Coalbed Methane Extraction Research Articles

Gas desorption diffusion behavior in coal particles under constant volume conditions: Experimental research and model development

ABSTRACT Understanding gas diffusion characteristics of coal particles is a significant theoretical basis for the exploitation of coalbed methane. Generally, diffusion experiments are performed under constant pressure conditions, and scholars seldom discuss gas desorption experiments with constant volume, but the recognition of this characteristic is also equally important. Based on this, the paper conducted coal particles desorption experiments under conditions of constant volume to explore gas diffusion characteristics under different adsorption equilibrium pressures and initial emission environmental pressures. The experimental data showed that the variation law of desorption amount with time under constant volume conditions conformed to the Langmuir-like equation. Meanwhile, the rising of initial emission environmental pressure reinforced the linear relationship between the limitation desorption amount and the adsorption equilibrium pressure. When the initial emission environmental pressure was 0.10 MPa, the correlation coefficient of linear fitting between them was as high as 0.9826. To better describe the diffusion process of this condition, a dynamic diffusion model of coal particles gas diffusion under constant volume conditions was established. The calculated values of this model had a preferable consistency with the experimental data, and the correlation coefficients were all over 0.99, which verified the reliability of the model. This research makes up for the theory of gas diffusion and provides a theoretical basis for coalbed methane extraction.

Improvement of Intensive In-Seam Gas Drainage Technology at Kirova Mine in Kuznetsk Coal Basin

One of the ways to resolve the “green energy-economic development” dilemma, in which the coal industry is situated, is by the improvement of technologies and the integrated use of extracted resources, including methane gas as a clean energy source. Using the example of the Kirova mine, located in Kuznetsk coal basin—one of the ecologically unfavorable coal mining regions of Russia—this article discusses an integrated technology for the extraction of coalbed methane (ECBM), which makes it possible to reduce greenhouse gas (methane) emissions and improve the safety and intensity of coal mining. The Kirova mine, with its 3 Mt production in 2019, is one of the coal mining leaders in Russia. The available mining equipment has the potential to significantly increase the output; however, gas is a limiting factor to this. The customary approaches to coal seam degassing have already been petered out. The miners and mine science are facing a challenge to validate and test an alternative technology to ensure effective in-seam gas drainage prior to vigorous mining. This article gives an account of the improvement track record of the in-seam gas drainage technology used to pre-treat coal seams for intensive and safe extraction. This technology suggests, at the first stage, hydraulic loosening of the target coal seam through wells drilled from the surface (SSHL), then hydraulic fracturing (HF) of the coal seam through the boreholes drilled from underground development headings, followed by methane extraction from the high-permeability coal-gas reservoir created through standard in-seam gas drainage underground wells. Results are presented in this paper of field testing of the improved SSHL technique. Findings are presented on the effective parameters of the HF technology. Methodological recommendations are offered for selecting viable in-seam gas drainage technology.

Effect of Proppant Distribution in Hydraulic Fractures on Coalbed Methane Extraction

Effect of Proppant Distribution in Hydraulic Fractures on Coalbed Methane Extraction

Exploring Win-Wins from Trade-Offs? Co-Benefits of Coalbed Methane Utilization for the Environment, Economy and Safety

With policy incentives for the coalbed methane in energy industry, coalbed methane from coal production has been effectively improved by technology innovations in coalbed methane extraction and utilization. The progress of coalbed methane promotes the clean construction of energy system and contributes to carbon neutrality target. To quantitatively measure the contributions of the coalbed methane in energy industry, this paper builds a carbon emissions accounting system for coalbed methane in China and assesses the historical co-benefits of coalbed methane utilization from the aspects of emissions reduction, safety and economy. By using the parameters of gas content, raw coal production, gas extraction rate and utilization rate over the years, emissions reduction potential and economic viability of coal seam gas are estimated and the safety benefits of coal mine gas extraction are analyzed by using data for gas accidents and economic losses. The results reveal that with the increase in raw coal production, the great emission reduction potential of coalbed methane is expected to benefit clean energy system and the development of carbon neutrality by means of policy incentives and technology innovations. The co-benefit evaluation indicates the huge profitability of coalbed methane from 2012 to 2015 and the significance of emissions reduction and safety gain internalization. Safety benefits are obvious in the negative exponential function between the annual drainage quantities of coalbed methane and annual death tolls from coal mine gas accidents. Based on these results, relevant suggestions are put forward for sustainable development of the coalbed methane in energy industry.

