Coalbed Methane Development Research Articles

Evaluation of recoverable potential of deep coalbed methane in the Linxing Block, Eastern Margin of the Ordos Basin

The deep coalbed methane (CBM) resources are widely developed in the Linxing Block. However, the evaluation of CBM geological areas suitable for CBM exploitation remains unexplored, hindering further development. This research optimizes the key geological parameters that influence the development of deep CBM from the perspectives of resource and development conditions. The evaluation system for deep CBM recoverability has been established, and the multi-fuzzy evaluation method has been used to perform the quantitative evaluation of recoverability. The results indicate that the resource conditions of No.8 + 9 coal seam are superior to those of No.4 + 5 coal seam. Favorable resource conditions are predominantly concentrated in the northeast and specific southern portions of the research area. Favorable development conditions for both coal seams are mostly concentrated in the northeastern area. Based on the classification standard of recoverable favorable areas, the Level II area is crucial for the development of No.4 + 5 coal seam. This area is primarily distributed in the northeast of the research area., Both Level I and Level II areas for the No. 8 + 9 coal seam are situated in the northeast. The Level III area is earmarked for deep CBM production and shows potential for exploration. Further analysis reveals that the resource conditions in the favorable area are generally superior to the development conditions. These areas are classified as Class A, including categories such as I-A, II-A, and III-A, indicating relatively complex reservoir transformation.

Investigation on the Nanopore Heterogeneity in Coals and its Influence on Methane Adsorption: A Multifractal Theory Study.

Coalbed methane (CBM) reservoirs constitute a distinct class of dense organic rocks characterized by extremely low porosity and permeability. Conducting an in-depth investigation into pore heterogeneity assumes paramount importance for the exploration and development of CBM. This study focuses on the multifractal analysis of the pores with diameters below 300 nm in six coal samples sourced from the Junggar Basin and the Qinshui Basin in China. The analysis is based on a series of experiments involving CO2 adsorption, low-temperature N2 adsorption/desorption, and CH4 isothermal adsorption. This work delves into the influence of pore heterogeneity on gas adsorption capacity by linking the structural parameters to CH4 adsorption properties. The results indicate that both the micropores, as assessed through CO2 adsorption, and mesopores to macropores, measured via N2 adsorption, exhibit multifractal behavior. In contrast to micropores, the mesopores and macropores display stronger heterogeneity and lower connectivity. Generally, uniform and well-connected nanopores are anticipated to positively contribute to gas adsorption. However, there is a positive correlation between the Langmuir volume and the heterogeneity degree of micropores. This phenomenon is ascribed to the fact that the greater surface complexity in micropores involves a larger specific surface area and a higher abundance of adsorption sites. This research contributes to a more profound and precise comprehension of the heterogeneous pore structure within CBM reservoirs, thereby establishing a theoretical foundation for the sustainable exploitation of CBM.

Evaluation of Favorable Area for Coalbed Methane Development Based on Local Variable Weight Theory.

The evaluation of a favorable area is crucial for the exploration and exploitation of coalbed methane (CBM) resources. In traditional evaluation methods, the weight of controlling factors for the evaluation of favorable area is often obtained from different models and calculation methods, and the constant weight is commonly used in the entire target area. The influence of the index value of controlling factors and the combination state of these values on the weight is consistently overlooked during the evaluation process. In view of this phenomenon, a new evaluation method based on variable weight theory was introduced to enhance the accuracy of the result from evaluation (i.e., favorable area for CBM development) in this paper. Based on the raw data of controlling factors, the evaluation area was divided into a finite number of regular grids; each grid could be seen as an evaluation unit, and different attribute values were assigned to them. The constant weights are determined by the analytic hierarchy process (AHP), while the variable weight of controlling factors in each unit was calculated by a partitioned variable weight model (VWM) which constructed based on variable weight theory. Finally, the VWM for the evaluation of favorable area was constructed and applied in the Weibei CBM field. The influence of variability in index values on the weight was taken into consideration in this model, which can complement the disadvantage of the constant weight model (CWM). The accuracy of the result from the evaluation of favorable areas for CBM development could be improved by using this VWM, which provides a reasonable idea and method for the selection of target areas in CBM fields.

