Coalbed Methane Recovery Research Articles

Recent Advances and Perspectives of CO2-Enhanced Coalbed Methane: Experimental, Modeling, and Technological Development

Carbon dioxide (CO2)-enhanced coalbed methane recovery (CO2-ECBM) is a critical way to increase methane production and reduce greenhouse gas (CO2 and CH4) emissions. As captured CO2 is continuously injected in the coal seams, a low cost of CO2 sequestration and high efficiency of CH4 recovery can be achieved via the flooding and replacing effects driven by the injected CO2 flow. Scientific insights into the complex process of CO2-ECBM in experiments, modelings, and technological developments need to be made to propose appropriate countermeasures. This review first highlights the progress of CO2-ECBM under laboratory conditions, e.g., the binary gas competitive adsorption and gas displacement experiments in the macroscale and porous structure tests using technologies of nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and computed tomography (CT) in the microscale. Then, the advances of mathematical models for changing in coal permeability and porosity during CO2-ECBM are reviewed, accompanying with the multi-field and multi-phase coupling responses of competitive sorption, diffusion, gas–water seepage, heat transfer, and solid deformation. Furthermore, the field pilot tests of CO2-ECBM in various countries and regions are also covered to reveal the key technical challenges confronted with the development of CO2-ECBM technology. The perspectives in experiments, models, and field pilots of CO2-ECBM are made, which include but are not limited to the following: conducting a core CH4/CO2 flooding test under in situ conditions, modeling CO2-ECBM with real fractures/faults and coal failure, developing a new method for gas migration and leakage monitoring in the field, and enacting relevant standards, laws, and regulations to promote CO2-ECBM.

Numerical Study on the Evolution of Reservoir Pressure and CBM Concentration Considering Hydraulic Fractures

Based on the theories of mass conservation and coalbed methane (CBM) adsorption/desorption, this paper first establishes a novel reservoir pressure model for CBM production, following which, the CBM concentration and production models are also proposed. Then, these models are programmed and solved by means of the finite element method. Taking the Hunchun CBM field in Jilin province, China, as an example, the reservoir pressure, gas concentration, and production characteristics under different hydraulic fracture forms are simulated and investigated. In conclusion, the reservoir pressure decreases very rapidly in a small region near the fracture tip, which we called the “reservoir pressure singularity”. The existence of a hydraulic fracture greatly reduces the reservoir pressure in the process of CBM exploitation. The permeability sensitivity coefficient of reservoir pressure, Rpk, is defined to quantitatively describe the influence of coal seam permeability on the evolution of reservoir pressure. Rpk decreases logarithmically as the distance from the CBM extraction well increases. The reservoir pressure and CBM recovery rate characteristics in the presence of multiple hydraulic fractures are also investigated. We believe these results could contribute to the design of hydraulic fracturing wells and the evaluation of gas production in a CBM reservoir.

Fracture propagation induced by hydraulic fracturing using microseismic monitoring technology: Field test in CBM wells in Zhengzhuang region, Southern Qinshui Basin, China

Hydraulic fracturing is an important technical measure for coalbed methane (CBM) development, and the propagation of fractures in the target coal seam induced by hydraulic fracturing is related to the stimulation performance in CBM recovery. Therefore, effective monitoring of fracture development during reservoir fracturing is critical for CBM engineering. In this paper, the microseismic technology was used to monitor the spatial and temporal characteristics of the fracture extension in the CBM well during hydraulic fracturing in Zhengzhuang Region, Southern Qinshui Basin, China. Based on the microseismic fracture scanning data and imaging processing, the three-dimensional shape of fractures in vertical wells after fracturing can be quantified, and for the vertical well ZHSY-1, the main fracture direction is identified as NE106°, and the fracture length is 426 m, and the fracture area of coal seam is 1.6 × 105 m2. It is found that fracturing does not develop continuously in time and space with fracturing fluid injection, and the fracture regions are scattered throughout the space, and the formation and development of fracture regions are intermittent. After fracturing with a large amount of fracturing fluid injection in Well ZHSY-1, the fracture area has been significantly increased, and the well gas production has been significantly improved, which is confirmed by the field CBM well data. This study provides a field application case for studying the effect of hydraulic fracturing fracture propagation using microseismic technology, which can be used as a reference for fracturing engineering in CBM development.