Experimental Study on the Effects of Different Heating Rates on Coalbed Methane Desorption and an Analysis of Desorption Kinetics.

Heat injection is an effective way to enhance coalbed methane (CBM) extraction. However, at present, the best way to inject that heat is not clear. To determine how the heating rate affects methane desorption, desorption tests at constant high (95 °C) and low (20 °C) temperatures and at three different heating rates (0.3, 0.6, and 0.9 °C/min to 95 °C) were conducted. The desorption content (the volume of gas desorbed per mass of coal) and the desorption rate under the constant 95 °C temperature were greater than those under the constant 20 °C temperature. For the heating rate tests, the total desorption contents under heating rates of 0.3, 0.6, and 0.9 °C/min were 1.322 times, 1.115 times, and 1.095 times that from the constant 95 °C temperature tests, respectively. The final desorption contents from the entire desorption process under heating rates of 0.3, 0.6, and 0.9 °C/min were 1.42 times, 1.30 times, and 1.20 times that from the constant 95 °C temperature tests, respectively. In the early parts of the heating stages, the desorption rates under the three heating rate tests were lower than those under the constant 95 °C temperature tests. When the heating stages ended, the desorption rates under the three heating rates were greater than those under the constant 95 °C temperature tests. After the heating ended, the desorption rates decreased rapidly. A higher heating rate was correlated with a faster decrease in the desorption rate. Kinetic analysis showed that heating coal to a high temperature before methane is desorbed did not suppress the diffusion coefficient decrease. Heating during desorption prevented the diffusion coefficient decrease. A lower heating rate is correlated with a slower diffusion coefficient decrease. Low heating rates were more effective for desorbing methane. The heat injection in the later stage of desorption had a more significant effect on promoting methane desorption than did the early desorption stage heat injection. An equation for calculating the optimal heat injection rate was proposed. These findings will offer significant references for the selection of a suitable way to inject heat to enhance CBM extraction.

Crack Propagation Characteristics of Coal Samples Utilizing High-Voltage Electrical Pulses.

The technique of high-voltage electrical pulses (HVEP) is a new method to enhance the permeability of coal seams and improve the efficiency of coalbed methane (CBM) exploitation. This paper is aimed at investigating the crack propagation characteristics of samples of different strengths, proposing the improved procedure of HVEP in field application, and proving that the electrohydraulic effect has a wide use in field application of CBM extraction. In this paper, an experimental system utilizing HVEP in water condition is established, coal samples with different strengths are crushed, and the extended processes of cracks are analyzed. According to the research results, the electrohydraulic effect has a good breakage on the coal; the number of main cracks is 2–3 and the length of the main cracks is about 30 cm in the vertical direction of the hard samples; and the formation of cracks is relevant to the discharge voltage, discharge times, and mechanical parameters of the samples. The results of scanning electron microscopy (SEM) demonstrate that the cracks and pore connectivity of the coal samples are improved obviously, and the permeability results show that the permeability of crushed coal samples is 20% greater than that of the raw coal sample. Meanwhile, the generation process of cracks can be divided into four periods: namely, fatigue damage accumulation, slow development, rapid development, and failure; the rapid development stage is the optimal phase in field application. Moreover, the shock wave produced by HVEP via electrohydraulic effect can crush the samples mainly; furthermore, the energy produced by bubble rupture also has a great influence on the formation of cracks. This study can provide a foundation for the HVEP to improve CBM exploitation.