Extreme massive hydraulic fracturing in deep coalbed methane horizontal wells: A case study of the Linxing Block, eastern Ordos Basin, NW China

Deep coal seams show low permeability, low elastic modulus, high Poisson’s ratio, strong plasticity, high fracture initiation pressure, difficulty in fracture extension, and difficulty in proppants addition. We proposed the concept of large-scale stimulation by fracture network, balanced propagation and effective support of fracture network in fracturing design and developed the extreme massive hydraulic fracturing technique for deep coalbed methane (CBM) horizontal wells. This technique involves massive injection with high pumping rate + high-intensity proppant injection + perforation with equal apertures and limited flow + temporary plugging and diverting fractures + slick water with integrated variable viscosity + graded proppants with multiple sizes. The technique was applied in the pioneering test of a multi-stage fracturing horizontal well in deep CBM of Linxing Block, eastern margin of the Ordos Basin. The injection flow rate is 18 m3/min, proppant intensity is 2.1 m3/m, and fracturing fluid intensity is 16.5 m3/m. After fracturing, a complex fracture network was formed, with an average fracture length of 205 m. The stimulated reservoir volume was 1 987×104 m3, and the peak gas production rate reached 6.0×104 m3/d, which achieved efficient development of deep CBM.

A Method for Evaluating Coalbed Methane Reservoir Productivity Considering Drilling Fluid Damage

In the process of coalbed methane development, drilling fluid and fracturing fluid cannot achieve absolute compatibility with formation. The incompatibility between the working fluid and reservoir will lead to the intrusion of working fluid into the reservoir and cause reservoir pollution. This is a very common phenomenon. There is a large amount of pulverized coal in the coal seam, and the intrusion of working liquid will be combined with the pulverized coal to form cement to block the seepage space in the reservoir. Since pressure relief and fracturing fluid backflow will be performed at the first time after fracturing, the intrusion range of the working fluid is small, generally reaching 10 m to 50 m. Compared with a conventional gas reservoir or shale gas reservoir, the working fluid loss during CBM development will seriously affect the subsequent production project and even make the gas well lose production capacity. On the other hand, in order to avoid this phenomenon, measures such as acidification or volumetric fracturing are sometimes used to improve the seepage environment near the well and near the fracture. The purpose of this study is to quantitatively evaluate the impacts of working fluid filtration and reservoir reconstruction on production. In this study, a single well productivity evaluation model and sensitivity analysis method considering drilling fluid filtration loss, fracturing fluid filtration loss, reservoir reconstruction and other processes is proposed. The formation mechanism of fluid loss during drilling and fracturing is described, and the productivity evaluation model considering fluid loss is combined with the Langmuir isothermal adsorption equation, steady-state diffusion law, Darcy’s seepage law and Duhamel convolution formation. Combined with the distribution of actual gas reservoir flow characteristics, the sensitivity of single well productivity to gas reservoir porosity, gas reservoir permeability, coal seam adsorption coefficient, working fluid filtration loss and reservoir reconstruction measures are analyzed. Through the analysis and fitting of the actual production data on site, the relationship curve can better fit the field production data, and the evaluation results are in line with the drilling and fracturing conditions at that time and the subsequent production conditions, with small errors. The obtained method is suitable for predicting the productivity of fractured vertical wells in different working conditions and provides a basis for the development and productivity prediction of CBM reservoirs in China and in international cooperation.

A Fully Coupled Gas–Water–Solids Mathematical Model for Vertical Well Drainage of Coalbed Methane

