Coalbed Methane Development Research Articles

In situ stress variation and coal reservoir permeability response of the Jurassic Yan'an formation in the southwestern Ordos basin, China: Its impact on coalbed methane development

The in situ stress (ISS) and permeability determine the coalbed methane (CBM) development method and extraction efficiency to a certain extent. Based on the data of the CBM well test and microseismic monitoring in the Jurassic Yan'an Formation, a prediction model of the ISS is established, and its influence on permeability and CBM development in the southwestern Ordos Basin, China, is discussed. The results show that the orientations of maximum principal stresses (Shmax) in the study area's eastern, central, and western parts are NE–SW, SEE–NWW, and NNW–SSE, respectively. With the increased burial depth, the main ISS parameters show an S-shaped variation. The study area's ISS state can be divided into Shmax > vertical stress (Sv) > horizontal minimum principal stress (Shmin) (<420 m), Sv > Shmax > Shmin (420 m–800 m), and Shmax ≈ Sv > Shmin (>800 m), and the transformation depth of the ISS field is 550 m and 1000 m. The lateral pressure coefficients (λ) ranged from 0.30 to 1.19, with an average of 0.69. The λ, Shmax/Shmin and Shmin/Sυ of the stress field are small, which indicates that the horizontal tectonic stress is weak. The coal reservoir's permeability is distributed between 0.0084 and 6.84 mD (av. 1.13 mD). Affected by the variation of the ISS state in the vertical direction, the coal reservoir permeability shows obvious zoning. The tensile stress field is beneficial to high-permeability coal reservoir formation, but it is constrained by the coupling constraints of a coal reservoir's confining pressure and effective stress. The coal reservoirs are in a normal fault stress state, which is beneficial for improving their hydraulic fracturing effect. Since the Shmin in the study area is always smaller than the Sυ, the height of the fracturing fracture should be controlled in the fracturing of the coal reservoir to avoid the overlying aquifer being connected by fracturing fractures. The above new insights have important guiding significance for the hyperpermeability CBM reservoir prediction, well-bore structure and hydraulic fracturing design in the Ordos Basin.

Research on influencing factors and remedial measures of coal seam water blocking effect

Coalbed methane development primarily relies on hydraulic fracturing and drainage gas extraction processes. The intrusion of external fluids and long-term drainage and extraction contribute to the water blocking effect, which becomes one of the crucial factors affecting production capacity. In this study, the coal reservoir of a certain coalbed methane in Hancheng, Shaanxi Province, was selected as the research object. The experimental method was employed to comprehensively evaluate the correlation between factors such as coal sample moisture content, permeability, porosity, properties of external liquids, and water blocking damage. The results indicated that there was a positive correlation between coal sample moisture content, liquid surface tension, and water blocking damage. On the other hand, coal sample pore permeability characteristics, liquid contact angle with rock samples, and water blocking damage showed a negative correlation. Building upon the aforementioned research, an investigation into remedial measures for water blocking damage was conducted. Additionally, a chemical treatment study using the compound surfactant system JSS-1 as a water-blocking agent was performed to manage the water blocking effect. The results demonstrated that JSS-1 as a water-blocking agent significantly reduced the surface tension of the wellbore fluid and improved its wetting behavior on the coal-rock surface. The laboratory experiments confirmed the capability of JSS-1 in managing the water blocking effect.