Response of Molecular Structures and Methane Adsorption Behaviors in Coals Subjected to Cyclical Microwave Exposure.

To better understand the methane adsorption behavior after microwave exposure, the importance of quantitatively characterizing the effect of cyclical microwave exposure on the molecular structures of coals cannot be overemphasized, with implications for enhancing coalbed methane (CBM) extraction. Thus, cyclical microwave exposure experiments of three different metamorphic coals were conducted, and the methane adsorption capacity before and after each microwave exposure (10 in total) for 120 s was evaluated. Fourier transform infrared spectroscopy analysis and peak fitting technology were applied to quantitatively characterize the changes in the structural parameters of coal molecules. The results showed that after modification, the structural parameters like aromatic carbon fraction (fa–F), aromaticity (I1 and I2), degree of condensation (DOC1 and DOC2), and the maturity of organic matter (“C”) gradually increased with increasing exposure times, while the length of the aliphatic chain or its branching degree (CH2/CH3) and the hydrocarbon generating capacity (“A”) showed a decreasing trend. The Langmuir volume (VL) of three different rank coal samples decreased from 29.2, 32.8, and 40.4 mL/g to 25.7, 29.3, and 35.7 mL/g, respectively; the Langmuir pressure (PL) increased from 0.588, 0.844, and 0.942 MPa to 0.626, 1.007, and 1.139 MPa, respectively. The modification mechanism was investigated by analyzing the relationship between the methane adsorption behaviors and molecular structures in coals. The release of alkane side chains and the oxidation of oxygen-containing functional groups caused by microwave exposure decreased the number of methane adsorption sites. As a result, the methane adsorption capability decreased. In addition, the decomposition of minerals affects methane adsorption behaviors in coals. This work provides a basis for microwave modification of coal as well as in situ enhancement of CBM extraction using microwave exposure.

Influence of active water on gas sorption and pore structure of coal

Active water fracturing is one of the commonly used technologies in area for enhanced coalbed methane (CBM) extraction. However, after the fracturing process, active water residues may block the pore of coal and ultimately reduce the production of CBM. To address this limitation, pore structures of Dongtan (DT) and Houwenjialiang (HW) coal samples were subjected to active water fracturing effect using active water with different concentrations of PAM prepared using polyacrylamide (PAM). Methane adsorption experiment was conducted to probe the effect of active water on the adsorption of methane in coal samples. Additionally, low-temperature nitrogen-adsorption experiments (LT-NAT) and nuclear magnetic resonance (NMR) were probed to characterize the discrepancies in the pore structure and chemical composition surface functional groups of the two coal-sample types. The results showed that considerable discrepancies existed in the pore structures of the two coal-sample types; however, their surface functional groups were similar. The active water treatment had contrasting effects on the adsorption behavior of methane features of the two coal-sample types, that is, as concentration of PAM in the active water increased, the methane adsorption of the treated HW coal sample increased whereas that of the treated DT coal sample gradually decreased. The PAM molecules in the active water residue could adsorb methane, and blockage of pores by the residue will reduce the amount of methane adsorption. The combined effect of the two dominated the influence of active water fracturing on methane sorption. Further optimizing the active water formulation to improve the flowback effect can be a very useful channel of reducing damage to the coal seam.

A non-Darcy gas flow model for coalbed methane in mine gobs

The expected extraction efficiency of coalbed methane (CBM) depends significantly on the laws considered to govern its gas flow. This study applies a non-Darcy gas flow model to describe the CBM migration in mine gobs; by mine gob, we mean that it is a fractured zone along with massive cracks and the primary place where gas flows after mining activities. A permeation experiment involving crushed sandstone is first conducted to prove the CBM undergoes Forchheimer-type non-Darcy flow. Subsequently, the three-dimensional continuous distribution functions of the permeability parameters are determined. The non-Darcy flow model includes the influence of inertial force on the gas flow, which is neglected in the Darcy model. A coupling model is established based on the experimental results and the gas flow characteristics in different regions. Thereafter, the model and distribution functions are applied to a series of numerical simulations of CBM extraction at the Sihe coal mine in China, to ascertain the most appropriate location for a ground borehole. These simulations involve boreholes placed in three different zones: the natural accumulation zone (NAZ), the load affected zone (LAZ), and the compaction stable zone (CSZ). The simulation results show that the total extraction quantity expected from the borehole in the NAZ is 2.4 and 13.5 times that from the boreholes in the LAZ and CSZ, respectively. This confirms that the NAZ is the most suitable zone for a borehole. This research ultimately provides a realistic gas flow model for CBM extraction from mine gobs.

