Coal Seam Reservoirs Research Articles

Application of Integrated Directional and Guide Drilling Technology in Deep Coalbed Methane

Abstract Deep coalbed methane is characterized by deep burial, complex and diverse microstructures, significant lateral variations in reservoirs, and the development of internal interlayers. Current seismic exploration methods suffer from severe high-frequency signal attenuation in coal layers, resulting in low seismic resolution, which severely limits the precision of geological models for coalbed methane and the encounter rate of reservoirs in horizontal wells. Based on this, this paper introduces an integrated directional and guide drilling technology and applies it to horizontal well operation, achieving excellent results. Firstly, in the pre-drilling phase, we utilize well-seismic integrated reservoir and structural prediction techniques to meticulously characterize coal seam distribution patterns and subtle geological structures. This enables the construction of a geo-steering model specifically for horizontal wells, optimizing the locations of target points, designing well trajectories, and selecting the optimal configuration of geological steering tools. Secondly, in the drilling operation stage, this system iteratively refines the depth-domain seismic section based on real-time measure while drilling (for short MWD). This process reconstructs the geological steering model, precisely predicts the position of coal seams at the bottom of the well, continuously optimizes the well trajectory design in real-time, and ultimately generates geo-steering instructions. Finally, after the completion of drilling operations, the system evaluates the coal seam area and iteratively refines the three-dimensional geological model. This process provides technical support for the deployment of the next well and the design of the target trajectory. After verification through multiple horizontal wells targeting deep coalbed methane, the guiding model reconstructed by this technology has achieved a fitting rate of over 95%, significantly enhancing the reservoir encounter rate to above 90%. This remarkable achievement underscores the technology’s precision and effectiveness in accurately guiding drilling operations toward targeted coal seam reservoirs.

Phase Behavior of Fluid Composition in Coalbed Methane Wells Pre- and Post-Workover: An Examination of the Panzhuang Block, Qinshui Basin, Shanxi, China

Workover operations significantly impact the service life and gas production capacity of coalbed methane (CBM) wells and are crucial for optimizing resource exploitation. To investigate workover operations’ impact on coal seam reservoirs, the authors designed a series of experiments and obtained the following results: (1) The workover operation induced a phase transition in the solid-liquid composition produced by the CBM well, indicating changes in the coal reservoir’s internal structure. (2) During the stable production stage before and after the workover, the proportion of Na+, Cl−, Ca2+, and Total Dissolved Solids (TDS) in the water samples showed a downward trend as a whole, while the HCO3−; after the workover, the Na+, Cl−, Ca2+, and TDS all increased suddenly, while the HCO3− decreased. (3) While inorganic minerals predominated in the precipitation material during the stable production stage pre-workover, their proportion decreased post-workover, with a noticeable shift in their qualitative composition. (4) It is an indisputable fact that workover operations cause physical and chemical damage to coal seam reservoirs. During workover operation, how to avoid damage and conduct benign reconstruction to the reservoir will be the direction of our future efforts. The experimental results provide valuable insights that can guide the optimization of CBM workover operations and inform the strategic planning of subsequent drainage activities.

Geological engineering double sweet spot evaluation of thin coal seam reservoirs based on facies control: A case study of tight sandstone gas in the Shenfu block of the Ordos Basin

Abstract The development of thin coal layers within the Shenfu tight sandstone reservoirs significantly impacts the control of hydraulic fractures over the actual sweet spots, leading to considerable inter-well production variability. This paper establishes a connection between individual well sweet spots and block sweet spots through a refined lithofacies model and conducts a comprehensive double sweet spot secondary optimization in conjunction with the characteristics of thin coal layer development. The research findings indicate the following: Class I sweet spots ≥0.46, 0.37≥ Class II sweet spots <0.46, 0.3≥ Class III sweet spots <0.37; fractures will be captured by the coal layer if the thin coal layer is more than 5 m away from the perforation point or if the coal layer thickness exceeds 3 m; the secondary sweet spot optimization can eliminate the influence of thin coal layers and establish a three-dimensional comprehensive double sweet spot model suitable for the study area.