An investigation into the modification of microwave-assisted oxidation in the macromolecular structure of coal via XRD and Raman spectroscopy

The macromolecular structure of coal governs the generation and expansion of micro/mesopores, contributing significantly to the storage and transport of coalbed methane (CBM). To reveal the modification mechanism of microwave-assisted oxidation at molecular scale, this paper presents a comprehensive analysis of the evolutionary characteristics of macromolecular structure of coal using X-ray diffraction and Raman spectroscopy tests. Results show that microwave irradiation triggers the conversion of ordered graphitic crystalline carbon to amorphous carbon, as well as the condensation and heterogeneous spatial arrangement of aromatic clusters. The oxidation of NaClO or Na2S2O8 destroys the initial aromatic structure and reduces the aromaticity of coal, whereas Fenton's reagent oxidation causes the cyclocondensation of macromolecular structure and contributes to the graphitization and crystallization of coal. Microwave-assisted oxidation not only facilitates the directional growth of aromatic rings and the cross-linking of aliphatic chains, but also induces the rotation, displacement, stacking and gradual parallel alignment of aromatic lamellae, thus improving the integrity of three-dimensional lattice of graphite structure and the orderliness of macromolecular structure. As the average size of microcrystalline structural units increases and align directionally to form molecularly oriented domains, the pore size and volume subsequently increases, thereby enhancing the gas transport capacity of coal and the extraction of CBM. This study provides theoretical guidance for the optimisation of microwave-assisted oxidant modification and the paramount consideration of CBM development strategy.

Changes of Permeability and Porosity of Tiefa Anthracite after Treatment with an Acetogenic Bacterium Clostridium sp.

Enhancement of coalbed methane (CBM) production by adding microbes or stimulating indigenous microbes within coalbeds is a promising method. Current research focuses on the study of mixed cultures for improving gas production. However, the application of single purified microbes has only attracted limited attention. In the present work, an acetogenic bacterium, Clostridium sp. was used to treat Tiefa anthracite from Liaoning Province, China. During the treatment, the changes of cell concentration and pH in the treatment solution, mineral components, porosity, surface area, surficial morphology, and permeability of the coal were analyzed. The results showed that pH decreased during the treatment of anthracite by the acetogen. The distance of the carbon layers the coal increased, while the number of aromatic layers (Nc), the packing degree (Lc), and elongation (La) decreased after treatment with the bacterium. Furthermore, compared with the raw coal, the permeability and pore volume increased, which might be improved by the further development of macropores. The present work revealed that acetogenic bacteria Clostridium sp. could change the physical properties of Tiefa anthracite by increasing its permeability and porosity. It offers an alternative method to promote in situ coalbed methane development in the future.

Quantitative Identification of Water Sources of Coalbed Methane Wells, Based on the Hydrogen and Oxygen Isotopes of Produced Water—A Case of the Zhijin Block, South China

The quantitative identification of water sources is an important prerequisite for objectively evaluating the degree of aquifer interference and predicting the production potential of coalbed methane (CBM) wells. However, this issue has not been solved yet, and water sources are far from being completely understood. Stable water isotopes are important carriers of water source information, which can be used to identify the water sources for CBM wells. Taking the Zhijin block in the Western Guizhou Province as an example, the produced water samples were collected from CBM wells. The relationships between the stable isotopic compositions of the produced water samples and the production data were quantitatively analyzed. The following main conclusions were obtained. (1) The δD and δ18O values of the produced water samples were between −73.37‰ and −27.56‰ (average −56.30‰) and between −11.04‰ and −5.93‰ (average −9.23‰), respectively. The water samples have D-drift characteristics, showing the dual properties of atmospheric precipitation genesis and water–rock interaction modification of the produced water. An index d was constructed to enable the quantitative characterization of the degree of D-drift of the produced water. (2) The stable isotopic compositions of produced water showed the control of the water sources on the CBM productivity. The probability of being susceptible to aquifer interference increased with the increasing span of the producing seam combination, reflected in the lowering δD and δ18O values and the decreasing gas productivity. (3) Three types of water, namely, static water, dynamic water, and mixed water, were identified. The characteristic values of the isotopic compositions of the static and dynamic water were determined. Accordingly, a quantitative identification method for the produced water sources was constructed, based on their stable isotopic compositions. The identification results have a clear correlation with the gas production, and the output of the static water contributes to the efficient CBM production. The method for the quantitative identification of the water sources proposed in this study, can help to improve the CBM development efficiency and optimize the drainage technology.