Optimal source–sink matching and prospective hub–cluster configurations for CO 2 capture and storage in India

Abstract At COP26, India announced strong climate commitments of reaching net-zero greenhouse gas emissions by 2070. Meeting this target would likely require substantial deployment of CO 2 capture and storage (CCS) to decarbonize existing large-point sources of CO 2 . This study attempts to evaluate opportunities for deployment of CCS in India in the forthcoming decades. A geographic information system-based approach was adopted for mapping existing sources of CO 2 with the sinks. The results show that regionally-appropriate ways of moving towards CCS at scale exist in both the power and industrial sectors. Coupled analysis of these sectors with sinks shows that eight clusters may be developed throughout the country to sequester 403 Mt-CO 2 annually. These clusters are concentrated near Category-I oil basins and the Category-I coalfields (Damodar Valley), which may also create suitable financial incentives by incremental oil and coalbed methane recovery, respectively. Furthermore, a first-order costing analysis evaluates that the cost of avoidance across basins may range from –$31 to $107/t-CO 2 , depending on the type of storage reservoir and the proximity to large-point sources. A total of 12 suitable hubs and clusters were created based on annual emissions above 1 Mt-CO 2 of each large-point source and their proximity with geological sinks.

Effects of Liquid CO2 Phase Transition Fracturing on Methane Adsorption of Coal

Liquid CO2 phase transition fracturing (LCPTF) is a kind of novel waterless fracturing technology for enhancing the coalbed methane (CBM) recovery. Relevant current studies are focused on exploring the transformed effect of LCPTF on the nanopore structure in coal. However, its influence on gas adsorption capacity has been rarely reported. This study addresses coal induced with LCPTF. The structure alterations of mesopores (2–50 nm) and macropores (>50 nm) and their effects on the gas adsorption capacity are evaluated with comprehensive measurements of a mercury intrusion porosimetry, low-temperature N2 adsorption, and isothermal adsorption. The results indicate that LCPTF has an enlarged effect on macropores, resulting in an increase in pore size and volume and a reduction of the pore specific surface area. The pore-enlarged-transformed effects of LCPTF cause an increase in pore size and a reduction of the pore volume and pore specific surface area of mesopores. The variations of the pore structure after LCPTF cause a reduction of the adsorption constant a and the increase in the adsorption constant b, indicating LCPTF’s reductive effect on adsorption capacity as well as its enhanced effect on desorption capacity. A novel effect evaluation method of LCPTF for improving CBM recoverability is proposed based on the variations of gas saturation and critical desorption pressure. This study examines, from the perspective of adsorption and desorption, the mechanism of LCPTF for enhancing CBM recovery, which provides theoretical guidance for LCPTF’s technical improvement and optimization of field application so as to secure a more reliable and efficient CBM recovery.

Experimental and numerical model of anisotropic permeability and CO2 injectivity of coal during CO2 enhanced coalbed methane recovery process

Guaranteeing CO2 injectivity has been the precondition for implementing the CO2-enhanced coalbed methane recovery (CO2-ECBM), however, it dramatically decreases during the CO2 injection process because it is influenced by the dynamic change of the anisotropic permeability of coal. To reveal and evaluate the CO2 injectivity of coal, the anisotropic permeability test and the CO2 injectivity simulation test were first conducted, then the corresponding numerical models were established and verified by the experimental data. The results show that the permeability of coal in parallel face cleat direction is the largest, followed by the permeability of coal in parallel butt cleat direction and that in vertical bedding direction is the minimum. The peak value of the instantaneous injectivity rate is enhanced and the injection time is prolonged with the increase of the CO2 injection pressure. The total CO2 injectivity rate is nonlinearly increased from 13.61 to 311.87 cm2/MPa min when the CO2 injection pressure raises from 2 to 10 MPa. The anisotropic permeability model is appropriate to describe the dynamic evolution of permeability under different boundary conditions, the CO2 injectivity prediction model can be used to evaluate the CO2 injectivity during the CO2 injection process. Increasing the CO2 injection pressure may temporarily promote CO2 injectivity, while the CO2 injection increment is limited. The CO2 fracturing by phase transition may be an available reservoir stimulation method for enhancing the CO2 injection and should be focused on in the future.