Research on high-pressure adsorption of supercritical CO2 and the characterization of coal structure change in anthracite

CO2-Enhanced Coal Bed Methane ( CO2-ECBM) Technology has been widely used in deep coalbed methane (DCBM) extraction and CO2 sequestration. CO2 is in the supercritical state ( ScC - O2 ) in deep coal seam reservoir, and has mechanical, physical and chemical effects on the coal body, making understanding ScCO2 adsorption crucial. Based on the self-developed supercritical isothermal adsorption device, ScCO2 high-pressure adsorption experiments were carried out in anthracite. Besides, using multiple characterisation methods to analyse the changes in microstructures of coal before and after ScCO2 adsorption and reveal the mechanism of processing. The results indicate that an obvious mutation zone in adsorption during high-pressure conditions is carried out, with the minimal adsorption capacity in the mutation zone; 2) The effect of ScCO2 on the microstructure of the coal mainly focuses on the micropores and mesopores (below 50 nm ), resulting in an improved coal adsorption capacity; 3)Providing three assessment indexes of potential of DCBM resource exploitation and CO2 sequestration includes the buried depth, porosity and fracture permeability. The above research outcomes provide important theoretical foundation for the development of DCBM resources, assessment of CO2 sequestration potential and engineering application.

Numerical simulation of fracture propagation in deep coal seam reservoirs

AbstractAs an unconventional oil and gas resource, coalbed methane has become a key area of unconventional oil and gas exploration worldwide, and hydraulic fracturing is the critical technology for the efficient stimulation of coalbed methane. However, due to the strong heterogeneity of deep coal seam reservoirs and the development of natural cleat fractures, the fracture propagation path is quite complex and difficult to capture. In this study, we propose a hydraulic fracture propagation simulation method using the ABAQUS Software for natural cleat fracture networks and a fluid–solid fully coupled two‐dimensional finite element model with the global embedded cohesive zone model (CZM). CZM is established to simulate the propagation of hydraulic fractures in the heterogeneous combination of coal rock and mudstone. The accuracy of this model is verified by using Blanton's experiment criterion. Finally, the model is used to simulate the influence of cleat angle, cleat density, length ratio of coal rock and mudstone combination, and construction displacement on the propagation behavior of hydraulic fractures. This study aims to deepen the understanding of the mechanism of hydraulic fracture propagation in deep coal seams and provides guidance for deep coal seam fracturing and hydraulic fracturing design.

Effects Of Matrix Swelling On Coal Permeability For Enhance Coalbed Methane (Ecbm) And Co2 Sequestration Assessment Part I: Laboratory Experiment

It has been reported that coal matrix swelling/shrinkage associated with CO2, adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2. The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory. Coal and reservoir properties which may impact upon this behaviour will be identified through extensive laboratory testing. This paper – first of two – presents the procedure for the laboratory tests as well as the findings. In the second part, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests, would be developed.

EFFECTS OF MATRIX SWELLING ON COAL PERMEABILITY FOR ENHANCE COALBED METHANE (ECBM) AND CO2 SEQUESTRATION ASSESSMENT PART I: LABORATORY EXPERIMENT

It has been reported that coal matrix swelling/shrinkage associated with CO2 , adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2 . The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory. Coal and reservoir properties which may impact upon this behaviour will be identified through extensive laboratory testing. This paper – first of two – presents the procedure for the laboratory tests as well as the findings. In the second part, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests, would be developed.

Effects Of Matrix Swelling On Coal Permeability For Enhance Coalbed Methane (Ecbm) And Co2 Sequestration Assessment Part Ii: Model Formulation And Field Application

In part II of this two-part paper series, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests presented in part I is presented. It has been reported that coal matrix swelling/shrinkage associated with CO2, adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2. The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate and develop numerical models based on the the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory.

Ecological risk assessment of soil and water loss by thermal enhanced methane recovery: Numerical study using two-phase flow simulation