Application of model-based acoustic impedance inversion in the prediction of thin interbedded coal seams: a case study in the Yuwang colliery, Yunnan Province

The Permian coal seams in eastern Yunnan and western Guizhou are thin, numerous, and staggered with other thin coal seams. Depicting the fine characteristics of coal reservoirs is pivotal for the safe and efficient exploitation of coal and coalbed methane (CBM), and is important for transparent mining. To improve the inverse resolution and accuracy of predicting reservoir thickness, this study used the model-based acoustic impedance (AI) inversion method that utilizes seismic and logging data. This method changes seismic data, reflecting stratigraphic interfaces, into AI data, reflecting lithologic structures. Moreover, it avoids the relevant assumptions of wavelets and reflection coefficients. Compared with other inversion methods, model-based AI inversion strengthens the description of thin reservoir horizontal and vertical changes. The results showed that comprehensively using intermediate-frequency seismic information and high-low-frequency logging data greatly broadens the seismic data frequency band and improves the dominant frequency of the reflected wave. Furthermore, the AI profile resolution and the prediction accuracy of the physical parameters for the target geological body can be improved. A cross-validation comparing the inverted thickness and measured thickness of borehole cores was applied to achieve fine prediction (error of appropriately 0.02–0.4 m), providing a basis for CBM development.

Thermal recovery of offshore coalbed methane reservoirs: Flow characteristics of superheated steam in wellbores

Methane in coalbed methane (CBM) reservoirs is mainly in the adsorbed state, and has both free and dissolved states. The proportion of adsorbed methane can be as high as 90% or more. Field practice shows that low methane desorption efficiency is one of the keys affecting CBM development. The adsorption of methane is significantly affected by temperature, and the desorption efficiency increases with the increase of temperature. In this paper, the wellbore flow during the development of CBM reservoirs by injection of superheated steam is taken as the research object, and the superheated steam flow model is established considering the influence of seawater disturbance caused by the phase change of superheated steam. The study found: (a) In the formation section wellbore, the superheated steam has the highest temperature value. (b) The temperature of the superheated steam is affected by both the Joule Thompson effect and the heat loss effect. (c) The longer the wellbore in the seawater section, the lower the temperature of the superheated steam in the wellbore. (d) Superheated steam is only suitable for shallow sea conditions.

Technical advances in well type and drilling & completion for high-efficient development of coalbed methane in China

Compared with the United States, Australia, Canada, and other countries, China's coalbed methane reservoir is featured by low formation pressure, low permeability, low gas saturation, and strong heterogeneity, which not only increases the difficulty of reservoir stimulation but also restricts the single well production and the recovery efficiency of coalbed methane (CBM). In view of this, the research and application of horizontal drilling and completion technology of CBM at home and abroad are reviewed to provide references and theoretical supports for the advancement of the horizontal well engineering technology of CBM in China. This paper introduces a series of horizontal drilling and completion technologies (e.g., the well type design, well structure optimization, well trajectory control, wellbore stability, completion optimization, reservoir protection, etc.). The advanced technologies and their adaptability, including screen pipe completion and stimulation with dual-layer tubular strings, radial horizontal well screen pipe completion and magnetic steering drilling, are dialectically discussed in this paper. In addition, a novel integrated engineering mode for the efficient and green development of deep CBM and coal through the underground “well factory” is proposed. The conclusions are as follows: (1) In view of the geological conditions and surface environment of the coal seam in China, the horizontal drilling and completion technologies of high rank coalbed methane should be further refined, and an integrated development for the coal measure gas is required; (2) More efforts should be made to strengthen the innovative research on horizontal drilling & completion technology for low-medium rank coals and the broken coal, so as to identify the efficient development mode for the complex coalbed methane reservoir; (3) When encountering major technical challenges during the green and efficient development of unconventional coal mines such as deep coal mines, low grade coal mines and thin coal seams in China, it is necessary to explore the development mode via in-situ conversion process based on underground “well factory” to realize the integrated green and efficient development of deep CBM and coal; (4) The high quality transformation with low-carbon and green energy in China will not only ensure national energy security, but also achieve the goal of “carbon peaking and carbon neutrality”.

