Coalbed Methane Recovery Research Articles

Hydraulic fracture to enhance coalbed methane recovery by using coated ceramsite

AbstractIn recent decades, coated ceramsite has become popular due to its low density, high compressive strength, and high flow conductivity, and it has been applied in many areas in construction field and chemical industry. In the area of hydraulic fracture, limited relevant literature can be found, especially on unconventional reservoirs with soft coals. In this paper, numerical simulation is performed to obtain migration regularity and granularity proportion using coated ceramsite, and it indicates that the biggest proppant concentration and strongest flow conductivity can be achieved under the granularity proportion of 1:6:2 with respective coated ceramsites of 40/60, 16/40, and 12/20 mesh. Then, a series of construction parameters are calculated and optimized to receive a best fracture performance by using mathematical methods, including total proppant volume, each proppant volume, fluid injection rate, fluid injection volume, and proppant concentration, etc. Results from micro seismic monitoring indicate that the actual fracture performance matches well with the simulated performance by using coated ceramsite and has an obviously longer fracture length and a bigger proppant concentration compared with performances by using quartz sand. Meanwhile, comparisons for gas productions fractured by coated ceramsite and quartz sand reveal that coated ceramsite can achieve a better drainage performance. All of the above can be a guide for hydraulic fracture and useful for enhancing coalbed methane recovery. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.

Mechanisms in CO2-enhanced coalbed methane recovery process

Injection of CO2 and subsequent desorption of CH4 is considered to be the most efficient enhanced coalbed methane (ECBM) recovery technique to date. Meanwhile, CO2-ECBM is an excellent option for CO2 geo-sequestration for an extended period. Despite ongoing research efforts and several field applications of this technology, the mechanisms of the process have yet to be fully understood. The coalbed heterogeneity, the fluid interactions with coal, the CO2 induced swelling, and the continuous pressure and composition changes require outright insights for optimal application of the technique. Furthermore, intermolecular interactions of CO2 and CH4, their competitive adsorption on the dry/wet coal surface, and the dispersion and advection processes play an important role in defining the CO2-ECBM recovery process. An attempt has been made here to understand the key mechanisms of CO2-ECBM recovery in coalfields, particularly the adsorption of CO2 in the supercritical state at the recommended sequestration depth. Cited as: Asif, M., Wang, L., Wang, R., Wang, H., Hazlett, R. D. Mechanisms in CO2-enhanced coalbed methane recovery process. Advances in Geo-Energy Research, 2022, 6(6): 531-534 . https://doi.org/10.46690/ager.2022.06.09

Microstructure evolution of bituminite and anthracite modified by different fracturing fluids

The intrusion of fracturing fluid into coalbed methane (CBM) reservoirs changes the microstructure of coal and affects CBM recovery. To study the microstructural evolution characteristics of bituminite and anthracite modified by different fracturing fluids, we prepared pure water (F1), slickwater (F2), and CO2 foam fracturing fluids (F3) to treat coals, and low-temperature N2 adsorption (LT-N2A), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) tests were conducted to characterize the variation in the microscopic pore structure, functional group structure, and mineral components of the modified coals, respectively. The results show that different fracturing fluids can significantly change the pore structure characteristics of bituminite and anthracite. The pore volume of bituminite and anthracite modified by F1, F2, and F3 decreases successively, while the average pore size and the percentage of macropore (>50 nm) volume increase. The average pore sizes of bituminite and anthracite modified by F3 are 16.78 nm and 19.38 nm, respectively, which increase by 18.59% and 30.89% compared with that of bituminite modified by F2 and F1, respectively; the average pore sizes of anthracite increases by 3.20% and 332.47%, respectively. The pore fractal dimension of coal was calculated based on the low-temperature N2 adsorption curve. It is found that the seepage pore fractal dimension (D1) of both bituminite and anthracite modified by F2 and F3 is smaller than that of F1 modification, with D1 of F2 modified coals being the smallest. Therefore, F2 and F3 can make the seepage pores of coal smoother and reduce the complexity. F2 and F3 can reduce the aromaticity (I) and the degree of condensation of aromatic rings (DOC) of bituminite and anthracite and increase the structural parameter of the oxygen-containing functional groups (‘C’). The three fracturing fluids have little effect on the aliphatic chain length (L). F2 and F3 can dissolve the carbonate and clay minerals of coal to generate new pores to increase porosity and permeability. The results provide theoretical guidance for the fracturing of medium- and high-rank CBM reservoirs.