Coal measure energy production and the reservoir space utilization in China under carbon neutral target

Coal measure energy production and reservoir space utilization are both important for energy security and can assist in achieving carbon neutrality. Coal measure energy production and reservoir utilization are therefore of scientific significance. This study focuses on the security of coal supply by discussing the possible technical approaches of combining low-carbon mining of coal measure resources and coal reservoir space utilization. Our study found that 1) the integration of underground coal gasification and carbon capture and sequestration (UCG-CCS) is an effective technology to improve energy efficiency and achieve the carbon neutral target in China; 2) CO2-enhanced coalbed methane (CO2-ECBM) recovery is an important way to reduce CO2 emissions during coalbed methane recovery; and 3) the integrated technology of solid waste filling (WF), coal bioconversion (CB), gas production (GP), and CO2 capture and sequestration (CCS) in coal goafs and abandoned coal mines can improve the energy production efficiency and reservoir space utilization and reduce geological disasters due to surface subsidence. These integrated technologies will become increasingly important for coal measure energy production and efficient reservoir space utilization as they will provide scientific theory and technical support to achieve carbon neutrality.

Coalbed methane accumulation, in-situ stress, and permeability of coal reservoirs in a complex structural region (Fukang area) of the southern Junggar Basin, China

The enrichment of coalbed methane (CBM), in-situ stress field, and permeability are three key factors that are decisive to effective CBM exploration. The southern Junggar Basin is the third large CBM basin in China but is also known for the occurrence of complex geological structures. In this study, we take the Fukang area of the southern Junggar Basin as an example, coalbed methane accumulation and permeability, and their geological controls were analyzed based on the determination of geological structures, in-situ stress, gas content, permeability, hydrology and coal properties. The results indicate that gas contents of the Fukang coal reservoirs are controlled by structural framework and burial depth, and high-to-ultra-high thickness of coals has a slightly positive effect on gas contents. Perennial water flow (e.g., the Baiyanghe River) favors gas accumulation by forming a hydraulic stagnant zone in deep reservoirs, but can also draw down gas contents by persistent transportation of dissolved gases to ground surfaces. Widely developed burnt rocks and sufficient groundwater recharge make microbial gases an important gas source in addition to thermogenic gases. The in-situ stress field of the Fukang area (700–1,500 m) is dominated by a normal stress regime, characterized by vertical stress > maximum horizontal stress > minor horizontal stress. Stress ratios, including lateral stress coefficient, natural stress ratios, and horizontal principal stress ratio are all included in the stress envelopes of China. Permeability in the Fukang area is prominently partitioned into two distinct groups, one group of low permeability (0.001–0.350 mD) and the other group of high permeability (0.988–16.640 mD). The low group of permeability is significantly formulated by depth-dependent stress variations, and the high group of permeability is controlled by the relatively high structural curvatures in the core parts of synclines and the distance to the syncline core. Meanwhile, coal deformation and varying dip angles intensify the heterogeneity and anisotropy of permeability in the Fukang area. These findings will promote the CBM recovery process in China and improve our understanding of the interaction between geological conditions and reservoir parameters and in complex structural regions.