Dual mechanisms of matrix shrinkage affecting permeability evolution and gas production in coal reservoirs: Theoretical analysis and numerical simulation

Matrix shrinkage is a factor that must be considered in the dynamic permeability model of coal reservoirs. The mechanism of matrix shrinkage affecting confining pressure (confining pressure mechanism) has been modeled by analogy with thermal expansion, and it is widely used in permeability model construction. However, the mechanism of matrix shrinkage affecting porosity (porosity mechanism) has not been widely recognized and modeled, and this mechanism independently controls porosity even though neither confining pressure nor pore pressure changes (only the replacement of different adsorbed gases occurs). The porosity mechanism and a permeability model that takes into account the dual mechanism have been modeled recently. This study compares the two mechanisms of matrix shrinkage by theoretical analysis of the mathematic relations in the permeability models considering different mechanisms and by finite element numerical simulations of coalbed methane development considering different mechanisms. Theoretical analysis shows that the effect of the porosity mechanism on permeability is more than 1.5 times that of the confining pressure mechanism. the numerical simulations results show that: considering the porosity mechanism and the confining pressure mechanism simultaneously allows for a larger and earlier improvement in permeability and a larger reservoir area to improve, and a significant improvement of 28% in gas production rate occurs compared with the case only the confining pressure mechanism were considered. This study reveals the importance of porosity mechanism in describing the dynamic evolution of reservoir permeability and production dynamics accurately, and provides a scientific basis for coalbed methane development.

In-situ stress of coal reservoirs in the Zhengzhuang area of the southern Qinshui Basin and its effects on coalbed methane development

In-situ stress is a critical factor influencing the permeability of coal reservoirs and the production capacity of coalbed methane (CBM) wells. Accurate prediction of in-situ stress and investigation of its influence on coal reservoir permeability and production capacity are significant for CBM development. This study investigated the CBM development zone in the Zhengzhuang area of the Qinshui Basin. According to the low mechanical strength of coal reservoirs, this study derived a calculation model of the in-situ stress of coal reservoirs based on the multi-loop hydraulic fracturing method and analyzed the impacts of initial fractures on the calculated results. Moreover, by combining the data such as the in-situ stress, permeability, and drainage and recovery data of CBM wells, this study revealed the spatial distribution patterns of the current in-situ stress of the coal reservoirs and discussed the impacts of the in-situ stress on the permeability and production capacity. The results are as follows. (1) Under given fracturing pressure, longer initial fractures are associated with higher calculated maximum horizontal principal stress values. Therefore, ignoring the effects of the initial fractures will cause the calculated values of the in-situ stress to be less than the actual values. (2) As the burial depth increases, the fracturing pressure, closure pressure, and the maximum and minimum horizontal principal stress of the coal reservoirs in the Zhengzhuang area constantly increase. The average gradients of the maximum and minimum horizontal principal stress are 3.17 MPa/100 m and 2.05 MPa/100 m, respectively. (3) Coal reservoir permeability is significantly controlled by the magnitude and state of the current in-situ stress. The coal reservoir permeability decreases exponentially with an increase in the effective principal stress. Moreover, a low lateral pressure coefficient (less than 1) is associated with minor horizontal compressive effects and high coal reservoir permeability. (4) Under similar conditions, such as resource endowments, CBM well capacity is higher in primary structural coal regions with moderate paleotectonic stress modification, low current in-situ stress, and lateral pressure coefficient of less than 1.

