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As the main technology of CBM development in the Qinshui CBM field, single-branch fractured horizontal wells have supported the breakthrough of 2 billion m3/year production of gas field. However, in the process of its large-scale application, there have been a series of problems, such as large difference in production among wells and poor adaptability in local areas, which seriously restrict the further improvement of CBM production in gas fields. On the one hand, although many key elements affecting the production of single-branch fractured horizontal wells have been identified, the core key elements are not clear. On the other hand, the understanding of the change rule of key elements on the plane is generally simple mathematical extrapolation or pure mathematical inversion, which lacks geological constraints and corrections, and has low reliability and resolution. Therefore, it is urgent to clear the core key elements that affect the production, and then establish targeted technical countermeasures on this basis. First of all, based on the objective geological conditions and the understanding of development and production practice in the Zhengzhuang block, the preliminary screening principle of the core key elements affecting the production of single−branch fractured horizontal wells was proposed, and nine elements were selected, including drilling length in pure coal seam, horizontal section inclination, total hydrocarbon, GR, number of fracturing stages, average sand gradient, average displacement gradient, average fluid volume gradient and desorption pressure gradient. Then, grey correlation analysis and single element analysis were used to determine the correlation degree between the nine elements of the preliminary screening and the production of single−branch fractured horizontal wells, and determine the internal mechanism of the elements' influence on the production. Secondly, from the three perspectives of resource base, reserve control and reconstruction, combined with the specific process of a single-branch fractured horizontal well deployment, reconstruction design and operation, nine elements were classified, and targeted technical countermeasures for production enhancement were established. In the stage of well location deployment, extrapolation data based on evaluation wells were used to initially identify the relatively enriched area, and the seismic data were processed by structure-oriented filtering + high−frequency coherence technology to delineate the local area for clearing hidden destructive structures in the area, and then straight/inclined wells were drilled to obtain GR and total hydrocarbons in the coal seam segment to determine the resource base of the well location deployment area. In the stage of well trajectory design and drilling, the waveform clustering technology was used to predict the distribution law of strongly deformed coal in the well location deployment area. The pre−stack depth migration of seismic data was processed to predict the thickness and fluctuation change law of coal seam in the well location deployment area, the well group mode was used to drill in batches, and the near−bit geological guidance technology was combined with the real drilling data to correct the local coal seam understanding in real time, to increase drilling length in pure coal seam, optimize well trajectory, ensure more high−quality fracturing stages, and improve reservoir control. In the reservoir reconstruction stage, GR (Gamma Ray), total hydrocarbon, well trajectory conditions and real drilling data were used to select fracturing stages, and the joint operation techniques of bridge plug setting, shooting and fracturing was used as a new fracturing technology to increase the displacement, sand and fluid volume, increase the reconstruction scale and strengthen the reconstruction, and improve resource utilization. Finally, relevant tests were carried out in the Zhengzhuang block by using the above countermeasures. A well group with 7 wells was deployed in the north of Zhengzhuang. Compared with the early single-branch fractured horizontal wells around the well group, the desorption pressure of the test wells is about 1-1.5 MPa higher than before, and the stable gas production is increased by nearly 5 000-10 000 m3/d. According to field practice tests, the core key elements affecting the production of single-branch fractured horizontal wells and the targeted production technical countermeasures established on this basis can greatly improve the stable gas production and desorption pressure of coalbed methane, which can help improve the production of the Qinshui coalbed methane field and provide important production practice experience and successful cases that can be learned from for peers who adopt the same main development technology.

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Accurate determination of gas content in deep coal mine is the premise for the prevention and control of gas disaster, as well as the efficient development and utilization of coalbed methane. Traditional coring technology of coal seam is generally open-ended and requires to obtain the amount of gas loss during the coring process by using estimation methods, making it difficult to ensure the accuracy and validity of the in-situ parameters of coalbed methane. Based on the academic idea of “in-site condition-maintaining coring”, the in-situ pressure- and gas-maintaining coring technology for deep coal seam was proposed, and the in-situ pressure- and gas-maintaining corer for deep coal seam was developed. The strength verification for corer outer tube, pressure-maintaining chamber and key weak parts were conducted based on three-dimensional numerical simulation. Meanwhile, the pressure-maintaining capacity of controller was measured and analysed relied on the self-developed pressure-maintaining coring laboratory simulation test platform, and the reliability of corer was verified through field tests in coal mine. The results shown that, the self-developed corer had the advantages of strong pressure-maintaining capacity, long pressure-maintaining time and stable anti-disturbance performance. The equivalent stress of the corer under internal fluid pressure of 20 MPa and torque of 1000 N·m was 121.1 MPa, which was much smaller than the yield strength of the material and met the strength design requirements. The equivalent stress of corer under the pressure of 20 MPa was 63.9 MPa, which met the strength design requirements. The corer can operate continuously and stably at the pressure of 19.4 MPa, which met the demand for gas testing in most of deep coal mines. The field test shown that, the pressure-maintaining controller of the pressure- and gas-maintaining corer closed well, with 100% of coring rate in the field. The research provides a theoretical, technical and equipment foundation for the accurate determination of gas content in deep coal seams, which is of great significance for the prevention and control of coal mine gas disasters and the exploration and development of coalbed methane.