The coupling relationship between the deformation field, the diffusion field, and the seepage field is an important factor in fluid transport mechanisms in the long-term coalbed methane (CBM) exploitation process. A mathematical model of gas–water two-phase fluid–structure coupling in a double-porosity medium in coal reservoirs is established in this paper. Taking Hancheng Block, a typical production block in Qinshui Basin, as the geological background critical desorption pressure, reservoir permeability anisotropy is considered in the model. COMSOL Multiphysics (COMSOL_6.0) was used to create the model. The accuracy and rationality of the model were verified by comparing field production data with the results of the simulation. Using the simulation, the influence law of various reservoir geological characteristics parameters (Langmuir strain constant, ratio of critical desorption pressure to reservoir pressure of coal seam (CDPRP), elastic modulus, initial water saturation, Langmuir pressure, etc.) on CBM productivity, reservoir pressure, and permeability ratio was discussed, and a thorough analysis of the factors affecting productivity was obtained using the orthogonal test method. The findings of this study indicate that the change in permeability is the result of the superposition effect of many factors. Different stages of drainage have different primary regulating factors. Rock skeleton stress has a consequence on coal matrix permeability in the early drainage stage, and coal matrix shrinkage is primarily impacted in the later drainage stage. Besides the initial water saturation, other reservoir geological parameters (e.g., CDPRP, Langmuir volume, Langmuir strain constant, elastic modulus) have a strong relationship with productivity. When the value of coal geological parameters increases, the degree of productivity release is higher (as the initial water saturation increases, the production decreases correspondingly). Different coal and rock parameters have varying levels of impact on the drainage stage of CBM wells. The influences of the CDPRP, Langmuir volume, Langmuir strain constant, and elastic modulus on gas production are mainly concentrated in the initial and intermediate drainage stages and begin to fall off during the last drainage stage. Per the multi-factor analysis, the main coal–rock parameters affecting the productivity release are the Langmuir strain constant, followed by the CDPRP and other parameters. The analysis findings can offer theoretical guidance for CBM well selection and layer selection and enhance the block’s overall CBM development level. The improved productivity prediction model for CBM, which is based on fluid–structure coupling theory, can offer a new technical benchmark for CBM well productivity prediction.

Experimental investigation on pore characteristics of vitrain and durain in low rank coal based on fractal theory

The macro petrographic compositions and its pore characteristics of coal reservoir play critical role in the accumulation and development of coalbed methane (CBM). In this paper, the pore characteristics of vitrain and durain were analyzed through the experiment and fractal theory. The results indicated that the micropores and microfractures develop in vitrain, and that transitional pores develop in durain. The pore volume and specific surface area (SSA) of vitrain are larger than those of durain, with the micropore SSA of vitrain being 35% higher than that of durain. The threshold pressure and tortuosity of vitrain are greater than that of durain, but the mean pore size of vitrain is smaller than that of durain. The fractal dimension D1 of vitrain is greater than that of durain, while the fractal dimension D2 is opposite, indicating that the pore surface of vitrain is coarser, and the pore structure of durain is more complex. The fractal dimension Dk of vitrain is larger than that of durain, the mean fractal dimension Ds of vitrain is smaller than that of durain, which shows that the diffusivity of vitrain is weak but the seepage capacity is strong due to the developed fractures. The difference in material composition and pore characteristics between vitrain and durain provides a new understanding for the development of CBM in low rank coal.

Research on the process of coal’s pores and fissures splitting during liquid nitrogen freeze-thaw action

ABSTRACT A clean and effective unconventional fossil fuel is coalbed methane (CBM). The development of CBM is essential to reducing the lack of oil and gas resources, preventing coal mine gas mishaps, and safeguarding the atmosphere. After the coal samples prepared in this study were frozen and thawed by liquid nitrogen, the change rules of coal surface and internal pores were investigated by metallurgical microscopy and carbon dioxide adsorption. The results showed that: 1) The values of specific surface area, characteristic adsorption energy, and total SF microvolume of coal samples increased after freeze-thawing of liquid nitrogen, but the CO2 adsorption experiments showed that the pore width decreased, which indicated that the mechanism of freeze-thawing fracture effect coal by liquid nitrogen mainly increased the number of micropores in coal. 2) The water saturation of coal samples rose with the degree of surface cracking, but when the water content reached a specific level, the rise in surface porosity of coal reduced with the water saturation. 3) The increase in the number of cycles has a greater effect on the extent of expansion of pores on the coal surface than on the expansion of pores in the coal.

Evaluating coal pore structure and gas sorption-diffusion behavior alteration induced by ultrasound stimulation using sorbing tests and matrix diffusion modeling