Experimental investigation of dynamic mechanical characteristics of inhomogeneous composite coal-sandstone combination for coalbed methane development

In deep resource exploitation, the coal seam and the strata are jointly loaded, forming a systematic combined structure that can have a significant effect on coalbed methane (CBM) development. Therefore, to understand the deformation and damage characteristics due to blasting load of the complex and real ground conditions, the 50 mm split Hopkinson pressure bar (SHPB) was used to study the dynamic performance and energy changes of coal-sandstone combination and sandstone-coal combination as inhomogeneous materials. In addition, high-speed photographic equipment was employed to determine the damage failure mechanisms. The results showed that the dynamic compressive strength and failure strains of two combinations showed polynomial relationships with increasing strain rates. The strain rate effects and likenesses of the two combinations' energy and energy rates were substantial. Furthermore, for the coal-sandstone combination, the initial damage fractures occurred at the end face near the bar, and at the interface for the sandstone-coal combination. Eventually, the improved constitutive model based on the ZWT was significantly consistent with the two combinations. The related theory can perform an effective and practical role in the mining of coal rocks under complex ground conditions for CBM development.

Study on the Electrical Control Mechanism of Gas Occurrence in a Microscale Coal Matrix.

The study of the gas occurrence mechanism in a microscale coal matrix is the basis of coalbed methane (CBM) reservoir formation mechanism analysis and its exploration and development scheme design, which has important scientific and engineering significance. Currently, many researchers are focusing on a specific coal type to explore the macroscopic adsorption characteristics of gas occurrence. However, the research on the microscale gas-solid coupling mechanism is relatively rare and the electrical control mechanism of gas occurrence is not reported in detail. This study focuses on the electrical mechanism of microscale gas occurrence using physical simulation experiments and molecular dynamics analysis. This study clarifies the "gas adsorption-electrical properties-functional group" linkage mechanism and explores the macroscopic performance of the microscale gas occurrence mechanism using electrical properties. The study reveals the following: (1) the coal reservoirs exhibit a weak negative potential at the nanoscale, and the trends of surface potential (SP) and surface electrical charging density (SECD) are fluctuated with the degree of coal rank increases; (2) there is a good correlation between the SP, SECD values, and the relative content of functional groups; and (3) the charge density on the coal's microscopic surface influences their gas molecule attraction capacity, affecting the gas adsorption capacity of coal reservoirs at the macroscale. This study presents a theoretical foundation for establishing the molecular force field superposition mechanism of gas occurrence in microscale coal matrix and has broad application prospects in the macroscale numerical simulation of CBM development.

Staged multicluster hydraulic fracture propagation mechanism and its influencing factors in horizontal wells for CBM development

AbstractStaged multicluster fracturing (SMCF) is crucial for the low‐permeability coalbed methane (CBM) reservoirs development. To reveal the fracture propagation mechanism and characteristics in SMCF and select appropriate hydraulic fracturing parameters, this study considers CBM exploitation in the Shizhuang block, Qinshui Basin in China as the engineering background. Then, XSite hydraulic fracturing software based on the discrete lattice theory and synthetic rock mass method is used to construct a three‐dimensional model. The influence mechanisms of the cluster spacing (CS) and lateral pressure coefficient (LPC) on the fractures propagation characteristics were investigated. With an increase in the CS, the stress interference between fractures gradually weakens, and the fracture deflection also decreases. Both the stimulated reservoir area (SRA) and fracture aperture of a single cluster increase with an increase in CS, but the SRA and fracturing efficiency exhibit limited growth when the CS is greater than 15 m. The SRA decreases exponentially with an increase in LPC. When the LPC increases from 0.4 to 1.4, the SRA decreases by 74.2%. The LPC plays a decisive role in the fracture propagation direction. When the LPC reaches 1.4, the fractures propagate along the horizontal direction. With an increase in LPC, the fracturing fluid pressure increases linearly, and the fracturing efficiency decreases exponentially. The results of this study reveal the fracture propagation law of SMCF and can provide a reference for engineering applications. In the SMCF at the 3# coal seam of the Shizhuang block, the CS can be set to approximately 15 m, and the CS can be appropriately reduced when the LPC is large.