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To assess feasibility of microwave radiation in CH 4 production, temperature rise of coals under microwave field and resultant alterations in physico-chemical properties were addressed. Results indicated that the temperature rise of coals is dominated by mineral compositions. The bulk temperature of coals containing strong microwave-absorbing minerals such as pyrite, smectite, and kaolinite rises markedly. The elevated temperature breaks alkyl side chains and aromatic rings of coals. Moreover, the total oxygen-containing functional groups decrease by radiation. Generally, the radiation reduces the micro- and mesopores of coals. Besides, the pore surface of low-rank coals after radiation becomes smooth, whereas that of medium- and high-rank coals becomes rough. Finally, the radiation decreases CH 4 maximum adsorption capacity by 6.74–36.69%. The radiation also diminishes CH 4 adsorption/desorption hysteresis for low-rank coals, while an opposite trend exists in medium- and high-rank coals. Such variations rely on pore structure responses to microwave. Overall, microwave radiation could promote CH 4 recovery. [Display omitted] • Temperature rise of coals under microwave field depends on mineral compositions. • Microwave radiation breaks alkyl side chains and aromatic rings of coals. • CH 4 adsorption/desorption on coals relies on pore structure responses to microwave. • Microwave radiation could enhance coalbed methane recovery. [ABSTRACT FROM AUTHOR]

Microwave irradiation could alter pores and surface chemistry properties of coal matrix, thereby affecting coalbed methane (CBM) production. The impacts of irradiation power on dominant pore structure parameters and functional group compositions of coals were elaborated in this study. The results show that the responses of micropore structure parameters of coals to irradiation depend on coal rank. Generally, micropore surface area and micropore volume of low-rank coals decrease after microwave irradiation. Moreover, a U-shaped trend exists between micropore parameters of low-rank coals and irradiation power. As for middle- and high-rank coals, micropore surface area and micropore volume increase after microwave irradiation. The irradiation power dependence of micropore parameters of middle-rank coal shows a decreasing trend, while that of high-rank coal shows an inverse U-shaped trend. Microwave irradiation decreases mesopores with diameter range of 2.00–32.00 nm for all coals, thereby reducing adsorption space of coal matrix for CH4. Besides, irradiation reduces C-O, C=O, and COOH of low-rank coals, while increases C-O and C=O of middle- and high-rank coals. Furthermore, microwave irradiation increases pyridine-N but decreases pyrrole/pyridone-N of all coals. Such alterations become pronounced with rising irradiation power. Overall, responses of pores and functional groups of coals to irradiation power rest with coal rank, therefore requiring further consideration in CBM production. [ABSTRACT FROM AUTHOR]

Most coal reservoirs show high gas content with relatively low desorption efficiency, which restricts the efficiency of coalbed methane (CBM) extraction and single-well productivity. This review highlights the desorption hysteresis mechanism and its controlling factors as well as methods and models to reveal desorption hysteresis and potential solutions. Methane adsorption and desorption can be recorded by both gravimetric and volumetric experiments. Although different adsorption models are used, desorption is generally considered with the Langmuir model. Desorption hysteresis is influenced by the petrophysical composition, thermal maturity, pore structure distribution of the coal, reservoir temperature, and moisture and water content. Methods for calculating desorption hysteresis include the area index, hysteresis index and introduction of a hysteresis factor and a hysteresis coefficient. Molecular dynamics simulations of methane desorption are mainly based on theories of kinetics, thermodynamics, and potential energy. The interaction forces operating among coal, water, and methane molecules can be calculated from microscopic intermolecular forces (van der Waals forces). The desorption hysteresis mechanism and desorption process still lack quantitative probe methodologies, and future research should focus on coal wettability under the constraints of liquid content, potential energy adjustment mechanism, and quantitative analysis of methane desorption rates. Further research is expected to reveal the desorption kinetics of methane through the use of the solid-liquid-gas three-phase coupling theory associated with the quantitative analysis of methane desorption hysteresis, thereby enhancing the recovery rate and efficiency of CBM wells. [ABSTRACT FROM AUTHOR]