Modelling multicomponent gas diffusion and predicting the concentration-dependent effective diffusion coefficient of coal with application to carbon geo-sequestration

Diffusion of multicomponent gases, which is a complex transport mechanism in coalbed methane (CBM) reservoirs, needs to be accounted for in models of enhanced coalbed methane recovery (ECBM) and geological CO2 storage. We develop a numerical model that accounts for diffusion of free gas, diffusion of adsorbed gas, and competitive sorption. Our model also reflects adsorption-induced swelling and the impact of moisture. The simulator for modelling multicomponent gas diffusion is first validated by comparing its computed concentration field against those yielded by the general-purpose finite element solver COMSOL and the available analytical solutions. The model is then validated against the experimental measurements from the published literature. Subsequently, our simulator is used to model the CO2 Enhanced Coalbed Methane (CO2-ECBM) scenarios. The simulation results quantify the influence of different types of diffusion on the effective diffusion coefficient of coal. We analyse diffusion of methane and carbon dioxide that must be considered when modelling CBM reservoirs and analyse performance-based sensitivity of surface diffusion, moisture, matrix deformation, and pressure. Finally, the model is applied to a CO2 storage case. We calculate the CO2 storage volume and the equilibrium time under various pore pressures. The simulation shows that as pore pressure increases, the amount of CO2 stored increases and the equilibrium time decreases. Overall, this model can be applied to estimate the effective diffusion coefficient, to analyse the impact of rock and gas parameters on multicomponent gas diffusion, to provide insight into the physics of multiple gas transport in coal, and to estimate the feasibility of CO2 storage under various pore pressures.

Experimental Study on Fractal Characteristics of Adsorption Pore Structure of Coal

The adsorption pore structure is the key affecting the technology of enhanced coal bed methane recovery (ECBM). In this paper, nitrogen adsorption measurement (NAM) and low-field nuclear magnetic resonance (NMR) methods are used to test the structural parameters of adsorption pores of four coal samples with different metamorphic degrees. Combining with the fractal theory, the applicability and the physical significance of fractal dimensions obtained from different models are analyzed. Finally, the main factors affecting the complexity of the pore size structure and the influence of fractal characteristics on the adsorption properties of coal are discussed. The results show that with the increase in the degree of metamorphism, the ranking order of the volume and the specific surface area of adsorption pores of four coal samples are long flame coal > anthracite > gas coal > coking coal. The fractal dimension D2 calculated by the Frenkel–Halsey–Hill (FHH) fractal model ranges from 2.5 to 2.9, representing the degree of the pore surface irregularity, and D3 calculated by the FHH fractal model ranges from 2.004 to 2.037, representing the complexity of pore size distribution. There is no clear quantitative relationship between the fractal dimension and the single structure parameters of adsorption pores. The more the pore diameter distribution is concentrated in the range of 2–5 nm, the larger the fractal dimension is, the higher the complexity of adsorption pore structure is; D2, which represents the irregularity of the surface of coal, has a good linear positive correlation with the maximum adsorption capacity of N2.

A Novel Permeability Prediction Model for Deep Coal via NMR and Fractal Theory

A quantitative description of the permeability of deep coal is of great significance for improving coalbed methane recovery and preventing gas disasters. The Schlumberger–Doll research (SDR) model is often used to predict rock permeability, but it has inherent defects in characterizing the pore structure of deep coal. In this study, a permeability model with fractal characteristics (FCP model) is established for deep coal based on nuclear magnetic resonance (NMR). The constants in the SDR model are theoretically explained by the relevant parameters in the FCP model. Centrifugation and NMR experiments were performed to determine the optimal centrifugal force and dual T2 cutoff values. The results show that the coal samples are mainly composed of micrometer and nanometer pores. The adsorption pores account for the largest proportion, followed by the percolation pores and migration pores. In addition, the prediction accuracy of the FCP model is significantly higher than that of the other three models, which provides a fast and effective method for the evaluation of deep coal permeability.

Synergistic Mechanism of Ultrasonic-Chemical Effects on the CH4 Adsorption-Desorption and Physicochemical Properties of Jincheng Anthracite.