Thermal enhanced methane recovery inevitably aggravates the soil and water loss, causing severe harm to the sustainability of groundwater environment and the surrounding ecosystem. Therefore, quantitative analysis of the effect of thermal enhanced methane recovery on groundwater loss and ecological risk of coalbed methane development zone is necessary. In this study, a coupling model of gas drainage and groundwater loss is established. The model considers the dynamic gas diffusion of coal matrix, the two-phase flow of water and gas, and the influence of temperature on such flow. Based on this model, characteristics of groundwater loss of coal seam reservoir caused by enhanced methane recovery are analyzed, and the ecological risk assessment of methane recovery zone is realized. Results indicate that during heat injection, the permeability of the coal seam increases with distance from the borehole due to the competition between two-phase flow and temperature. High temperature develops the permeability, gas production, and water production of the reservoir. The change rules of water and gas productions are similar with initial increases and subsequent declines. The influence of coal gas diffusion on groundwater loss has a certain time lag. In the early stage, the dynamic attenuation of gas diffusion is not apparent. In the later stage, the supplement rate of gas from matrix to fracture decreases. The initial saturation has a significant influence on the water production rate in the early stage. A large Langmuir volume constant not only strengthens the peak value of gas drainage rate but also the gas drainage rate itself in the later declining period. Large scale coalbed methane development will face ecological risks such as water environment pollution, habitat destruction and soil degradation, which is the key aspect of ecological environment management and risk prevention.

Production Analysis for Fractured Vertical Well in Coal Seam Reservoirs with Stimulated Reservoir Volume

The development and utilization of coalbed methane is of great significance to reduce carbon dioxide emission. Through the research, this paper presents a fast analytical solution method for the productivity of coalbed methane reservoir with finite-conductivity fractured well and stimulated reservoir volume region. Based on the dual-porosity flowing mechanism, combined with the Langmuir adsorb equation, Fick diffusion law, and Darcy law, a mathematical model considering diffusion in matrix and transport in natural fracture system is established, using spherical matrix to describe the transient steady-state sorption, and using cubic matrix to describe the pseudosteady-state sorption. Then, combined with the inner system and outer system, the analytical solution was obtained. Furthermore, the accuracy of the solution was validated against a numerical simulation. According to the Duhamel principle, the effect of wellbore storage and skin factor was got. Due to the SRV region, the linear flow and radial flow will appear before the pressure wave reach the outer region. And then, based on the pressure analysis result, we will have made the sensitivity analysis with different influence parameter. The result reveals that storage coefficient and conductivity factor mainly influence the early time; the permeability ratio and dimensionless SRV region radius mainly influence the property of SRV region. Finally, the analytical solution of the new model was applied to field history match. This model takes into account the adsorption and desorption characteristics of coalbed methane, as well as the SRV zones generated during fracturing. The calculation speed of the new model is increased while the calculation accuracy is retained, and the intensity of software application is reached. The model achieves the purpose of rapid evaluation and accurate prediction of gas well productivity and obtains a set of productivity evaluation method suitable for coalbed methane reservoir with fractured vertical well, which provides a basis for the development and productivity evaluation of coalbed methane reservoir in domestic and international cooperation.

Influence of In Situ Stress on Well Test Permeability and Hydraulic Fracturing of the Fanzhuang Block, Qinshui Basin

In situ stress is an important factor affecting the permeability and hydraulic fracturing of a coal seam reservoir. In this study, the in situ stresses of 166 coalbed methane (CBM) wells in the Fan...

Coal seam failure during primary/enhanced gas production: How failure develops in fields

Coal failure has been observed in many coal seam reservoirs during gas production, which generated significant impacts to coal permeability and to the efficiency of gas recovery. In this paper we develop a flow-geomechanical-coupled numerical approach and use two genuine field examples to discuss the relevant large-scale coal failure behaviours in fields. One example is with the CBM production in Cedar Hill reservoir, San Juan Basin; and the other one is with the CO2-injected ECBM process in Allison Unit, San Juan Basin. Major observations from the present work include: 1) Laboratory tests and analytical analyses based on uniaxial strain conditions may have considerable deviations in prediction of the coal failure during gas production in fields, because such a condition would not hold there because of non-uniform pressure distributions in the field due to gas production/injection. 2) Field-scale coal strength parameters (e.g., the friction angle) should be remarkably weaker than its counterparts measured with core samples because, if otherwise, the coal failure that has been observed would not occur. 3) In CBM processes, coal failure usually occurs first in the immediate vicinity of a wellbore, and then may expand to the whole field with continuing gas depletion. 4) ECBM processes can have complex strain history including inelastic coal failure due to primary production as well as elastic unloading due to CO2 injection.

