Coalbed Methane Extraction Research Articles

Evaluation and intelligent deployment of coal and coalbed methane coupling coordinated exploitation based on Bayesian network and cuckoo search

Coal and coalbed methane (CBM) coordinated exploitation is a key technology for the safe exploitation of both resources. However, existing studies lack the quantification and evaluation of the degree of coordination between coal mining and coalbed methane extraction. In this study, the concept of coal and coalbed methane coupling coordinated exploitation was proposed, and the corresponding evaluation model was established using the Bayesian principle. On this basis, the objective function of coal and coalbed methane coordinated exploitation deployment was established, and the optimal deployment was determined through a cuckoo search. The results show that clarifying the coupling coordinated level of coal and coalbed methane resource exploitation in coal mines is conducive to adjusting the deployment plan in advance. The case study results show that the evaluation and intelligent deployment method proposed in this paper can effectively evaluate the coupling coordinated level of coal and coalbed methane resource exploitation and intelligently optimize the deployment of coal mine operations. The optimization results demonstrate that the safe and efficient exploitation of coal and CBM resources is promoted, and coal mining and coalbed methane extraction processes show greater cooperation. The observations and findings of this study provide a critical reference for coal mine resource exploitation in the future.

Technology opportunity discovery based on patent analysis: a hybrid approach of subject-action-object and generative topographic mapping

ABSTRACT An incomplete understanding of the technical details in a firm’s technology selection can lead to a failure in the process of the technology opportunity discovery (TOD) and cause a series of R&D problems. This study proposes an approach for the automated TOD by combining the subject-section-object (SAO) and the generative topographic mapping (GTM), which concentrates on the role of the semantic information in TOD process. First, the semantic information of the technology components in a target field is extracted and the topics of different semantic structures are defined. Second, the GTM-based patent map is established to discover technology opportunities based on a vector matrix composed of patents and topics. Finally, the degree of semantic similarity is applied to measure the technology novelty and to identify promising technology opportunities. The case of the coal-bed methane extraction technology demonstrates that the automated approach based on the semantic information can help understand the concrete details of technology opportunities and improve the accuracy of TOD.

Permeability Response Characteristics of Primary Undeformed Coal and Tectonically Deformed Coal under Loading-Unloading Conditions in Huainan Coalfield, China.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Experimental Study of the Pore Structure and Gas Desorption Characteristics of a Low-Rank Coal: Impact of Moisture.

Coal–water interactions have a prominent impact on the prediction of coal mine gas disasters and coalbed methane extraction. The change of characteristics in the microscopic pores of coal caused by the existence of water is an important factor affecting the diffusion and migration of gas in coal. The low-pressure nitrogen adsorption experiments and gas desorption experiments of a low-rank coal with different equilibrium moisture contents were conducted. The results show that both the specific surface area and pore volume decrease significantly as the moisture content increases, and the micropores (pore diameter <10 nm) are most affected by the water adsorbed by coal. In particular, for a water-equilibrated coal sample at 98% relative humidity, micropores with pore sizes smaller than 4 nm as determined by the density functional theory model almost disappear, probably due to the blocking effects of water clusters and capillary water. In this case, micropores with a diameter less than 10 nm still contribute most of the specific surface area for gas adsorption in coal. Furthermore, the fractal dimensions at relative pressures of 0–0.5 (D1) and 0.5–1 (D2) calculated by the Frenkel–Halsey–Hill model indicate that when the moisture content is less than 4.74%, D1 decreases rapidly, whereas D2 shows a slight reduction as the moisture content increased. In contrast, when the moisture content exceeds 4.74%, further increases in the moisture content cause D2 to decrease significantly, while there is nearly no change for D1. The correlation analyses show that the ultimate desorption volume and initial desorption rate are closely related to the fractal dimension D1, while the desorption constant (Kt) mainly depends on the fractal dimension D2. Therefore, the gas desorption performances of coal have a close association with the pore properties of coal under water-containing conditions, which indicate that the fluctuation in moisture content should be carefully considered in the evaluation of gas diffusion and migration performances of in situ coal seams.