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.

3D hybrid coupled dual continuum and discrete fracture model for simulation of CO2 injection into stimulated coal reservoirs with parallel implementation

This work presents the development of a 3D hybrid coupled dual continuum and discrete fracture model for simulating coupled flow, reaction and deformation processes relevant to fractured reservoirs with multiscale fracture system, e.g., coal and shale, efficiently and accurately. In this hybrid model, the natural fracture network and coal matrix are described together by a dual continuum approach and the large fractures are represented explicitly by the discrete fracture approach. A combination of different types of elements is used for spatial discretization. Large fractures are discretised with lower-dimensional interface elements and continuum domains with higher-dimensional elements. The coupling between the two models is achieved via the principle of superposition. To reduce computational time of simulations for complex and large-scale problems, a hybrid MPI/OpenMP parallel scheme is implemented in this work. The developed model is applied to investigate coupled thermal, hydraulic, and mechanical processes associated with CO2 sequestration and enhanced coalbed methane recovery. The results demonstrate capabilities of the model to adequately capture the effects of multiscale fracture system and their coupled behaviour during CO2 injection and methane recovery from coal reservoirs. Performance of the proposed parallelisation scheme was tested by comparing computation times of serial and parallel implementations. A good performance improvement was achieved, the speedup using parallelized scheme reaches up to about 10 times along with satisfactory scalability for considered application example. The findings of this work support developments and improvements of efficient advanced numerical models to study coupled THCM behaviour in fractured porous geomaterials.

Laboratory experiments of CO2-enhanced coalbed methane recovery considering CO2 sequestration in a coal seam

CO2-enhanced coalbed methane (CO2-ECBM) recovery has the triple benefits of recovering clean resources, ensuring coal mine safety, and achieving CO2 geological sequestration. To study the synergistic effect of CH4 recovery and CO2 sequestration, we designed a large-scale multifunctional apparatus that can realistically simulate the environment of CO2-ECBM technology. The results showed that the reservoir pressure dropped rapidly during conventional CBM recovery, but remained at a high level for a long time during CO2-ECBM recovery. The reservoir temperature close to the injection well was dominated by CO2 adsorption, whereas that close to the production well was dominated by CH4 desorption. The recovery efficiency of CH4 increased from 66.67% during conventional CBM recovery to 90.86%–93.50% after CO2-ECBM recovery. However, the sequestration efficiency of CO2 decreased from 67.89% to 43.98% with increasing CO2 injection pressure from 1.0 to 1.6 MPa. We then proposed a displacement index to reflect the synergistic effect of CH4 recovery and CO2 sequestration. This index first increased then decreased during CO2-ECBM recovery, dropping significantly at the last stage under increased injection pressure. Therefore, we suggest that a dynamic pressure injection mode may be more effective for ensuring efficient CH4 recovery and CO2 sequestration effect.

Coupled adsorption-hydro-thermo-mechanical-chemical modeling for CO2 sequestration and well production during CO2-ECBM

Injecting CO2 into the methane-bearing coal seams can not only store CO2 but improve coalbed methane recovery. In this paper, a fully coupled adsorption-hydro-thermo-mechanical-chemical (AHTMC) model for CO2 enhanced coalbed methane (CO2-ECBM) is constructed, including the multiple fields with the coupling relationships of competitive adsorption of CO2 and CH4 in coal matrix (A), gas-water two-phase flow and gas diffusion in formation water (H), heat transfer and conversion (T), coal deformation (M), geochemical reactions of various anions and cations and the mineral dissolution in formation water (C). The model is verified by cored coal sample experiments in the laboratory and applied to simulate a CO2-ECBM pilot project in Qinshui Basin with different CO2 injection rates. The simulation results show that CH4 adsorption capacity decreases and CH4 production rate increases significantly with CO2 injection. When CO2 breaks through production well, the CO2 storage rate decreases and the pH in drainage decreases significantly. The CO2 injection rate has a significant influence on CO2-ECBM. Importantly, 25 t/d of CO2 injection is available for the pilot project with a CH4 recovery growth rate of 14.89% and a CO2 storage ratio exceeding 96% in a CO2 injection duration of 8000d.