Investigation of Hydraulic Fracturing Crack Propagation Behavior in Multi-Layered Coal Seams

Coalbed methane is not only a clean energy source, but also a major problem affecting the efficient production of coal mines. Hydraulic fracturing is an effective technology for enhancing the coal seam permeability to achieve the efficient extraction of methane. This study investigated the effect of a coal seam reservoir’s geological factors on the initiation pressure and fracture propagation. Through theoretical analysis, a multi-layered coal seam initiation pressure calculation model was established based on the broken failure criterion of maximum tensile stress theory. Laboratory experiments were carried out to investigate the effects of the coal seam stress and coal seam dip angle on the crack initiation pressure and fracture propagation. The results reveal that the multi-layered coal seam hydraulic fracturing initiation pressure did not change with the coal seam inclination when the burial depth was the same. When the dip angle was the same, the initiation pressure linearly increased with the reservoir depth. A three-dimensional model was established to simulate the actual hydraulic fracturing crack propagation in multi-layered coal seams. The results reveal that the hydraulic crack propagated along the direction of the maximum principal stress and opened in the direction of the minimum principal stress. As the burial depth of the reservoir increased, the width of the hydraulic crack also increased. This study can provide the theoretical foundation for the effective implementation of hydraulic fracturing in multi-layered coal seams.

Experimental investigation of effects of CO2 injection on enhanced methane recovery in coal seam reservoirs

Enhanced coalbed methane (ECBM) recovery performed by injecting CO2 into coal reservoirs has been found to be a viable option for methane recovery, while CO2 injection into coal seams, especially unmineable ones, has great potential to sequestrate large volumes of CO2 to mitigate greenhouse gas accumulation in the atmosphere. This study was therefore initiated to investigate the effect of CO2 flooding on ECBM recovery and CO2 storage in coal. A series of CO2 core flooding tests was carried out on a high rank bituminous coal core with CO2 injection pressures from 6 to 10 MPa. CO2 injection rate, methane and CO2 production rates, methane displacement efficiency and the volumetric strains of the core were calculated and recorded to enable interpretation of the results. It was observed that, compared with natural methane production in which only around 51.73% of methane is recovered, CO2 injection provides excellent methane displacement efficiency (over 90%), especially for high injection pressures (greater than 8 MPa), which drive methane completely out of the coal sample. Although higher CO2 flooding pressure favours greater CO2 injection rates and methane production rates for any coal types, high rank coal shows faster and greater methane production and CO2 storage capacity. The observed rapid breakthrough of CO2 in the gas produced by elevated CO2 injection, especially for high injection pressures, may complicate the gas separation process. Adsorption-induced volumetric strain is approximately proportional to the CO2 storage capacity in coal which increases with CO2 pressure, and the strain reduces coal permeability and therefore CO2 injectivity and methane production.

Experimental and numerical evaluation of CBM potential in Jharia Coalfield India

Geophysico-mechanical characterization of coal data are important in the economic success of CH4 extraction as well as a CO2 injection in deep coal seam reservoir. The heterogeneous nature of coal makes the CH4 removal quite challenging because of the complex behaviour of the seam at in situ as well as applied stress level. Coal matrix behaviour depends on several parameters as permeability, porosity, pore pressure, gas content, structural features, etc. plays a leading role in methane extraction. Therefore, extensive laboratory investigation is handiest approached to anticipate the behavior of coal effectively. This paper presents the results of coal characterization, gas permeability, adsorption/desorption capacity of coal as well as the performance of CBM production well in the replicated model of JH-MD-XVI-T coal seam at a depth of 580 m. The coal characterization was determined to evaluate the prospects of methane in the study area. The gas permeability was determined in a triaxial experimental set up using Darcy’s approach to in situ conditions. The decrease in permeability with an increase in confining as well as gas pressure was observed in all coal samples due to the crushing of grain, coal deformation and narrowing of fractures as well as cleats leading to hinder the flow of fluid through it. The well performance was evaluated to determine the gas rate as well as cumulative gas volume over twenty-five years of well life. Mutual relation between permeability, in situ confining pressure as well as gas pressure, has been established statistically.