Mechanical damage mechanism of frozen coal subjected to liquid nitrogen freezing

Anhydrous liquid nitrogen cracking low-permeability coal seams can improve the efficiency of coalbed methane extraction. Under normal pressure, liquid nitrogen (-196℃) contacts the coal body, and the temperature stress and frost heave force generated will change the mechanical properties of the coal body, cause internal structural damage, and increase the seepage channel of coalbed methane. In order to study the influence of different liquid nitrogen freezing variables on the mechanical properties of coal, Brazilian splitting, uniaxial compression, and ultrasonic velocity measurement tests of saturated frozen coal were carried out. The results show that a single freezing of liquid nitrogen has a certain enhancement effect on the mechanical properties of coal: the tensile strength increases by 64.0%, the uniaxial compressive strength increases by 54.6%, but as the freezing time increases, the coal strength increases first and then decreases The strength of coal under the action of freeze–thaw cycles shows an exponential decline in varying degrees: tensile strength has dropped by 81.3%, uniaxial compressive strength has dropped by 68.9%, and the degree of decline is positively correlated with the number of freeze–thaw cycles. The change in mechanical strength changes the elastic stage. The freezing enhancement factor I and the freeze–thaw damage factor D are defined by the change of elastic modulus. I presents a quadratic function relationship that first increases and then decreases with freezing time, and D continues to decrease as the number of freeze–thaw cycles increases. The longitudinal wave velocity first increases and then decreases with the freezing times, and decreases with the increase of the number of freeze–thaw cycles. The porosity is negatively correlated with the wave velocity. The wave speed decreases, the porosity increases, and the internal pores of the coal are connected to form a fracture network, which accelerates the damage of the coal and reduces the mechanical properties of the coal. Through analysis, it is concluded that when liquid nitrogen acts on coal, the change in mechanical properties is the result of the combined action of multiple forces.

Investigation on the CBM Extraction Techniques in the Broken-Soft and Low-Permeability Coal Seams in Zhaozhuang Coalmine

To enhance the coalbed methane (CBM) extraction in broken-soft coal seams, a method of drilling a horizontal well along the roof to hydraulically fracture the coal seam is studied (i.e., HWR-HFC method). We first tested the physical and mechanical properties of the broken-soft and low-permeability (BSLP) coal resourced from Zhaozhuang coalmine. Afterward, the in situ hydraulic fracturing test was conducted in the No. 3 coal seam of Zhaozhuang coalmine. The results show that (1) the top part of the coal seam is fractured coal, and the bottom is fragmented-mylonitic coal with a firmness coefficient value of less than 1.0. (2) In the hydraulic fracturing test of the layered rock-coal specimens in laboratory, the through-type vertical fractures are usually formed if the applied vertical stress is the maximum principal stress and is greater than 4 MPa compared with the maximum horizontal stress. However, horizontal fractures always developed when horizontal stress is the maximum or it is less than 4 MPa compared with vertical stress. (3) The in situ HWR-HFC hydraulic fracturing tests show that the detected maximum daily gas production is 11,000 m3, and the average gas production is about 7000 m3 per day. This implies that the CBM extraction using this method is increased by 50%~100% compared with traditional hydraulic fracturing in BSLP coal seams. The research result could give an indication of CBM developing in the broken-soft and low-permeability coal seams.