Ultrasonic is a new method to enhance coalbed methane recovery. A deeper comprehension of the synergistic mechanisms of combined ultrasonic-chemical modification on the CH4 adsorption-desorption capability and physicochemical properties of coal is necessary for potential field implementation, as the modification of coal reservoirs frequently necessitates the addition of chemical reagents. This paper evaluated the CH4 adsorption-desorption properties of anthracite modified by sodium dodecyl sulfate (SDS) solution, ultrasonic modification, and combined ultrasonic-SDS modification. Fourier transform infrared spectroscopy, low-temperature nitrogen adsorption, and micro-CT were applied to elucidate the synergistic mechanism of the combined modification. The research results show that the SDS solution reduces the saturated adsorption capacity of anthracite and increases its final desorption rate by dissolving clay minerals and the physical adsorption masking effect of SDS micelles on the coal surface. Some surface groups with low bond energy are broken or evaporated under mechanical vibration and thermal effects generated by ultrasonic. The original fractures are expanded and connected, which changes the adsorption-desorption properties of anthracite. The synergistic effect of the combined modification of ultrasonic-SDS can promote the penetration range and chemical reaction efficiency of the SDS solution, which expands the effective range of ultrasonic. After combined modification, the amount of aromatics, oxygen-containing functional groups, and aliphatic hydrocarbons on the surface of coal is reduced. The connected porosity of coal samples accounts for 91.5% of the total porosity. As a result, the saturated adsorption capacity of anthracite reduces by 26.7%, and the final desorption rate increases by 28.0%. The effect of the combined ultrasonic-chemical modification is better than that of a single modification.

Modeling and optimization of recovery enhancement efficiency by CO2/N2 mixture displacement in stimulated coal seams by destressing boreholes

Binary mixture (CO2/N2) displacement can effectively enhance coalbed methane recovery. A small gas injection amount in low-permeability coal seams leads to poor recovery enhancement efficiency (REE) by gas displacement. Constructing pressure-relief chambers inside the targeted coal seam can effectively improve the gas injection amount, yet the correlation between heterogeneous coal failure and the recovery enhancement by binary gas mixture remains unclear. Artificial parameters that influence recovery enhancement, including the diameter of pressure-relief chambers, drilling spacing, CO2 concentration, gas injection pressure and initial gas pressure, can be controlled or optimized in field practice. However, these parameters are not independent of but correlated with each other. This study aims to enhance gas recovery in deep mining coal seams. To this aim, a model of multi-component gas flow and geo-mechanics was established to model the gas displacement process in destressed coal seams. Next, advantages of the model considering coal failure and heterogeneity were revealed by comparing effective breakthrough radius and gas recovery amount, respectively. Subsequently, the evolution law of multi-physics was analyzed and the conclusions are as follows: (1) the model considering heterogeneous coal failure can well exhibit the storage and migration of multi-component gas; (2) the gas displacement influencing region can be divided into strong, weak and pianissimo displacement zones according to the regions that binary mixture acts on. Moreover, the pre-breakthrough REE and the post-breakthrough REE were proposed based on the characteristics of different gas injection stages. The respective and coupling effects of the artificial parameters on REE were further investigated by response surface methodology. The results reveal that the above five parameters (pressure-relief chamber diameter, drilling spacing, CO2 concentration, gas injection pressure and initial gas pressure) are dominant factors controlling the post-breakthrough REE, yet the latter three parameters have a notable impact on the pre-breakthrough REE. In addition, the pre-breakthrough REE and the post-breakthrough REE can hardly be improved at the same time and it is necessary to improve the REEs according to different gas injection stages. Finally, an injection-extraction alternating and multi-level optimization technology was proposed to further enhance gas displacement in low-permeability coal seams. The research results provide a theoretical reference for engineering practice.

Potential effect of carbon dioxide injection on the functional groups of medium volatile bituminous coals analysed using in-situ diffuse reflectance Fourier-transform infrared spectroscopy