Improving the production efficiency of high rank coal bed methane in the Qinshui Basin

There are abundant high-rank coal bed methane (CBM) resources in China, accounting for one third of total CBM resources. Its efficient development and utilization is of great significance to guarantee the national energy strategic security, diminish the hidden danger of coal mine production and reduce carbon dioxide emission. In order to solve the "four lows" problem (i.e., low effective utilization ratio of proved reserves, low productivity targeting ratio, low single-well production rate and low development profit) restricting the development of high-rank CBM industry in China, this paper deeply analyzes the core problems restricting the development of high-rank CBM. Based on this, several new methods of production control, area selection and evaluation are put forward by taking multiple measures, such as paying the same attention on theoretical research and technological research & development, carrying out laboratory research and field test in parallel and conducting large-scale construction and benefit development simultaneously. And the following research results are obtained. First, the geological difference between CBM and coal mine, the difference in reserves recoverability, the adaptability of engineering technology and the scientificity of production are the main factors restricting CBM development effect. Second, "Four-element" production control theory, methane-leading engineering transformation method and methane-leading production control theory are proposed, which provides guidance for the development of a series of technologies for the efficient development of high-rank CBM. Third, in practice, the control degree of quality reserves is increased from 32% to 80%, the success ratio of development wells is increased from 60% to 95%, the average single-well daily gas production of vertical wells is increased by about 1100 m3, the drilling cost of horizontal wells is reduced by 50%, and the operation cost per cubic meter of gas is reduced by 24%. In conclusion, the established technology series for the efficient development of high-rank CBM actively promote the efficient CBM development in the Qinshui Basin. The yearly CBM production of PetroChina Huabei Oilfield Company is expected to reach 20 × 108 m3 in the middle of the "14th Five-Year Plan", which promotes the strategic development of CBM industry in China.

Drilling and completion technologies of coalbed methane exploitation: an overview

Coalbed methane (CBM) drilling and completion technologies (DCTs) are significant basis for achieving efficient CBM exploration and exploitation. Characteristics of CBM reservoirs vary in different regions around the world, thereby, it is crucial to develop, select and apply the optimum DCTs for each different CBM reservoir. This paper firstly reviews the development history of CBM DCTs throughout worldwide and clarifies its overall development tendency. Secondly, different well types and its characteristics of CBM exploitation are summarized, and main application scopes of these well types are also discussed. Then, the key technologies of CBM drilling (directional drilling tools, measurement while drilling, geo-steering drilling, magnetic guidance drilling, underbalanced drilling and drilling fluids), and the key technologies of CBM completion (open-hole, cavity and under-ream completion, cased-hole completion, screen pipe completion and horizontal well completion) are summarized and analyzed, it is found that safe, economic and efficient development of CBM is inseparable from the support of advanced technologies. Finally, based on the current status of CBM development, the achievements, existing challenges and future prospects are summarized and discussed from the perspective of CBM DCTs.

Coalbed methane reservoir dynamic simulation comparisons using the actual steeply inclined model and ideal steeply inclined model