The variation in produced gas composition from mixed gas coal seam reservoirs

This paper investigates predictive modelling of gas production from coal seam reservoirs that contain mixtures of CO2 and CH4. A series of laboratory experiments are presented that are analogues of the gas production process where gas is drained at a controlled rate from a coal core sample with a known initial gas content and composition. The experiments were performed at two initial gas compositions and for two outflow rates. There was excellent agreement in the mass balance of the experiments with the difference between the initial gas volume and cumulative outflow ranging between 0.8 and 1.8% of the initial volume. During the early stages of gas desorption the CO2 concentration of the outflow gas was significantly lower than that in the adsorbed gas but increased significantly at low pore pressures. Thus, the CH4 was preferentially desorbed relative to CO2. For example, in an experiment in which the initial gas content of the coal was ~30% CO2, the CO2 concentration of the produced gas was only ~13% and did not increase significantly until the pore pressure fell below 0.8 MPa, ultimately increasing to ~95%. A reservoir simulator was used to match the desorption observations, and the results indicate, in a finding consistent with previous work, that the extended Langmuir adsorption model based on pure gas isotherms does not provide accurate predictions. A modified extended Langmuir approach was developed, based on using measurements of the initial gas content and its composition, provided a more accurate fit to the experimental observations of gas desorption. A fully predictive procedure using the 2D Equation of State adsorption model with the pure gas adsorption isotherms provided good agreement with gas desorption observations. A series of hypothetical reservoir simulation case studies are presented that illustrate changes of CO2 concentration with time for a variety of initial compositions.

Reduced Order Modelling for Estimating CO2 Storage and Enhanced Coalbed Methane of Unconventional Coal Seam Reservoirs

Carbon capture, utilization, and storage (CCUS) is a promising technology for mitigating climate change while providing opportunities for the continued use of traditional fuel sources. While difficulties remain for employing carbon capture technologies at a competitive cost, the injection of carbon dioxide (CO2) into underground storage reservoirs is a well-understood process. Still, continuing the development of resource storage estimates for methane (CH4) production and CO2 storage in hydrocarbon reservoirs at a regional scale is an important aspect for incentivizing industrial-scale implementation of CCUS technologies. There are multiple methods for developing CO2 storage and utilization resource estimates with varying degrees of uncertainty. Although static volumetric models can provide quick reservoir estimates, they necessarily include high levels of uncertainty. On the other side of the spectrum, dynamic reservoir simulations can reduce the uncertainty but require significant resources of time and data that may not be available. The Sequestration of CO2 Tool for Enhanced Coalbed Methane (SCO2T-ECBM), a reduced-order model developed in this study, utilizes sensitivity and uncertainty analyses of dynamic reservoir simulations to generate proxy models for estimating both CO2 storage and enhanced coalbed methane (ECBM) resources in unconventional coal seam reservoirs. We compare CO2 storage and utilization resource estimates from SCO2T-ECBM with regional estimates from static, volumetric models. The results indicate that static models may be overly optimistic, particularly in the mid- and high-range estimates. This result is consistent with other studies that have compared static estimates with dynamic simulations, and may suggest a re-evaluation of CO2 storage and utilization resource estimates is needed. SCO2T-ECBM has promise as a useful tool in generating CO2 storage and utilization resource estimates to aid in the deployment of CCUS technology.

Experimental investigation of CO2 injection into coal seam reservoir at in-situ stress conditions for enhanced coalbed methane recovery

CO2 geological sequestration is an effective method to reduce the concentration of CO2 in the atmosphere. The injection of CO2 into CH4-containing coal seams also named CO2-ECBM (CO2 Enhanced Coalbed Methane Recovery), allows the storage of CO2 and the enhancement of CH4 recovery. Due to complex geological environment of the site, dynamic variation of CH4 and CO2 can hardly be monitored in real time during CO2 injection into coal seams. Therefore, this paper conducted experimental studies were carried out on cuboid coal samples with a size of 300 ∗ 70 ∗ 70 mm via a self-developed experimental device which can simulate in-situ stress and temperature conditions. In the experiment, different adsorbed gases (CH4 and CO2) were applied to determine the variation of permeability values under various load stresses and pore pressures. Results proved that the coal had greater adsorption capacity to CO2 than to CH4. The injected CO2 occupied the original adsorption site of CH4, thus leading to the rapid discharge of CH4. Meanwhile, real-time dynamic monitoring was also performed on gas parameters in coal under different gas injection pressures to obtain variation laws of pore pressure, gas flow rate and concentration at the outlet with the increment of injected CO2. The results reveal that with the increase of injected CO2, pore pressure tended to equilibrium while CH4 flow rate and concentration at the outlet gradually fall to zero. Therefore, CO2 injection into the coal mass can effectively improve the recovery of CH4. In addition, variations of gas flow rate and concentration at the outlet with the displacement ratio of gas volume under different gas injection pressures suggested that the enhancement of gas injection pressure boosted discharge efficiency of CH4 from the coal mass and CO2 consumption. Furthermore, inspired by the competition relationship between engineering efficiency and CO2 consumption, we think a reasonable and effective economic cost model can be established as an effective method to guide the selection of gas injection pressure in the future. Hence, the experimental results have guiding significance for better understanding and application of CO2-ECBM technology.