Effects of pore structure changes on the CH4 adsorption capacity of coal during CO2-ECBM

The effects of supercritical CO2 (scCO2) on pores in coal play a critical role in determining the CH4 adsorption and desorption characteristics and CO2 capacity of coal seams during CO2 geological storage-enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of pore structure changes caused by scCO2 on the CH4 adsorption capacity of coals, six coal samples with different ranks are exposed to scCO2 and water for 240 h at 62.5 °C and 15 MPa using a scCO2 geochemical reactor. Then, gravimetric isothermal adsorption tests, mercury intrusion porosimetry, N2 adsorption tests, and CO2 adsorption tests are performed to determine the CH4 adsorption capacity, pore volume, and pore specific surface area (SSA) of the coal samples with and without scCO2-H2O treatment. Finally, the influences of scCO2 on the pore structure and the effects of pore structure changes on the CH4 adsorption capacity of the coal samples are analyzed. The results show that the CH4 adsorption capacity of coal and its changes after scCO2-H2O treatment are strongly affected by density effects and pore selection effects. The CH4 adsorption capacity of coal is determined by the micropore volume and SSA; however, their changes only affect the CH4 adsorption capacity in the micropore filling stage when the pressure is <5 MPa with weak effects on the total CH4 adsorption capacity, which is related to their weak changes after scCO2-H2O treatment compared with macropores and mesopores. The changes in CH4 adsorption capacity mainly depend on the changes in macropore and mesopore SSAs after scCO2-H2O treatment; scCO2 mainly changes the CH4 adsorption capacity in the transitional stage, when the pressure is 5–10 MPa, and the monolayer and multilayer adsorption stage, when the pressure is >10 MPa. Meanwhile, the effect of pore structure changes on the CH4 adsorption capacity of coal with different ranks is complex. The CH4 adsorption capacity of subbituminous coal and medium-volatile bituminous coal increases after scCO2-H2O treatment due to the increases in macropore SSA, that of high-volatile bituminous coal slightly decreases, and that of semianthracite coal and anthracite coal slightly increases because of the changes in mesopore SSA after scCO2-H2O treatment. However, the CH4 adsorption capacity of low-volatile bituminous coal increases, which is the result of changes in the SSA of the mesopores and macropores due to slight changes in the micropore volume and SSA.

Experimental Study on the Influence of Effective Stress on the Adsorption–Desorption Behavior of Tectonically Deformed Coal Compared with Primary Undeformed Coal in Huainan Coalfield, China

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs.

Improved Permeability Model of the Binary Gas Interaction within a Two-Phase Flow and its Application in CO2-Enhanced Coalbed Methane Recovery.

In this paper, we develop a dual-porosity dual-permeability model for binary gas migration to explore the permeability evolution in the matrix and fracture in the process of a gas–water two-phase flow during CO2-enhanced coalbed methane (CO2-ECBM) recovery in coal reservoirs. This mechanistic model accommodates the effects of elastic deformation caused by the effective stress change in the matrix and fracture, the swelling/shrinkage deformation of the matrix caused by adsorption/desorption, the convection and diffusion of gas, and the discharge of water. Specifically, the time-dependent matrix swelling, from initially completely reducing the fracture aperture to finally affecting the coal bulk volume, is considered by the invaded volume fraction involving binary gas intrusion. The model is validated through laboratory data and applied to examine the permeability evolution of CO2-ECBM recovery for 10 000 days. Furthermore, we analyze the sensitivity of some selected initial parameters to capture the key factors affecting CO2-ECBM recovery. Our modeling results show that the permeability evolution can be divided into two stages during the process, where stage I is dominated by effective stress and stage II is dominated by adsorption/desorption. Increasing the injection pressure or initial permeability advances the start of stage II. The decrease in initial water saturation causes the permeability to change more drastically and the time of stage II to appear earlier until a time long enough, after which little effect is seen on the permeability results.

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.