Coalbed methane development has great significance to ensure the safe and efficient coal mining, clean energy supplementation and carbon emission reduction. Ultrasound stimulation can effectively modify the coal pore/fracture structure and change the gas ad/de-sorption process and diffusion kinetics characteristics, thus altering gas production for coal deformations. This study estimated the pore structure damage and gas sorption-diffusion of coal before and after ultrasound stimulation with gas sorption tests and gas sorption-diffusion modeling. The results show that the maximum nitrogen adsorption capacity increases by 181.2%–218.9% in coal after ultrasound stimulation, and the pore volume and specific surface area of the ultrasound stimulated coals are 1.5–2.0 times and 5.6–7.6 times of those of the raw coals, respectively. Ultrasound stimulation (20 kHz) has a significant modification effect on pores with size of 2∼50 nm. Fractal analysis of pore structure indicates that the pore surface roughness reduces after the ultrasound treatment, and the development of coal pores has become more complex, and the pore heterogeneity has increased with ultrasound stimulation. The sorption tests and gas sorption-diffusion modeling indicate that gas sorption and diffusive transportation increases after the ultrasound treatment, especially in the initial period. In the initial pressure boosting stage (0∼0.5 MPa), the cumulative methane adsorption amount increases by 20.94%, the diffusion coefficient K increases by 17.32%, and the gas adsorption constant b increases by 62.66%, which is attributed to the “pore-forming” and “pore-expanding” effect caused by ultrasound vibration and cavitation. Ultrasonic stimulation technology has a special improvement performance on coal seams with poor matrix gas deliverability, which is beneficial for the life cycle production enhancement of coalbed methane wells.

Key Techniques for Fine Three-Dimensional Geological Modeling of Coalbed Methane

China's proven coalbed methane reserves exceeding 30 trillion cubic meters, and two major coalbed methane industrial bases have been established in the Qinshui and Ordos Basin. Among them, the eastern margin of the Ordos Basin is one of the main blocks for shallow coalbed methane development, holding immense exploration and development potential. Nowadays, the reservoir characteristics of the eastern margin of the Ordos Basin remain unclear, necessitating further research to study reservoir features through fine reservoir description studies. Threedimensional geological modeling can effectively characterize reservoir heterogeneity, especially for predicting geological features between wells and in undrilled gas reservoirs. This study primarily focuses on conducting fine characterization of gas reservoirs and establishing a fine threedimensional geological model through techniques such as multi-attribute fusion in the eastern margin of the Ordos Basin, providing guidance for making and adjustment of development schemes.

Experimental research on settlement and migration law of pulverized coal in vertical well of coalbed methane

In view of the serious problem of pulverized coal production in the process of coalbed methane development, the experimental conditions were set up by using the production characteristics of coalbed gas wells, and the static sedimentation experiments and dynamic sedimentation and migration experiments of pulverized coal particles in the wellbore of coalbed methane wells were carried out. The static settling velocities of pulverized coal particles with different sizes were obtained in the static settling experiment of pulverized coal. Discovery by dynamic transport experiments, when the drainage speed is greater than 0.035m/s, 40-400 mesh pulverized coal will be carried and transported. The research results have important guiding significance for optimizing on-site drainage and mining parameters.

An Advanced Early-Stage Production Forecasting Model for Middle-High Rank Coal Development

Reasonable production prediction of coalbed methane (CBM) is of great significance for improving the economic benefit of CBM reservoirs. Current prediction methods for CBM production focus on the later stages of development, with few studies on early production forecasting. The objective of this work is to provide a reliable new idea for the early production prediction of CBM through various analyses and demonstrations. First, the CBM development modes are classified according to the production characteristics of the Panhe demonstration block of Shaanxi Province, China. Second, an efficient and feasible early production prediction model is established based on the geological potential and development potential. Finally, using the proposed model, different modes’ production characteristics and optimization strategies are analyzed. The research shows that the gas production profiles can be divided into two modes: single-peak mode (SPM) and multipeak mode (MPM). The peak production and average EUR of the SPM are 49.6% and 32.4% higher than those of the MPM, but the stable production period is only 0.2~1 year. In terms of the geological potential of CBM wells, the gas content, critical desorption pressure, and formation coefficient of the SPM are 6.7%, 13.3%, and 37.8% higher than those of the MPM, and the gas wells are mainly located in the high part of the coal seam (the average height difference is about 20 m). Besides, the concept of quasidesorption degree Pdq is innovatively introduced to characterize the development potential of gas well. The Pdq has an exponential relationship with CBM production, and the coefficient of the exponential term in SPM is approximately 22% larger than that in MPM. Moreover, the production of gas wells is greatly affected by the continuity of production. In the process of gas production, the influence of factors such as equipment shutdown should be minimized. To examine the applicability of the proposed method, the model is applied to an actual CBM well in Panhe, and the prediction accuracy is higher than 85%.