CO2 sequestration with enhanced coalbed methane recovery (CO2–ECBM) is a promising technology for mitigating CO2 emissions. To implement CO2–ECBM, complex interactions must be elucidated between CO2 and coal during CO2 sequestration, but the mechanism through which coal and CO2 interact remains controversial mainly because coal contains a complex macromolecular structure. Therefore, we used in-situ high-pressure diffuse reflectance Fourier-transform infrared spectroscopy to investigate the potential impact of the CO2 injection on the functional groups of medium volatile bituminous (MVB) coals. The results of this study showed that CO2 physisorption could be found near the MVB coal macromolecular CH bonds, and the interactions between the MVB coal macromolecular CH bonds and CO2 kept strengthening with increasing CO2 injection pressure. Injecting CO2 into the MVB coal disrupted the weak interactions between aromatic layers, and supercritical CO2 (ScCO2) could disrupt the physical association between small molecular compounds and the coal macromolecular structure, which extracted hydrocarbon and oxygen-containing compounds, and thus reduced the content of oxygen-containing functional groups and -CH2- in the MVB coal. The coal macromolecular CO bonds were not conducive to methane adsorption on the coal surfaces, injecting high-pressure CO2 (P = 7 MPa, 8 MPa) into the MVB coal matrix irreversibly damaged the coal macromolecular CO bonds, which enhanced the CH4 adsorption capacity and was not conducive to CH4 desorption. The results of this study showed that injecting ScCO2 into the MVB coal could change the coal macromolecular structure and reduce the content of oxygen-containing functional groups and -CH2-, thus affecting the properties of MVB coal reservoir, such as porosity, permeability, and mechanical characteristics, which could advance knowledge of the theoretical usefulness for CO2-ECBM recovery and sequestration in the MVB coal reservoirs.

Heterogeneous structural responses of high-rank coal pores to hydraulic fracturing based on low-field nuclear magnetic resonance

Pores in coal are not only the main space for coalbed methane (CBM) occurrence but also the space to be opened during CBM recovery. Therefore, the analysis of the impact of hydraulic fracturing on coal pores, especially the change of adsorption pores before and after hydraulic fracturing, is of great significance to the evaluation of the hydraulic fracturing effect and CBM recovery. Hydraulic fracturing experiments and low-field nuclear magnetic resonance technology were used to analyze changes in the T2 curve, adsorption pore, and the effects of distance and in situ stresses on pore modification in coal samples of the Sihe (SH) and the Chengzhuang (CZ) mines before and after hydraulic fracturing. The results show that hydraulic fracturing can affect pores <10 nm. The CZ coal samples exhibit stronger heterogeneity than the SH coal samples after hydraulic fracturing, and the pore size distribution (PSD) anisotropy of the CZ samples is increased. For the SH coal samples with poor heterogeneity, the effect of hydraulic fracturing on pore transformation depends more on in situ stresses. The smaller horizontal in situ stresses difference facilitates the establishment of complex pore networks. After hydraulic fracturing, when the pore diameter is 30–100 nm, the pore volume proportions of the SH samples in the directions of maximum horizontal principal stress (σH), minimum horizontal principal stress (σh), and vertical stress (σV) increase from 43.73% to 64.84%, 59.79%, and 60.16%, respectively. Hydraulic fracturing increases the anisotropy of the PSD of the CZ samples.

Productivity enhancement in multilayered coalbed methane reservoirs by radial borehole fracturing

Coalbed methane (CBM) is an important unconventional natural gas. Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate. Radial borehole fracturing, which integrates radial jet drilling and hydraulic fracturing, is expected to create complex fracture networks in multilayers and enhance CBM recovery. The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers. First, a two-phase flow and multi-scale 3D fracture network including radial laterals, hydraulic fractures and face/butt cleats model is established, and embedded discrete fracture model (EDFM) is applied to handle the complex fracture networks. Then, effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing. Finally, two field CBM wells located in eastern Yunnan-western Guizhou, China were presented to illuminate the stimulation efficiency by radial borehole fracturing. The results indicated that compared with vertical well fracturing, radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production, approximately 2 times higher. Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing. Furthermore, the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme. The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.