Numerical simulation is an efficient method to quantitatively describe the reservoir dynamics of coalbed methane (CBM) reservoirs. The ideal steeply inclined model (ISIM), assumed to be a steeply inclined plate, has been widely applied in steep coalbed methane reservoir modeling, although the ISIM cannot accurately reflect the actual reservoir geological conditions. In this paper, the dynamics of CBM production and reservoirs using the ISIM and actual steeply inclined model (ASIM) were compared, taking the steep coal in the Fukang mining area located in northwestern China as an example, with the purpose of revealing the differences and applicability of the ASIM and ISIM. The ASIM and ISIM were established by Petrel software, and CBM production was matched and predicted by Eclipse software. Data reflecting reservoir dynamics, such as water saturation, reservoir pressure, and gas content, were extracted. The dynamic changes in the reservoir physical properties of the ASIM and ISIM were also compared. The results showed that: 1) multiple gas production peaks occurred in both ASIM and ISIM. The maximum daily gas production of ASIM occurred earlier than the maximum daily gas production of ISIM. The peak gas production and cumulative gas production of ASIM were both greater than the peak gas production and cumulative gas production of ISIM. 2) Due to the variations in grid shape and dip angle with each grid in the ASIM, the production effect of the ASIM was better than the production effect of the ISIM in the third stage (4–10 years) of drainage. 3) In the third stage (4–10 years) of drainage, the decrease rate of reservoir pressure of ASIM was larger than the decrease rate of reservoir pressure of ISIM because of the relatively better production performance of ASIM. 4) Differentiation of gas and water dominated the variation trend of gas content, and in the third stage (4–10 years) of drainage, the ASIM has higher recovery efficiency compared with ISIM. Compared with ISIM proposed by previous scholars, the ASIM was more helpful to monitor the dynamic behavior of coal reservoirs, and ASIM can provide a more reliable basis for guiding coalbed methane development.

Impact factors and contributions of commingled production in multiple coal seams

AbstractThe multilayer development of coalbed methane (CBM) is affected by many factors, and the gas production contribution of each seam is unclear. In this study, 12 geological and 10 engineering parameters were adopted to reveal the key factors affecting the combined‐layer drainage productivity of CBM wells before using numerical simulation to clarify the contribution of each coal seam. The results show that the coal reservoir in the Laochang mining area exhibited the advantages of high gas content, intact coal body structure, and large thickness of the combined coal seams, and it was deficient with low saturation and ratio of critical desorption pressure to reservoir pressure. The medium permeability after fracturing and the influence of a small range of fracturing leads to the steep and deep pressure drop funnel of the coal reservoir, and most of the reservoir pressure drop is ineffective; this may be the reason for the unstable production and rapid attenuation of most CBM wells in the study area. The gas production of the Nos. 13 and 18 coal seams is stable and makes a significant contribution, and they can be used as the key objects of follow‐up CBM exploration and development in this area. Geological factors, such as coal body structure, ratio of critical desorption pressure to reservoir pressure, thickness of combined coal seams, and gas saturation, as well as engineering factors, such as permeability and half length of fractures after fracturing, are the main factors affecting the productivity of the CBM wells in the Laochang mining area.

Impact of Coal Orthotropic and Hydraulic Fracture on Pressure Distribution in Coalbed Methane Reservoirs

Coalbed methane (CBM) shows tremendous in situ reserves, attracting a great deal of research interests around the world. The efficient development of CBM is closely related to the dynamic pressure distribution characteristics in the coal seam. As the dominant component of the geological reserve for CBM, the adsorption-state gas will not be exploited until the local coal pressure becomes less than the critical desorption pressure. Therefore, although the CBM reserve is fairly large, the production performance is generally limited, with a poor understanding of the dynamic pressure field during the CBM production. In this work, in order to address this issue properly, the coal’s inherent properties, the coal’s orthotropic features, as well as artificial hydraulic fracturing are considered, all of which affect pressure propagation in the coal seam. Notably, to the current knowledge, the impact of coal’s orthotropic features has received little attention, while the coal’s orthotropic features are formed during a fairly long geological evolution, changing the dynamic pressure field a lot. Numerical simulation is performed to shed light on the pressure propagation behavior. The results show that (a) coal’s orthotropic features mitigate the depressurization process of CBM development; (b) the increasing length of a hydraulic fracture is helpful for efficient decline in the average formation pressure; and (c) there exists an optimal layout mode for multi-well locations to minimize the average pressure. This article provides an in-depth analysis upon pressure distribution in CBM reservoirs under impacts of coal orthotropic feature and hydraulic fractures.