Dynamic Evolution of Gas Flow during Coalbed Methane Recovery to Reduce Greenhouse Gas Emission: A Case Study.

Gas pre-extraction technology in a coal reservoir can not only reduce greenhouse gas (GHG) emissions but also effectively recover coalbed methane (CBM). In this work, we use a geomechanical-coupled gas flow (GCF) model to simulate and analyze the pre-extraction effect of a mining-disturbed coal seam. First, the simulation results of the GCF model are compared with field test data to verify the correctness and reliability of our model. Then, the evolution law of the stress field, permeability field, and gas flow field in the extraction process is analyzed through a case study. The results show that the first principal stress of coal in a mining area increases first and then decreases slowly and reaches the peak value at 5 m. The third principal stress increases gradually at first and becomes stable after 10 m. As the distance from the mining face increases, the permeability and gas pressure of the coal seam show continuous and asymmetric “U”-shaped and “n”-shaped distribution characteristics, respectively. In addition, the recovery effect and abnormal emission factors of CBM are discussed. This study can provide theoretical guidance for optimizing the CBM recovery effect and reducing GHG emissions during mining.

Enhanced coalbed methane recovery by the modification of coal reservoir under the supercritical CO2 extraction and anaerobic digestion

Coal reservoir is considered as the ideal site for coalbed methane (CBM) occurrence, biomethanation and CO2 sequestration. Four coal samples with different ranks were selected to conduct supercritical CO2 (Sc-CO2) extraction and anaerobic digestion (AD). X-ray photoelectron spectroscopy, mercury intrusion, and isothermal adsorption experiments were performed on the raw coal samples and the remaining coal samples after Sc-CO2 and Sc–CO2–AD to analyze the mechanism of enhanced CBM recovery by reservoir modification. The results showed that coal pore volume and porosity are greatly increased after Sc-CO2 extraction. Meanwhile, the reduction of the oxygen-containing functional groups and the micropore specific surface area determined the weakening of the adsorption capacity of coal. After Sc–CO2–AD, the macropore volume and porosity of coal continued to increase, and the permeability of coal was effectively improved. And the oxygen-containing functional groups and micropore specific surface area continued to decrease, which determined that the adsorption capacity of coal for methane continued to decrease. Sc-CO2 and AD can mutually promote the modification of coal reservoirs. These effects are quite favorable for gas desorption, migration and production during CBM exploitation, which provide powerful supports for CO2 emission reduction, CO2 geological sequestration and CBM production enhancement.

Precise distinction of mechanical behavior stages of coal under temperature-pressure constraints and its permeability and energy consumption characteristics

The mechanical behavior and permeability (K) of coal reservoirs are important research directions in the development of coalbed methane recovery. In this study, the raw coal was collected from the Baode block, China. The pseudo-triaxial tests examined the coal mechanical behavior and K under high temperature-pressure (T-P) conditions. We proposed a new micro-slope method to accurately distinguish the deformation stages of coal under high T-P conditions. The whole deformation process is divided into five stages, and the dynamic permeability is divided into six stages. The results show that the K is closely related to the deformation stage and exhibits a U-shaped change with the axial strain, and its minimum value occurs at the end of the elastic deformation. The T and P control the evolution of coal mechanical behavior and K. The peak stress (σmax) and peak strain (εmax) decrease with the T but increase with the P. The initial permeability (K0) decreases with the increase in P and T in the 10–30 MPa and 30–80 °C range, respectively. On the other side, the minimum permeability (Kmin) is least affected by the T and P. The kinetic properties of energy dissipation show that, the T and P control the dynamic changes of K by affecting the energy at different stages inside the coal. This study provides a reference for understanding the mechanical behavior of deep coal reservoirs.