Effects of Injection Pressure and Duration on Alternate High-Pressure Water-Gas Sequestration of Coalbed Methane

To solve the problems of high ground stress, low permeability and low coalbed methane recovery, this paper proposes an optimization technique for improving coal seam permeability, i.e., water-gas alternating displacement enhances coalbed methane recovery (WGA-ECBM). The seepage field of gas-water two-phase flow is derived and a multiphase fluid-solid coupling model between physical parameters is established. This paper studies the effects of three displacement methods (water injection, air injection and water-air alternating injection) on methane recovery. The influence of injection pressure and injection duration on the WGA-ECBM technique is also studied. The V C H 4 (methane volume fraction) of the three methods decrease by 17.84%, 12.72% and 44.62% in the borehole, respectively. The influence ranges (diameter) are 24.68 m, 149.73 m and 135.21 m, respectively. Pore pressures and displacement area also increase as injection pressure increases in a 5 MPa gradient. And V C H 4 in the borehole decrease by 35.57%, 43.82%, 46.78% and 51.72% from 10 MPa to 25 MPa, respectively. With the increase of injection duration, the influence range expands significantly but the pore pressure distribution changes little. And V C H 4 decreases slightly in the displaced area and the displacement velocity gradually slows down. This technique promotes multiphase flow migration in coal seam, and increases the displacement range and methane recovery efficiency.

Evaluation of carbon dioxide geological sequestration potential in coal mining area

Coal bases are the concentration areas of coal exploitation and use in China in which the carbon emission sources are concentrated, and carbon emission reduction is challenging. However, the carbon emission sources and geological sequestration sinks typically correlate well with coal bases, providing favorable conditions for clustering carbon capture, utilization, and storage (CCUS). CCUS is the only way for coal bases to achieve near-zero carbon emissions in the future. This study established the constitutive equations of CO2 sequestration capacity for unexploited deep coal seams, residual coal in producing and abandoned mines, and mined-out areas based on their CO2 sequestration form, geological background, and coal mining status. Then, the geological sequestration potential of one of the most important coal bases in east China was discussed according to the constitutive equations of CO2 sequestration capacity by considering matching relationships between sequestration sources (coal-fired power plants) and sinks. The results showed that the coal base has excellent CO2 geological sequestration potential. The total CO2 sequestration capacity in coal seams in the coal base is approximately 0.85 billion tons, including 0.76 billion tons in unexploited deep coal seams, 5.17 million tons in residual coal in producing and abandoned mines, and 82.45 million tons in mined-out areas. The source–sink matching relationships of CO2 geological sequestration in the coal base provide favorable conditions for CCUS, and the average distance between the CO2 sequestration sources and target sequestration sinks is only 10 km. The unexploited deep coal seams with buried depths of 1000–1500 m are primary reservoirs for CO2 geological sequestration in the coal base. In the next 30 years, CO2-enhanced coal bed methane recovery (ECBM) in unexploited deep coal seams and CO2 sequestration in mined-out areas should become a special CCUS technology in the coal base due to their great CO2 geological sequestration potential. CO2-ECBM and CO2 sequestration in mined-out areas are also significant for other coal bases in China.

Optimization of coalbed methane recovery from extraction borehole using novel plastic spraying material: A field application and evaluation

High-quality gas extraction is confronting enormous difficulties due to the leakage of air from the roadway through the coal wall into the extraction system, particularly in the boreholes along the coal seam. In this work, a new set of material and technology for coal wall spraying and sealing was developed to block the air leakage channel and enhance the concentration of extracted gas. The material has excellent air-tightness, resilience, and flame retardancy, and the feasibility of the implementation of the technology was demonstrated by the application in Hexi Coal Mine. The results showed that: the gas-extracted concentration was lower than 10% after 70 days under the conventional borehole sealing method. However, the new material and technology could increase gas-extracted concentration by 26.9∼43.7% with a maximum increase multiple of 4.090 times. Moreover, the average gas-extracted concentration can be increased to 76.7% and maintain a relatively stable state for 80 days. This technology could not only reduce the air leakage, but also strengthen the internal and external pressure difference of the borehole, so that more gas can be extracted at a high concentration, which is expected to provide a reference for the gas disaster control and high-quality gas source guarantee.

Research and application of high-pressure water jets drilling and repairing failure drainage borehole for enhancing coalbed methane recovery in underground coal mines

Research and application of high-pressure water jets drilling and repairing failure drainage borehole for enhancing coalbed methane recovery in underground coal mines