Geological and Geochemical Responses to Productivity of CBM Wells in the Baiyang River Block of the Southern Junggar Basin, China

The southern Junggar Basin, Xinjiang, has abundant coalbed methane (CBM) resources. Currently, the Baiyang River development pilot test area (BYR block for short) in the Fukang east block has achieved large-scale CBM development, but the productivity characteristics and its controlling factor are still unclear. Based on the field production data of the BYR block and experimental tests, this paper summarizes the gas and water production characteristics and presents the analysis results of the geological and geochemical responses to the productivity of CBM wells. The productivity of CBM wells in the BYR block was generally characterized as medium-to-low yield. The productivity was jointly controlled by the burial depth, structure condition, thickness and number of co-production coal seams, and hydrogeological conditions. The gas production first increased and then decreased with the increase in the burial depth of the coal seam, and a burial depth between 750 and 1000 m was the most beneficial to increasing the gas production due to the good gas preservation conditions and suitable permeability and stress conditions. The total thickness of the co-production coal seams had a positive effect on the productivity of gas wells, but the productivity was also affected by the number of co-production coal seams and interlayer interference. In the BYR block, the co-production of the nos. 41 and 42 coal seams was the most favorable combination form for CBM drainage. The productivity of CBM wells had a good response to the Na+, K+ and HCO3− concentrations but a poor response to δD-H2O and δ18O-H2O. Based on the concentrations of the main ions and TDSs of the coal seam water, a productivity response index δ* was established, and there was a good positive correlation between the productivity and δ*.

Geological Characteristics of Low-Yield and Low-Efficiency CBM Wells and Practical Measures for Production Increase in the Qinshui Basin.

Many low-production and low-efficiency wells in the Zhengzhuang, Fanzhuang, Lu'an, and Yangquan blocks of the Qinshui Basin seriously hinder the development of coalbed methane in China. Through in-depth research on the geological conditions and development technology of coalbed methane, it was found that the main reasons for the existence of a large number of low-production and low-efficiency wells are the fragmentation of coal structure, poor adaptability of vertical well types, sizable well spacing, and mismatched stimulation measures. On this basis, it is proposed to adopt an L-shaped horizontal well and staged Fracking technology in the block with a complete coal structure but low permeability. For stress-concentration areas, the first fracturing of a vertical well has a single crack and a small coverage area; After a period of drainage and production, the use of repeated fracturing technology can increase the complexity of fractures and increase gas production; For the fractured area of coal structure, the use of roof fracturing technology effectively solves the problem of coal fragmentation. In natural fracture development areas, new fractures are formed using fracture turning technology to achieve the effect of increasing production. The above technologies have achieved good results in the Qinshui Basin engineering experiment. Therefore, in-depth research on the geological conditions of coalbed methane and matching related development technologies are the main ways to solve low-efficiency and low-production wells in coalbed methane development blocks.

Pressure-dependent characteristics of the diffusion coefficient during the gas desorption process in coal cuttings: Numerical modelling and analysis

Determining the diffusion coefficient is crucial in coalbed methane development. Existing models have limitations, as they often apply only to spherical systems, cannot describe nonlinear adsorption, and do not quantify the impact of gas pressure on diffusion coefficients. Additionally, traditional trial methods struggle to accurately compute diffusion coefficients in numerical solutions. To address these challenges, our study builds two unipore diffusion models (EPDDM and PPDDM) by introducing exponential and power pressure-dependent diffusion coefficients, considering three-dimensional coordinates and Langmuir adsorption. And then proposes a diffusion coefficient determination method using a particle swarm optimization algorithm. Through desorption experiments, numerical modelling, and model comparisons, we analyze the evolution law of diffusion coefficient, gas pressure, and desorption volume during the desorption process. Our results show that the proposed models quantitatively depict the positive correlation between diffusion coefficient and gas pressure. Furthermore, they accurately reflect changes in desorption volume. During early-stage desorption, the diffusion coefficient of EPDDM is 1.63–1.85 times greater than that of PPDDM, while their desorption volumes are similar. This observation motivates the proposal of a more versatile Logistic pressure-dependent diffusivity model, aiming to unify the above two models. These achievements are anticipated to serve as a foundational framework for exploring the mechanisms governing gas diffusion.