Feasibility investigation of enhanced coalbed methane recovery by steam injection

Heat injection is an effective technique to enhance unconventional oil and gas recovery. As an excellent heat transfer medium, steam has a great potential for coalbed methane (CBM) recovery enhancement. To investigate the feasibility of enhanced CBM recovery by steam injection, infrared thermal imaging (ITI) and thermocouples were employed to study the heating efficiency of steam on three ranks of coal, and nuclear magnetic resonance (NMR) was applied to analyze water transport within the coal after steam treatment. The results showed that the internal temperature of the coal samples reached about 80 °C in 17 min of steam treatment and thermal cracking of coal were induced, hence steam injection could facilitate gas desorption and provide more channels for gas migration. NMR test results indicated that steam preferentially condensed in coal micropores due to the capillary condensation effect, hence steam might have the potential to displace methane. The T2 spectra of the coals after steam treatment exhibited only the first two peaks without the third peak, indicating that steam injection might not cause the water blocking effect. The outcomes of this study are expected to provide valuable insights into the field application of steam injection to enhance CBM production.

A Comparative Analysis of Pore Attributes of Sub-Bituminous Gondwana Coal from the Damodar and Wardha Valleys: Implication for Enhanced Coalbed Methane Recovery

A Comparative Analysis of Pore Attributes of Sub-Bituminous Gondwana Coal from the Damodar and Wardha Valleys: Implication for Enhanced Coalbed Methane Recovery

Enhancing the Efficiency of Coal Bed Methane Recovery by Injecting Carbon Dioxide Based on an Anthracite Coal Macromolecular Model and Simulation Methods

Enhancing the Efficiency of Coal Bed Methane Recovery by Injecting Carbon Dioxide Based on an Anthracite Coal Macromolecular Model and Simulation Methods

Effect of rarefied proppant monolayer on coal permeability

Effective gas drainage and intensification of coalbed methane recovery is a challenging problem in underground mining. One of the methods to increase coal permeability is hydraulic fracturing. Long-term conductivity of hydraulic fractures is ensured using wedging materials (proppants). The effect of partial filling of an artificial fracture by proppant on coal permeability is studied. The permeability tests of samples with a longitudinal fracture filled by a rarefied monolayer of proppant were performed under conditions of triaxial compression at 1-5 MPa. It is found that with a decrease in the surface density of the proppant by 2-4 times, the coal permeability is preserved and corresponds to the values obtained for a continuous proppant monolayer. The compressibility coefficients of the natural and artificial fractures in coal are estimated based on the permeability test results. The compressibility coefficient in the propped fractures is one order of magnitude lower than in the natural cracks.

CO2 Adsorption/Desorption, Induced Deformation Behavior, and Permeability Characteristics of Different Rank Coals: Application for CO2-Enhanced Coalbed Methane Recovery

CO₂ Adsorption/Desorption, Induced Deformation Behavior, and Permeability Characteristics of Different Rank Coals: Application for CO₂-Enhanced Coalbed Methane Recovery

Nonlinear Diffusion-Based Multifield Coupling Model for Thermally Enhanced CBM Recovery

Thermal stimulation is a supplementary technique for enhancing gas recovery from coalbed methane (CBM) reservoirs that has received considerable attention worldwide. Investigating gas and heat transfer in coal seams during thermally enhanced CBM recovery is of great significance for predicting gas production and optimizing the extraction method. Gas diffusion in the coal matrix and coupled multiphysics are two of the most important aspects when analyzing gas migration and heat transport. However, previous studies either neglected the nonlinear diffusion process of gas or only assumed that the diffusion coefficient varies with production time. However, considerable experimental results indicate that the gas diffusion coefficient is not only determined by time but also by temperature, which strongly impacts multifield interaction during gas recovery. Thus, prior diffusion models and coupled models must be modified. In this paper, a time-and-temperature-dependent gas diffusion model is established based on fractal theory and experimental data. The proposed diffusion model is embedded into the coupled thermal-hydro-mechanical model to comprehensively describe the behavior of coal deformation, gas migration, and heat transport during CBM recovery. Additionally, both new diffusion model and coupled model were validated with experimental results or field test data, showing that these developed models are applicable for modeling long-term gas diffusion and gas production. Finally, the coupled model was implemented into COMSOL Multiphysics software, and a series of numerical simulations were conducted. The calculation results showed that heat injection could promote gas desorption and diffusion in the matrix while inhabiting gas flow in fractures near the injection well. Gas dynamic diffusion could inhabit gas migration in both matrix system and fracture system at a later production stage. This also means that ignoring the gas nonlinear diffusion process leads to a severe overestimation of coal permeability and gas production.