The effect of leakage characteristics of liquid CO2 phase transition on fracturing coal seam: Applications for enhancing coalbed methane recovery

Liquid carbon dioxide phase-transition blasting (LCO2-PB) is a promising alternative method for enhanced coalbed methane (ECBM) extraction and prevention of coal and gas outbursts. During the process of the LCO2-PB fracturing the coal seam, the improper application of fracturing model will reduce the efficiency of gas drainage and cause gas disasters. To clearly understand the ECBM modes of LCO2-PB fracturing coal seam process, this paper employed experiments to investigate the effect of leakage characteristics of the LCO2-PB on fracture models of coal cubes, including release pressure (Pr), filling level (V), release diameter (Dr) and blasting hole diameter (Db). We propose the combination I and II of the LCO2-PB leakage parameters and applied two methods to the in-situ tests. The results revealed that the gas flow of the boreholes of the LCO2-PB with the combination I is 0.368 m3/min, which is 2.07 times that of the combination II (0.178) in the soft coal seam. Besides, the average methane flow of the boreholes with combination II is 0.796 m3/min, which is 1.78 times the combination I (0.448) in the hard coal seam. The adjustment of leakage parameters of the LCO2-PB technology can be helpful for ECBM and reduce the risk of coal and gas outbursts in the soft and hard coal bed.

Gas desorption characteristics and related mechanism analysis under the action of superheated steam and pressurized water based on an experimental study

As a kind of unconventional natural gas, efficient and clean energy, CBM (Coalbed methane) exhibits great potential in adjusting energy structure and formulating energy development strategy. Desorption of methane from coal is essential for CBM production. And the desorption characteristics of methane in coal under different conditions and how to improve the desorption capacity have always been a hot issue of interest to researchers. So, research in this aspect becomes urgent. In this study, a series of desorption experiments of coal samples with various particle sizes under different adsorption pressures were carried out and the influence mechanism of pressurized water and superheated steam on the desorption was analyzed. The results were as follows: 1) The gas desorption capacity is more influenced by the adsorption pressure than the particle size, the particle size mainly affects gas release rate of the desorption process. It can be found that the lower the adsorption pressure, the more obvious the enhancement effect of superheated steam on desorption. 2) The inhibition of desorption by water injection can be attributed to four inhibition mechanisms caused by water, i.e. Water blocking effect, Wetting effect, Capillary effect, and Jamin effect. 3) Superheated steam can provide the energy required for the desorption of methane, intensify the thermal movement of molecules, and can also carry and displace methane, which together increases the desorption capacity and effective diffusion coefficient of gas in coal. These research results can provide a theoretical basis for hydraulic measures and heat injection techniques related to on-site CBM extraction.

Experimental research on the effect of plasma on the pore-fracture structures and adsorption-desorption of coal body

This paper is aimed at investigating the influence of plasma technology on desorption and adsorption features of coal in the fracture scale and proving that plasma technology boasts a wide prospect in the field of coalbed methane extraction. In this paper, CT was combined with low-temperature liquid nitrogen adsorption to analyze the pore and fracture variations of coal before and after plasma breakdown. More importantly, a self-designed desorption experimental system was used to explore variations of gas desorption with time in the fracture scale. According to the research results, the fragmentation effect of plasma is reflected by the generation of cracks in the macro-scale and the improvement of pores in the micro-scale; the fracture is generated from the section near the electrode, and the fracture growth rate after breakdown can reach 18–41 times; the action of plasma can crush coal into little pieces and promote the porosity by over hundreds of times. The appearance of semi-open pores in the liquid nitrogen adsorption test shows that part of closed pores in the coal body are opened by plasma. Moreover, the plasma channel can weaken the adsorption capacity of coal to gas, which is similar to the temperature effect. Meanwhile, the plasma channel increases the desorption amount of coal by 50%–70% and greatly improves the desorption rate of coal due to the influence of interconnected cracks.