Coalbed methane reservoir characteristics of coal seams of south Karanpura coalfield, Jharkhand, India

Coalbed methane has emerged as a viable natural gas resource in India since 2007. The understanding of gas genesis kinetics, storage mechanisms and the geological controls is vital for exploration and successful recovery of gas in a cost-effective manner. In this respect, a multidisciplinary analytical approach including gas content, stable isotopes (δ13C1), hydrocarbons distribution, reconstruction of original organic matter, chemical properties, sorption kinetics and the role of petrographic constituents have been assessed. The volatile matter content varies from 20.2 to 32.1 wt%; indicating medium volatile to high volatile bituminous rank of coal with the maximum vitrinite reflectance (VRo%) ranging from 0.63 to 0.98%. The studied coals are vitrinite rich and contains vitrinite group macerals in the range of 41–65 vol%. Dendritic micro-fractures are confined to vitrite microlithotype, and secondary mineralization helps preserve fracture connectivity. The average gas content of coal seams is 2.06 cc/g. The increase in gas content values per 100 m is about 0.38 cc/g emphasizing escape and migration of hydrocarbons during restructuring and tectonic activities in the basin. The relationship of C3/C1 and C2/C1 ratios is indicating that the hydrocarbons in desorbed gas originated from the thermogenic process. The large concentration of methane in desorbed gas and its stable isotope signatures (δ13C1 < −40‰) also indicates the thermogenic origin of gases in coal seams. Few desorbed gas samples have been found in the mixed gas region, which may be due to the influx of fresh water carrying bacteria leading to the generation of biogenic gases and subsequent mixing with the sorbed thermogenic gases. The values of S1 (0.19–0.78 mg HC/g) and S2 (10.31–25.63 mg HC/g) indicate excellent source rock potentials. The positive correlation of original hydrogen index (HIO) with present-day hydrogen index (HIPD) and original organic content (TOCO) with present-day organic content (TOCPD) suggests uniform consistency in organic carbon conversion to hydrocarbons.The gas content compared with sorption capacity reveals the undersaturation of coal seams. The experimental results summarize that the low gas content is a critical issue. Similarly, the low, quality and quantity of CH4 may affect the exploration and recovery of methane in the long term into this basin. But, the low values of sorption time (0.12–6.65 days) signifies good diffusion characteristics that may support the recovery of gas. Finally, it is concluded that depth of occurrence, maturity and pore/cleats associated with microconstituents control the gas accumulation and transport in coal seam reservoirs of South Karanpura coalfield.

Experimental investigation of fracture propagation induced by carbon dioxide and water in coal seam reservoirs

Because of the low or ultra-low permeability of coal seams, hydraulic fracturing technology has been widely used to improve the permeability of coal seams in both ground and underground extraction. Recently, liquid carbon dioxide (L-CO2) as an environment-friendly fracturing fluid has drawn attention from researchers and engineers. In this study, a true tri-axial system was adopted to perform hydraulic and L-CO2 fracturing experiments on coal specimens from the South Sichuan Basin in China. The effects of geological and engineering factors (horizontal stress ratio, injection rate, and fracturing fluid) on induced fracture propagation were investigated in this study. The various geological and engineering factors can synthetically, rather than unilaterally, impact the fractures propagation to generate three types of stimulation mechanism. Based on our experimental results, we succeeded in approximately predicting the stimulation mechanism for induced fracturing under different conditions. Generally, the effect of pre-existing natural fractures on the fracture propagation was enhanced, whereas the effect of the geo-stress was reduced, when L-CO2 was adopted as the fracturing fluid.