An authority-enterprise equilibrium differentiated subsidy mechanism for promoting coalbed methane extraction in multiple coal seams

Because of strict technical requirements and high operational costs, exploiting deep coal seam resources is generally uneconomic under traditional uniform subsidy policies, which has resulted in an inadequate extraction of coalbed methane (CBM) and many safety risks or potential emission problems. To deal with these problems, an authority-enterprise equilibrium differentiated subsidy mechanism is proposed to promote CBM extraction in multiple coal seams. Specifically, the differentiated subsidy mechanism ensures that deep resources get more unit subsidies than shallow resources. A bi-level multi-objective optimization model is developed to resolve the interactive relationships between the authority and CBM enterprises, which also seeks a comprehensive balance between increased energy extraction and fiscal expenditure. A case application indicates that implementing differentiated subsidies could effectively promote multiple coal seam CBM extraction. The further discussion showed that a low energy output target and a moderate subsidy can accelerate deep coal seam extraction. Our proposed differentiated subsidy mechanism can be applied to the CBM industry in regions that own abundant deep coal seam resources. Our confirmed appropriate subsidy policy can be used as a reference for the government to promote energy extraction in multiple coal seams.

Study on key influencing factors of competitive adsorption of coalbed methane by carbon dioxide displacement.

The extraction of coal bed methane (CBM) by injecting CO2 into deeply buried unmined coal seams in competition with CH4 adsorption to provide a clean fuel is known as enhanced coal bed methane recovery (ECBM) and has proven to be an effective technological strategy to address global warming. The study of the interaction of coal with CO2 and CH4 under multi-physical field conditions is particularly necessary. In this work, a series of experiments were conducted on a home-made test system to investigate the competing sorption patterns of high and medium ash coal samples subjected to variables such as gas pressure, temperature, nodulation and lateral limit constraints. The results show that there is a sorption isotherm relationship between coal samples and exposure time. The adsorption capacity sorption of CH4/CO2 varied considerably for different ash coal samples. As the CO2 pressure increased from 2.3 to 5.5 MPa, the strain on the coal samples increased from 0.082 to 0.4%. The deformation in the vertical laminae direction is always greater than that in the parallel laminae direction. A correlation coefficient K exists between 1 and 2, and there is an internal expansion pattern in the adsorption deformation of coal. This paper can contribute to the improvement of ECBM efficiency.

Insight on microscopic mechanisms of CH4 and CO2 adsorption of coal with different ranks

By conducting research on the microscopic mechanisms of CH4 and CO2 adsorption in different rank coals, the CO2-enhanced coal bed methane recovery (CO2-ECBM) technique can be further developed. In this paper, low-pressure N2 adsorption (LPGA-N2), low-pressure CO2 adsorption (LPGA-CO2), isothermal adsorption, Fourier transform infrared (FTIR) experiments, and molecular simulations were applied to analyze the mechanisms. The results demonstrated that the pore volume and specific surface area (SSA) of coal showed a U-shaped trend with the increase of metamorphic. The volume and SSA of the micropore were positively correlated with the ultimate adsorption volume, indicating that micropores were the main sites for adsorption. With the increased metamorphic degree, the aliphatic chains and oxygenated functional groups shed, and the aromatic rings polymerized. These changes in chemical functional groups directly affected the adsorption capacity of coal and indirectly affected the adsorption capacity by affecting micropore evolution. Compared with the CH4 molecule, the CO2 molecule had a smaller structure and a stronger intermolecular force, shown as the larger adsorption energy and closer distance to the coal molecule in the molecular simulation. These findings can provide the data and theoretical basis for the CO2-ECBM technique, coal bed methane extraction, and effect evaluation of extraction.

Evolutionary Mechanism of Coal Breaking by High-Pressure Water Jets Based on Numerical Simulation