Electrochemical Modification on CH4 and H2O Wettability of Qinshui Anthracite Coal: A Combined Experimental and Molecular Dynamics Simulation Study

The wettability of gas and liquid on the coal surface is one of the fundamental factors that affect the depressurization process during the coalbed methane (CBM) extraction. The wettability of coal surface changed after electrochemical modification, leading to the change in methane adsorption/desorption and water movement in coal reservoirs. Thus, the CH4 adsorption amount, desorption ratio, and coal–water contact angle of raw and modified anthracite samples were measured and simulated. The mechanism of electrochemical modification was analyzed by functional groups, surface free energy, pore characteristics, interaction energies, and coal swelling. The experimental results showed that the saturated adsorption amount of methane decreased from 41.49 to 34.72 mL/g, and the simulation results showed that the saturated adsorption amount of methane decreased from 2.01 to 1.83 mmol/g. The coal–water contact angle also decreased from 81.9 to 68.6°. Electrochemical modification mainly affects the wettability of CH4 and H2O by changing the functional groups and pore structures of anthracite, and the influence on functional groups of coal surface is greater. This work provided a basis for enhancing CBM extraction by electrochemical modification.

Fracture and pore development law of coal under organic solvent erosion

Promoting fracture development of deep low-permeability coal seams is one of the necessary means to improve the efficiency of coalbed methane extraction. Organic solvent erosion is an efficient method to promote fracture and pore development in coal. However, the law of fracture and pore development in the process of solvent erosion has not been clarified yet. In this study, the law was discussed from four aspects, namely macro-fracture, micro-fracture, internal fracture and pore. The results show that organic solvent treatment is pretty effective in promoting the fracture and pore development. After solvent treatment, soluble molecules dissolve in the solvent and separate from coal to form the extract. Through the removal of soluble molecules, primary fractures and pores widen, and new fractures and pores develop in large numbers. Then, part of the insoluble coal becomes broken and falls off, forming the abscission. Different solvents exert varying effects which are related to their extraction rates on coal. Under a high extraction rate, secondary fractures are distributed around the main fractures, and fractures are interconnected to form a complex fracture network. Finally, the effects of organic solvents erosion on coal were classified.

Liquid nitrogen fracturing efficiency as a function of coal rank: A multi-scale tomographic study

This study explored the potential of liquid nitrogen fracturing in three different coal ranks. X-ray computed tomography results in micro- (μ-CT) and macro (medical-CT) scales revealed a poor performance of LN2 fracturing in anthracite. In contrast, porosity evolution for bituminous and sub-bituminous coal in microscale was 14 % and 119 %, respectively. The porosity enhancement in macroscale for these coals was also significant. Interestingly, in both coals, thoroughgoing fracture planes originated from the initial cleat network following freezing. Furthermore, connectivity analysis through skeletonization and Euler number analysis suggested that sub-bituminous connectivity increased by 20-fold for small pores (<3 μm), while it doubled in bituminous. SEM and digital microscopy also revealed the inefficiency of LN2 fracturing in anthracite, but promising efficiency in the other two coals. In macroscale, a large thoroughgoing fracture (140 μm opening) appeared following treatment in bituminous coal. Sub-bituminous coal also illustrated numerous new cracks and interconnections. Another visualisation analyses yielded the appearance of fracture bridges in cleat network. Further, the results suggested that the mechanical properties of the original coal revealed a direct relationship with coal rank, increasing from 2.13 GPa for sub-bituminous to 5.46 GPa for anthracite. The reduction in mechanical properties due to damage was remarkable for lower coal ranks, at 18 % and 31 % in the treated bituminous and sub-bituminous, respectively, but only 4 % in anthracite. Finally, experimental permeability evolution analysis showed a more than double increase in sub-bituminous (0.026–0.064 mD) and bituminous (0.011–0.027 mD) permeability, while anthracite passed no flow. Numerical permeability study, through Lattice Boltzmann simulation, showed a similar trend for the coals. Therefore, this study presented the results of LN2 fracturing technique in a range of coal ranks, which can be useful in terms of long-term enhanced gas recovery plans for coalbed methane extraction in different seams.