Experimental analysis of pore structure and fractal characteristics of soft and hard coals with same coalification

Accurate and quantitative investigation of the physical structure and fractal geometry of coal has important theoretical and practical significance for coal bed methane (CBM) development and the prevention of dynamic disasters such as coal and gas outbursts. This study investigates the pore structure and fractal characteristics of soft and hard coals using nitrogen and carbon dioxide (N2/CO2) adsorption. Coal samples from Pingdingshan Mine in Henan province of China were collected and pulverized to the required size (0.20–0.25 mm). N2/CO2 adsorption tests were performed to evaluate the specific surface area (SSA), pore size distribution (PSD), and pore volume (PV) using Braunuer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and Density Functional Theory (DFT). The pore structure was characterized based on the theory of fractal dimensions. The results unveiled that the strength of coal has a significant influence on pore structure and fractal dimensions. There are significant differences in SSA and PV between both coals. The BJH-PV and BET-SSA obtained by N2-adsorption for soft coal are 0.029–0.032 cm3/g and 3.523–4.783 m2/g. While the values of PV and SSA obtained by CO2-adsorption are 0.037–0.039 cm3/g and 106.016–111.870 m2/g. Soft coal shows greater SSA and PV than hard coal, which is consistent with the adsorption capacity ({V}_{mathrm{L}}). The fractal dimensions of soft and hard coal are respectively different. The Ding coal exhibits larger D1 and smaller D2, and the reverse for the Wu coal seam is observed. The greater the value of D1 (complexity of pore surface) of soft coal is, the larger the pore surface roughness and gas adsorption capacity is. The results enable us to conclude that the characterization of pores and fractal dimensions of soft and hard coals is different, tending to different adsorption/desorption characteristics. In this regard, the results provide a reference for formulating corresponding coal and gas outburst prevention and control measures.

Study on Mechanical Properties and Crack Propagation of Raw Coal with Different Bedding Angles based on CT Scanning.

The deformation and damage characteristics of coal are the important foundation that affects the fracturing potential of coal reservoirs and the development plan of coalbed methane (CBM). To reveal the influence regulation of primary fractures and the bedding angle of coal on its failure and provide theoretical basis for CBM development, raw coal samples of no 16 coal seam in Wenjiaba Coal Mine, Zhijin County, Bijie City with different bedding angles were selected as the research object, and uniaxial compression tests were carried out on them, and CT scanning and crack reconstruction before and after sample failure were carried out. The results show that (1) the compressive strength, elastic modulus, and Poisson’s ratio of coal show a strong bedding angle effect, and the changing trend of each index is basically the same. The coal samples with bedding angles of 0 and 90° are the highest, while the coal samples with bedding angles of 30° are the lowest, and the overall distribution is an approximate “U” with the increase in bedding angle. With the increase in bedding angle of 0–90°, the failure modes of coal samples are tension-shear combined failure, shear-slip failure, and splitting tension failure in turn. (2) The observation of raw coal and CT scanning show that the primary cracks in coal samples are well developed, especially in the lower part of 0° samples, the cracks in 30° samples, 60° samples, and 90° samples are evenly distributed and develop at a certain angle with the weak bedding surface, and microcracks parallel to and nearly perpendicular to the weak bedding surface are developed in 45° samples. At the same time, banded minerals in coal and rock samples are also well developed. (3) The characteristics of crack propagation and evolution in coal samples with different bedding dip angles are significantly different. The bedding dip angles and primary cracks of coal seam have a great influence on crack propagation. With different bedding angles, the propagation modes are different. The crack propagation mainly includes two ways: forming a certain angle with bedding and extending along the bedding plane. (4) The fracture characteristic parameters of coal in the primary state and after failure have the same law with the bedding dip angle, showing a trend of high at both ends and low in the middle, which is an irregular “U”-shaped distribution and has a similar law with mechanical characteristic parameters.