different moisture contents were tested. Changes in these four freeze-thaw variables changed the petrophysical properties of the frozen-thawed coal samples; the pore structure, porosity, and permeability of the coals were modified. Of these variables, the number of freeze-thaw cycles had the most substantial effect on modifying the coal’s petrophysical properties. The degree of modification on the coals of different rank was affected by the coal’s initial porosity. In general, lignites were modified the most, anthracite coal was modified less, and bituminous coal was modified the least. The study analyzed three of the classic NMR transforms for determining permeability and found that the Schlumberger-Doll Research (SDR) model matched the measured gas permeabilities most consistently. Based on this SDR permeability model, equations suitable for predicting the permeability of frozen-thawed low-rank coals were derived. In addition, results from scanning electron microscope studies showed that a fracture network with fracture widths of as much as 32.3 μm was formed in the coal after 30 freeze-thaw cycles. Additionally, micron-size particles falling from the coal surface gradually increased as the number of freeze-thaw cycles increased, indicating that freeze-thaw using LN 2 materially modified the physical properties of the coal. [ABSTRACT FROM AUTHOR]

Abstract: This paper, first of a two-part series, discusses the results of a laboratory-scale study completed to establish the permeability variation trend with continued production of methane from coal-gas reservoirs. The field condition of uniaxial strain, assumed in the analytical models developed for permeability prediction, was replicated in the study. The results showed that the permeability of coal increases continuously, the rate of increase accelerating at low pressures. The primary reason for the increase appears to be the decrease in horizontal stress resulting from the sorption-induced volumetric strain, the so called “matrix shrinkage” effect. In the second part, experimental data is used to validate the commonly used permeability prediction analytical models and present a modification for one to improve its ability to predict permeability changes. [Copyright &y& Elsevier]

Abstract: Significant changes occur in the absolute permeability of coalbed methane (CBM) reservoirs during primary depletion or enhanced recovery/CO2 sequestration operations. In order to project gas production, several analytical models have been developed to predict changes in coal permeability as a function of stress/porosity and sorption. Although these models are more transparent and less complicated than the coupled numerical models, there are differences between the various analytical models and there are several uncertainties. These are discussed briefly in this paper. A new model is then proposed, which is based on the volumetric balance between the bulk coal, and solid grains and pores, using the constant volume theory. It incorporates primarily the changes in grain and cleat volumes and is, therefore, different from the other models that lay heavy emphasis on the pore volume/cleat compressibility values. Finally, in order to demonstrate the simplicity of the proposed model, a history matching exercise is carried out using field data in order to compare the different models. The modeling results suggest that the agreement between the predicted permeability using the existing models and the one proposed here is very good. The merit of the proposed model is its simplicity, and the fact that all input parameters are easily measurable for any coal type with no uncertainties. [ABSTRACT FROM AUTHOR]

Abstract The permeability of coalbed methane (CBM) reservoirs is typically very low and it is challenging and essential to effectively increase coal permeability to maximize CBM recovery. An exploratory study on enhancing coal porosity/permeability using freeze−thaw cycling with liquid nitrogen (LN 2) was conducted. The changes of fracture and porosity in coal with the freeze−thaw treatment using LN 2 were evaluated using nuclear magnetic resonance (NMR). After freeze−thaw treatment, the coal pore size tended to increase and new pores/fissures were generated. The growth rate of the pore size was positively correlated with the LN 2 freezing duration. The effective porosity had a positive correlation with the freezing duration, but the correlation for residual porosity was negative. This means that the volume of irreducible fluid in the coal decreased while the amount of free fluid increased. Scanning electron micrograph studies indicated that the maximum fracture width in the coal samples grew from 5.56 μm at a T freezing = 1 min to 100 μm for T freezing = 60 min, matching the NMR findings. This study provides a scientific basis and guidance for engineering application of freeze−thaw using liquid nitrogen to enhance coalbed methane extraction. Highlights • NMR was used to conduct the evolution of pore structure in coal samples. • Coal pore size and effective porosity increase with increasing LN 2 freezing time. • A method of LN 2 frozen-thawed fracturing for CBM extraction was proposed. [ABSTRACT FROM AUTHOR]