Currently, coal bed methane (CBM) resources are in urgent need of development to alleviate the problem of resource constraints. This paper proposes a new method for detecting CBM gas by using the electroluminescence optical properties of the material to establish an electro-luminescence analysis method and the effect of CBM gas on the electro-luminescence intensity of nanomaterials to explore the electro-luminescence properties of nanomaterials and their influencing factors. In this paper, we investigate the high-pressure water jet drill bit for coal rock crushing by theoretical analysis, numerical simulation and rock breaking and punching tests, from the mechanism of coal rock crushing by jet, drill bit structure design, drill bit internal structure optimization and drilling and punching tests. By introducing acoustic emission technology, online monitoring of the coal seam crushing process was achieved. Physical tests and numerical simulations focusing on high-pressure water jet impact studied the crushing mechanism and damage characteristics of concrete under high-pressure water jet impact, improved the accuracy and safety of high-pressure water jet crushing technology, and investigated the crushing process of concrete and the damage distribution near the erosion hole. The extraction of CBM through very small radius radial drilling technology is one of the main trends in current exploration and development. The core component of radial drilling technology is the water jet bit, which has a decisive influence on the drilling efficiency and extraction results of radial drilling technology. Among many types of jet bits, the high-pressure water jet bit has the advantages of stability and reliability, strong drilling capacity, etc. It is suitable for the technical requirements of radial drilling, but the research on the mechanism of coal rock fragmentation and its flow field characteristics needs further study.

Numerical simulation of coalbed methane under the mining conditions based on dynamic virtual well and variable permeability field

After coal seam mining,stress change and rock movement in adjacent rock strata, make permeability and other fields change in adjacent coal seam. At the same time, due to the exploitation of coal seams, material exchange may occur in the adjacent coal seams. It also caused desorption of near coal seam near the mining working face. Recent years, with the development of coalbed methane mining technology under pressure relief in mining areas, it is necessary to carry out numerical simulation under coalbed methane mining conditions to realize the optimization of coalbed methane extraction simulation under mining conditions. The coupling effect of dynamic virtual well and variable permeability field advancing with the mining face is used to simulate the output of coalbed methane under mining conditions: the dynamic virtual well is used to advance with the mining face to characterize the influence of the working face on the material exchange of adjacent coal seams in the mining process; the use of variable permeability field to realize the influence of the permeability of the adjacent coal seam during the advancement of the working face. Finally, through the dynamic coupling of variable permeability field and dynamic virtual well with the mining face, the rapid pressure relief effect of high speed and short time on the adjacent layer under the influence of mining is characterized. The simulation results of the computer simulation software written by the algorithm show that when the mining effect is not considered, that is, when the algorithm does not consider the dynamic virtual well production and the variable permeability field, it is basically consistent with the commercial software simulation results, but for the consideration of mining condition is a situation that commercial software cannot simulate, and the simulation results of this algorithm are more consistent with the actual monitoring data of the mine. This method achieves numerical simulation of CBM under mining conditions by improving the mature algorithm, which can provide effective guidance for future CBM capacity prediction under mining conditions.

Effective stress effect and slippage effect of gas migration in deep coal reservoirs

The deep coal reservoir is in a high in-situ stress regime, and the drop in pore pressure during the coalbed methane extraction can induce significant slippage effect and effective stress change. Both of them play a competitive role in permeability evolution. In our research, coal seepage experiments under variable bulk stress and pore pressure paths are performed to investigate the slippage effect and effective stress effect during gas migration in deep coal seams. The results show that the gas flow pattern changes from effective stress–control to slippage effect–control during the increase of bulk stress. Because of the dual pore structure and rich clay, coal presents microscopically inhomogeneous assemblage in mechanical and seepage properties. It makes the effective stress coefficient (ESC) for permeability, κ, always greater than 1, up to 1.94–2.64. By substituting the elastic modulus of the bedding for that of the clay in the coal, the clay-particle model can be used to explain the κ > 1. The linear-Klinkenberg equation is no longer applicable to the correction of gas slippage effect in deep coal reservoir, but conforms to the double-slip-Klinkenberg equation. Slippage effect weakens the stress sensitivity of permeability. As the effective stress increases, the slippage coefficient b increases accordingly, and the slippage effect becomes more pronounced. The gas flow pattern in deep coal reservoir is basically in the region of slippage flow, and the slippage effect cannot be ignored in gas migration. Furthermore, the anisotropy of κ is weakened in deep coal reservoirs, and the intrinsic permeability of coal, k∞, has the strongest stress sensitivity to σ3.