Controlled mechanism of dynamic behavior of water-based ultrasonic cavitation in coal pore microstructure

Coal pores are sealed as a result of water lock effect, which in turn inhibits coalbed methane (CBM) from desorbing and diffusing. It seriously restricts the efficient development of CBM in the complex geological environment. Relieving pore water lock (RPWL) in coal reservoirs is a scientific problem that puzzles CBM development. Currently, the widely-interested technology of enhancing coalbed methane recovery (ECBM) can not effectively break through the pore water lock. Ultrasonic cavitation can generate shaped charge micro-jets, high temperature, and high pressure, which may be a potential strategy to realize RPWL in coal reservoirs. It has been maturely applied in numerous fields such as medicine, chemical industry, and manufacturing industry. Therefore, water-based ultrasonic cavitation-enhanced coalbed methane recovery (WUC-ECBM) was proposed in this study. This technology has enormous application potential for the efficient and green development of CBM. However, as the theoretical key to the successful application of WUC-ECBM, its RPWL mechanism is still blank. In this paper, X-ray nondestructive testing and fluid invasion methods were used to reconstruct the digital model of coal pore microstructure. Through ultrasonic cavitation numerical simulation, the dynamic behavior mechanism of water-based ultrasonic cavitation was studied in coal pore systems, revealing the microscopic RPWL mechanism of WUC-ECBM. Results show that low metamorphic coal has a larger and fuller pore system, which reflects a stronger topological advantage. Small pores and throats are the main locations of ultrasonic cavitation. This spatial feature is consistent with pore water locking phenomenon. During the formation of a cavitation bubble, ultrasonic energy instantly weakens the surface tension of the water film and activates RPWL. When the cavitation bubble collapses, the micro-jets, high temperature, and high pressure wreak mechanical damage and thermal shock to pore water, accelerating the RPWL process. Ultrasonic frequency affects the cavitation threshold. Increasing ultrasonic power will significantly boost cavitation intensity in pores, particularly in high metamorphic coal. The research results contribute to the green, efficient and intelligent development of CBM in the harsh geological environment, and provide theoretical guidance for WUC-ECBM.

Physical Characteristics and Controlling Factors of Coal Gas Reservoir in Pingdingshan No. 10 Coal Mine

The physical properties of coal reservoirs are the main restrictions to exploration and development of Coalbed methane (CBM). The study of the physical characteristics of coal reservoirs and their controlling factors is of great significance to the safe and efficient development and utilization of CBM resources in the mining area. The Pingdingshan No. 10 coal mine was chosen for this study because of its better gas production effect at the test wells. The reservoir properties of the main coal seams of the No. 10 coal mine were tested and analyzed to comprehensively evaluate the reservoir properties, and the physical characteristics and controlling factors of the coal reservoir in Pingdingshan No. 10 coal mine were studied. The results indicate the following: (1) The gas content and methane purity of the No. 4 coal seam were significantly higher than the No. 2 seam, and therefore they have better development potential than the No. 2 seam. (2) The average adsorption time of the No. 2 coal seam was less than that of the No. 4 seam, making it easier to reach the peak production capacity of CBM wells in the short term, but was not conducive to long-term stable production of CBM. The Langmuir volume of the No. 2 coal seam samples was significantly greater than for the No. 4 coal seam; however, the No. 4 coal seam contains about three times the amount of gas in the No. 2 seam. (3) All three coal seams have high porosity, which was favorable for large amounts of CBM adsorption and storage. Micropores predominated; transitional pores were less frequent, and a few mesopores occurred; macropores were the least common. Samples from the No. 4 seam contained the highest proportion of micropores. (4) Organic pores were common in all the coal samples, with pore diameters not more than 30 µm, mainly concentrated between 50.5 and 1000 nm. Microfractures with apertures less than 70 nm were relatively frequent, mainly in the 50–65 nm range. A large number of the nanoscale microfractures were curved or jagged. (5) Fractures in the No. 2 and No. 4 coal seam samples with widths of 50 nm to 20 µm were more developed, and many were filled with kaolinite, quartz, and other minerals. (6) The samples contained mostly layered silicate minerals (kaolinite), with hard granular minerals (quartz) next, and a very small quantity of iron-type minerals, such as siderite and pyrite in dendritic form. The results of this study can provide a reference basis for the large-scale development and utilization of subsequent CBM wells.