Study on Temperature Variation and Pore Structure Evolution within Coal under the Effect of Lilquid Nitrogen Mass Transfer.

Liquid nitrogen freezing, which is an effective permeability enhancement technology, has been applied to the extraction of oil, shale gas, and coalbed methane (CBM). This study is aimed at revealing the effect of liquid nitrogen mass transfer on the temperature variation and pore structure evolution within coal. To achieve this aim, first, temperature measurement tests under the action of liquid nitrogen freezing were conducted on saturated and dried coal samples, respectively. Next, the coal samples were subjected to nuclear magnetic resonance and computer tomography tests before and after liquid nitrogen cold soaking to further explore the mechanism of coal temperature variation from a microscopic perspective. The results show that the action of liquid nitrogen mass transfer can accelerate coal temperature variation through coal pore structure and pore water phase change. The thermal stress and frost heave force generated by liquid nitrogen cold soaking exceed the tensile strength of the coal sample, which directly causes crack initiation, expansion, and connection. The mass transfer of liquid nitrogen has a significant promoting effect on pore development. This study provides the technical support necessary for the efficient exploitation of CBM resources and the improvement of CBM extraction rate.

Effect of Electrolyte pH on CH4 Adsorption and Desorption Behavior in Electrochemically Modified Lean Coal.

Practical application of the electrochemical method for enhancing coalbed methane (CBM) extraction is limited due to the insufficiency of in-depth experimental and theoretical research. Therefore, H2SO4, Na2SO4, and NaOH electrolytes with different pH values were selected to electrochemically modify lean coals and their CH4 adsorption and desorption behaviors were tested. The experimental results showed that the amount of CH4 adsorption decreased after modification using H2SO4 and Na2SO4 electrolytes, and increased after electrochemical modification using the NaOH electrolyte. The ratio of CH4 desorption increased from 83.20 to 90.10 and 87.84%, respectively, after electrochemical modification using H2SO4 and Na2SO4 electrolytes, and decreased to 81.71% after electrochemical modification using the NaOH electrolyte. The pore volume and average pore size increased, and the specific surface area decreased with the decrease of electrolyte pH. Lower pH electrolytes perform better and the mechanism was analyzed by the change of surface energy, surface groups, and pore structures. The work will provide a basis for the reasonable selection of electrolyte pH when using in situ electrochemical methods for enhancing CBM extraction.

Numerical Simulation Research on Hydraulic Fracturing Promoting Coalbed Methane Extraction

Although hydraulic fracturing technology has been comprehensively investigated, few scholars have studied the influence of hydraulic fracturing on the effect of coalbed methane (CBM) extraction, and few considered the interaction between water and CBM in the research process, which is not conducive to guiding the engineering design of hydraulic fracturing wells. In this work, a hydraulic-mechanical-thermal coupled model for CBM extraction in hydraulic fracturing well is established; it combines gas-liquid two-phase infiltration, where nonisothermal adsorption is also considered. The COMSOL Multiphysics software is used to carry out the numerical simulation study of the CBM extraction process in hydraulic fracturing well and analyze the influence of coalbed permeability, initial methane pressure, and fracture length on CBM extraction in hydraulic fracturing well, and the results show that the hydraulic-mechanical-thermal coupled model for CBM extraction can be used for CBM extraction research in hydraulic fracturing well. The initial coalbed permeability, initial gas pressure, and fracture length all affect the migration speed of CBM to surface well in different ways and have a greater impact on the CBM production rate of hydraulic fracturing well. The greater the initial coalbed permeability and methane pressure are, the longer the fracture length is and the greater the CMB production rate of hydraulic fracturing well is. The change trend of coalbed permeability during the extraction process of surface fracturing well is directly related to the state of the reservoir. The factors of stress, temperature, and CBM desorption jointly determine the increase or decrease of coal seam permeability.