Geomechanics impact due to in-situ and induced stresses during drilling of horizontal and highly deviated coal bed methane wells

Hydrocarbon exploration is the arduous task. Urgency of hydrocarbon and avengement of technology push operators to recover the unexplored unconventional hydrocarbon like coal bed methane (CBM) which has been discontinued. Limitations such as misunderstanding of geomechanical stress, formation behaviours- abrasiveness, mud properties etc; impaired the drilling performance and leads to well failure. Therefore, wellbore stability due to geomechanical stresses is considered as one of the major stages in well planning and required extensive study. The dominant elements of fascinating the CBM extraction are cost and the currently available technology and poor understanding of reservoir as compare to conventional reservoir. CBM wells fails due to exceeding the limit of tensile and shear strength which includes wellbore collapse, pipe sticking, caving, loss circulation and leads to ruin the operator’s money and time. The stress regimes in induced, in-situ stresses are natural and cannot be change; however study of these stresses and implementation of findings are essential to implement while planning the well and monitoring stresses behaviour during drilling of highly deviate or horizontal wells are key element to successfully develop the coal bed methane reservoir. Effect of geomechanical stresses are experienced while drilling horizontal and highly deviated wells. By considering the effects of in-situ and induced stresses; suitable operational window can be design to reduce the CBM wellbore failure. The present work analyse the geomechanical in-situ and induced stresses which are contributing towards instability of wellbore along with matrix shrinkage effect, outcome of this study can be utilized for the efficient planning of failure-free wellbore operating envelopes for CBM wells.

Experimental study on improving coalbed methane extraction by chemical treatment using acetic acid or ammonium persulfate

AbstractAd‐/desorption and diffusion processes are the key factors controlling methane production from coal. The experiments used samples from methane‐producing coal seams with different contents of carbonate mineral and different ranks. Acid treatment using 1.0 mol/L acetic acid (CH3COOH) and oxidant treatment using 1.0 mol/L ammonium persulfate ((NH4)2S2O8) were conducted on these coal samples to observe the changes in surface functional groups that act as primary sorption sites and pore structure that determines diffusivity by Fourier transform infrared spectrometer, in situ scanning electron microscopy (SEM) imaging and energy dispersive spectroscopy. The results showed that the acidizing‐related mass loss of coal samples is in the range of 3.78%–7.57% due to the complete dissolution of pore‐ and fracture‐filling carbonate minerals. For the coal samples after oxidizing treatment, dissolution evidence can be observed in both carbonate minerals and organic matter, with a mass loss ranging from 2.01% to 3.0%. Acidizing or oxidizing induced dissolution can destroy aliphatic functional groups and the cross‐linked structure of coal, resulting in a decrease in methane adsorption capacity that is beneficial to methane desorption. More opened pores and interconnected fractures can be observed in SEM images due to the dissolution of carbonate minerals, suggesting an alteration of methane diffusion pathways in the coal matrix. Although only some of the carbonate minerals were dissolved following oxidant treatment, coal organic matter is partially oxidized and generates some dissolution pores, which provides more methane diffusion pathways in the coal matrix. Therefore, all the treated coal samples showed a reduction in methane adsorption capacity and an enhancement of desorption capacity and diffusivity. This result suggests that chemical treatment in coal seams using acetic acid or oxidant has the potential to improve coalbed methane extraction.

Dielectric anisotropy effects on the microwave-induced thermodynamic response of coal: Numerical simulations and experiments

Microwave heating technology is a promising solution in underground engineering, such as mining, coalbed methane extraction, and oil/gas production, that primarily depends on the conversion of microwave energy due to dielectric properties. However, inadequate understanding of the effects of dielectric properties on microwave irradiation limits further development of microwave techniques. In this study, based on the dielectric anisotropy of coal, an electromagnetic-thermomechanical coupled model was used to numerically investigate the thermodynamic response under microwave irradiation at 2.45 GHz and 1 kW for 180 s. We performed experimental verification via indoor test. Results show that the sensitivity of microwaves to the dielectric direction in the TE10 mode is x > z > y. In different dielectric anisotropy models, the temperature and maximum principal stress increase synchronously and linearly with increasing heating time, and the curve growth rate is positively correlated with the equivalent dielectric permittivity. There are also marked temperature and stress concentrations in the sample. The compressive stress is concentrated in the center of the sample, while the tensile stress is around the outer edge. For bedding coal, tensile fractures occur under microwave irradiation along the bedding, and shear fractures run through the bedding. With the increase of the input microwave energy, the failure mode of coal gradually expands from single fracture to fracture network.