Coalbed Methane Enrichment Regularity and Model in the Xishanyao Formation in the Santanghu Basin, NW China

The Santanghu Basin is a typical low-rank coal-bearing basin in northwest China, with abundant coalbed methane (CBM) resources. However, the understanding of the main controlling factors and reservoir formation models of CBM in low-rank coal is still insufficient, which has restricted the exploration and development of CBM in this region. In this paper, the CBM enrichment controlling factors and enrichment models are analyzed based on the aspects of sedimentary environment, reservoir characteristics, sealing conditions, and hydrogeological conditions after systematically analyzing the geological characteristics of coal measures. The research results indicate that the coal seams of the Xishanyao Formation in the Santanghu Basin are stably developed, with the main macerals being vitrinite and a lower degree of coalification belonging to low-rank coal; the highest content of CBM can reach 7.17 m3/t, and the methane is mainly composed of biogenic gas supplemented by thermogenic gas; the roof lithology of the coal seam is mainly mudstone and siltstone, with good sealing conditions. Finally, two enrichment modes of coalbed methane in slope zones are proposed, namely, the CBM enrichment in the slope zone and the CBM enrichment by fault-hydraulic plugging. The results of this study can serve as a guide for the exploration and development of the deep-buried coalbed methane in the low-rank coal areas.

Characteristics and Influence Factors of Natural Desorption in Coal Bodies from Fukang Mining Area, Xinjiang, China.

Coal body desorption characteristics are one of the key factors that influence the development of coalbed methane (CBM). In this study, 91 coal core samples from 11 CBM wells in the Fukang mining area were collected from Xinjiang, China, and the coal quality, high-pressure mercury compression, gas content, and natural desorption characteristics measurements were launched. With the detailed analyses of the differences in cumulative desorption volume, desorption ratio, and on-site average desorption rate for the coal samples with different body structures and macrolithotypes, the influence of the maximum reflectance of vitrinite, microscopic coal rock composition, and coal quality and pore characteristics on CBM desorption characteristics were discussed. The results showed that the cumulative desorption volume, desorption ratio, and desorption rate of cataclastic structure-bright coal are higher than those of primary structure-semibright coal. With the increase of RO,max and vitrinite content, the adsorption capacity of coal increases, and the increased methane concentration difference during desorption leads to an increase in cumulative desorption volume and on-site average desorption rate. The higher contents of moisture and ash yield would occupy the adsorption sites and hinder gas diffusion, which would decrease the desorption of coalbed methane. The greater porosity/pore volume ratio of medium and large pores can enhance the connectivity of pores, which increases the desorption ratio and the average desorption rate, while the higher micropore porosity/pore volume ratio can increase the gas adsorption space and the cumulative desorption volume. The pore characteristics have the most significant effect on the cumulative desorption volume and desorption ratio. The results of the study can help guide coal mine gas management and CBM development from middle-and low-rank coal reservoirs in Xinjiang.

Ion and Stable Isotope Geochemistry of Produced Water in Coalbed Methane Wells in the Tiefa Basin.

The potential geochemical information in the produced water of coalbed methane (CBM) wells is conducive to the exploration and development of CBM in the case that the produced water is primitive formation water. A total of 58 produced water samples collected from 13 CBM wells in the Daxing Mine, Tiefa Basin, were investigated. Ionic composition tests and stable isotope analysis were conducted to explore the geochemical characteristics and sources of produced water as well as the method for determining whether the produced water is primitive formation water. The results suggest that the fracking fluid for CBM stimulations is the main factor affecting the ion change of the produced water in the initial stage of drainage. The concentrations of Cl- and Ca2+ + Mg2+ could be taken as the indices to identify whether the produced water is primitive formation water. When the Cl- concentration is lower than 20 mEq/L and the Ca2+ + Mg2+ concentration is lower than 1 mEq/L, the produced water is close to the pristine formation water. Biogenic methanogenic activity may result in a high δ13CDIC and high concentrations of HCO3- in the pristine formation water in the Tiefa Basin. The data of δD and δ18O in the study area suggest that the formation water might come from atmospheric precipitation, which is later affected by evaporation and the water-rock reaction. The hydrogen isotope values in the produced water derived from the lower coal group display a substantial elevation compared to those from the upper coal group. This disparity in the hydrogen isotope composition presents an opportunity to utilize δD in produced water as a tool for distinguishing the formation water between these two groups.