Coalbed Methane Industry Development Framework and Its Limiting Factors in China

The current average daily gas production of single coalbed methane (CBM) wells in China is less than 1,300 m3. The low production rate of CBM wells dramatically limits the sustainable development of the CBM industry in China. To promote CBM industry development, three universal geological factors, i.e., gas sources, channels, and driving force of the low production wells are analyzed, considering the various and complicated geological conditions of CBM reservoirs in China. Additionally, we find that the growth in proven CBM reserves should be greatly promoted as the CBM proven rate is only less than 3% by the end of 2020. The widespread CBM extraction mode based on depressurization via dewatering generates very limited effects in coal seams with notable deformation, high in situ stress, or little water. Therefore, a specific design of CBM development modes is proposed, according to the applicable geological conditions of different CBM development modes. It covers depressurization via dewatering, depressurization via stress release, and coal and CBM comining. Moreover, the regional governments play a key role in the development of the regional CBM industry, and favorable policies should be formulated to promote the multipoint development conditions across China. This study is concluded to provide a recommended development framework involving sustainable CBM production in China and its development processes.

Cryogenic fracturing of synthetic coal specimens under true-triaxial loadings-An experimental study

Cryogenic fracturing with liquid nitrogen (LN2) has been recognized as a potential and promising waterless fracturing technique for coalbed methane extraction, owing to the environmental harmlessness and low-temperature feature. Though it showed spirited performances in some field pilots in the late 20th century, it remains poorly understood, especially when LN2 is used directly as the fracturing fluid. In this work, we evaluated the performance of LN2 fracturing on cubic synthetic coal specimens under true-triaxial loadings. The study also included two subtasks that examined the necessity of a pre-cooling step and the effect of water saturation. Hence, the specimens are classified into dry and wet specimens, and the LN2 treatments are divided into two modes based on whether being pre-cooled. The results showed that the pre-cooling improved the fracturing performance of LN2 such that the breakdown pressures of the pre-cooled specimen are about 40% lower than the non-precooled ones. Based on the semilog pressure–time analysis, the pre-cooled specimens showed higher permeability enhancement (2.9- to 7.9-fold) than the non-precooled ones (1.4- to 1.7-fold). The dry specimens after LN2 fracturing gave a fracture along the direction perpendicular to the minimum horizontal stress, whereas fractures of the wet ones deviated from the expected direction. It indicates that the expansion pressure caused by the frost heaving may change the fracture orientation. In addition, small thermal cracks were observed around the borehole in the open-hole section from the cross-section of the wet specimen, which agrees with the widely accepted thermal damage theory.

Mathematical Model and Numerical Simulation of Coalbed Methane Migration Considering the Adsorption Expansion Effect

The influence of gas adsorption and desorption on the volumetric strain of coal was measured by a self-designed, fluid–solid coupling triaxial coal adsorption deformation experimental system. The experimental results show that coal deformation has a threshold value with an increase in gas content. Before the gas content reaches the threshold value, coal deformation is not obvious. When the gas content reaches the threshold value, the deformation will increase sharply. At the same time, coal volume strain changes with coal gas content in accordance with exponential law ε = ε0(eϒc-1). Based on the experimental results, considering the coupling effects of heat transfer, water seepage, coal and rock mass deformation, and coalbed methane desorption and seepage, a mathematical model of heat transfer–deformation–seepage coupling coalbed methane migration was established, and a numerical simulation study was carried out on the heat injection-enhanced coalbed methane mining project. The results show that (1) With continuous heat injection, the gas in the coal seam is rapidly desorbed, and the adsorbed gas content forms an elliptical funnel that extends from the extraction hole to the deep part of the coal body and takes the fracturing crack as the center to the upper and lower boundaries of the coal seam. At day 30, the adsorbed gas content in the whole drainage area has decreased to 0.1 m3/t within 2m from the fracture zone. (2) On the basis of considering the gas adsorption expansion effect, the strain of the coal body decreases from roof to floor. With the increase in the heat injection time, the strain value will also increase, and the strain increase will decrease. When the heat injection is 30 days, the maximum strain at the roof is 0.015. The research results have important guiding significance for predicting coal rock deformation and determining gas extraction efficiency in the process of heat injection-enhanced coalbed methane extraction.

A FRACTAL PERSPECTIVE ON STRUCTURAL DAMAGE AND FRACTURE CHARACTERISTICS OF COAL SUBJECTED TO LIQUID NITROGEN COOLING AT LABORATORY-SCALE

To explore the influence of liquid nitrogen (LN[Formula: see text] cooling on the structural damage and failure characteristics of coal, a series of structural experiments, uniaxial compression tests, and crack morphology characterization experiments were carried out in this study. Based on the fractal theory, the pore structure damage and failure characteristics of the coal before and after the LN2 cooling treatment were compared. After the LN2 treatment, the structural damage inside the coal increases. Compared to the untreated coal, the fractal dimensions of the total pores, seepage pores, and adsorption pores in the LN2-treated coal decrease by 0.88%, 0.24%, and 5.16%, respectively. The results indicate that the LN2 cooling treatment can increase the connectivity of pores, especially adsorption pores. The internal structure damage of the coal induced by the LN2 treatment reduces its compressive strength and elastic modulus by 27.86% and 32.62%, respectively. Simultaneously, it can be found that the fragments fractal dimension of the coal decreases by 9.53%, and the fractal dimension of the induced fracture surface increases from 2.134 to 2.151 after the LN2 treatment. By the LN2 treatment, the distribution of fragments becomes more uniform, and the complexity of the generated fracture surface increases. Therefore, the LN2 cooling treatment can increase the effective seepage path and the contact area between the crack and the coal matrix to promote the gas flow and enhance the gas desorption in the coal matrix. The experimental results also show that the damage of the single LN2 treatment on the coal is limited. LN2 cycling treatment is suggested to use to improve the efficiency of the coalbed methane extraction because that it can give full play to the multiple cracking effects of LN2 on the coal.

An Improved Coupled Modeling Method for Coalbed Methane Extraction after Hydraulic Fracturing

This paper proposes an improved coupled modeling method for coalbed methane extraction after hydraulic fracturing. Based on the damage mechanics theory, we introduce a state damage variable to record the damage changes at different times and construct the coupling governing equations of the seepage field and solid mechanics field in the hydraulic fracturing process. Then, the results of hydraulic fracturing simulation are used as initial values to perform coupling modeling for coalbed methane extraction. The method is implemented by the combination of different modules in COMSOL to solve the state damage variable, displacement, and pressure. Additionally, two 2D numerical examples are used to verify the precision of this method, and a 3D coalbed methane extraction simulation is performed to analyze the extraction effects and the evolution characteristics of the permeability before and after hydraulic fracturing. Numerical examples show that the proposed coupling modeling method can not only simplify the calculation steps of the CwM method and reduce the numerical errors caused, but also characterize the damage, porosity, permeability, and extraction effect caused by hydraulic fracturing in coal seams with permeability anisotropy.

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

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

The Young’s Modulus and Poisson’s Ratio of Hard Coals in Laboratory Tests

The Young’s modulus and Poisson’s ratio, parameters reflecting the elastic response of a rock to stress, are the key parameters used in many engineering activities, such as hard coal mining and natural gas extraction. The objective of this paper was to present the results of complex laboratory measurements of the static and dynamic Young’s modulus and Poisson’s ratio for Upper Carboniferous hard coals from the Upper Silesian Coal Basin. The coals differed in geologic age (Mudstone Series—younger coals; Upper Silesian Sandstone Series—older coals) and petrographic structure (vitrain, clarain, and durain lithotype). Elastic parameters of the coals were determined following compression tests under a complex state of stress, as well as using the ultrasonic method in reservoir conditions. On this basis, linear functional dependences between parameters such as UCS, differential stress, confining pressure, strain rate, P- and S-wave velocities, and the static and dynamic Young’s modulus and Poisson’s ratio were determined. These dependences turned out to be linear, with strong and very strong correlation, as indicated by the high coefficients of determination, R2. These new results significantly broaden the knowledge of elastic properties of Carboniferous hard coals, especially in the field of geoengineering, underground coal gasification, and reservoir stimulation for coal bed methane extraction.