Coalbed gas non-Darcy flow has been observed in high permeable fracture systems, and some mathematical and numerical models have been proposed to study the effects of non-Darcy flow using Forchheimer non-Darcy model. However, experimental results show that the assumption of a constant Forchheimer factor may cause some limitations in using Forchheimer model to describe non-Darcy flow in porous media. In order to investigate the effects of non-Darcy flow on coalbed methane production, this work presents a more general coalbed gas non-Darcy flow model according to Barree-Conway equation, which could describe the entire range of relationships between flow velocity and pressure gradient from low to high flow velocity. An expanded mixed finite element method is introduced to solve the coalbed gas non-Darcy flow model, in which the gas pressure and velocity can be approximated simultaneously. Error estimate results indicate that pressure and velocity could achieve first-order convergence rate. Non-Darcy simulation results indicate that the non-Darcy effect is significant in the zone near the wellbore, and with the distance from the wellbore increasing, the non-Darcy effect becomes weak gradually. From simulation results, we have also found that the non-Darcy effect is more significant at a lower bottom-hole pressure, and the gas production from non-Darcy flow is lower than the production from Darcy flow under the same permeable condition. [ABSTRACT FROM AUTHOR]

Coal fines are commonly generated as by-product during coalbed methane production mainly due to the interaction of coal with inseam water flow. A portion of the created coal fines may settle and plug the coal cleats and hydraulic fractures due to the gravity and coal pore size constraint. This could result in the reduction of coal permeability and blockage of coalbed methane wells or gas drainage boreholes. Despite the increasing awareness of the importance of understanding coal fines, limited research has been carried out on the characterization of coal fines creation. This study aimed to numerically characterize the generation process of coal fines in micro-scale coal cleats. The Scanning Electron Microscopy (SEM) images for a coal sample from Bulli Seam of the Sydney Basin in Australia were obtained and analysed to determine the actual cleat geometries and the characteristics of coal fines distribution. Then a fully coupled fluid-structure numerical model was developed to identify the creation process of coal fines at micro-scale. The impact of pertinent production conditions on coal fines generation was studied, including production pressure drawdown, temperature, coal fines Young's modulus and strength. The SEM images revealed that the particle size distributions of the coal fines in the examined cleats were in the order of hundreds of nanometres to several microns. The results of the numerical studies showed the coal fines production increased with pressure build-up, and decreased with increasing coal fines strength with more sensitivity compared with pressure. Critical values for production pressure drawdown were obtained, above which failure area began to expand; threshold values were also determined, below which remarkable reduction of coal fines production was achieved. Coal cleat geometry plays an important role in determining coal fines production. It was noted that exposed microstructures, cleat elbow regions and micro-fracture tips are more likely to generate coal fines. Based on these findings, guidance can be provided on the control of production conditions to mitigate coal fines issue, and new insight into where and how coal fines are created by inseam water flow can be achieved. [ABSTRACT FROM AUTHOR]

The Hancheng area of the southeastern Ordos Basin is a sample area for coalbed methane (CBM) development in China. There are three target coal seams for CBM development in this area, but the gas production of the different coal seams varies significantly. This study investigates the controlling factors of CBM production in the Hancheng area. The results indicate that permeability has the main impact on CBM production in the Hancheng area. Coal types and stress are closely related to fracture development and further impact on the permeability of coal reservoirs in this area. According to the macroscopic coal petrography description, mercury porosimetry experiments and X-CT scanning, bright coal has the largest pore volume and average pore size diameter than those of the other coal types and developed fractures with good openness and connectivity, whereas dull coal has the most undeveloped macropores and fractures. The No. 3 coal seam always has a higher productivity because it mainly consists of bright coal and semi-bright coal and has a relatively shallow buried depth. Therefore, the No. 3 coal seam had the best physical properties as a whole compared to the other coal seams, providing the most favourable conditions for CBM production. [ABSTRACT FROM AUTHOR]

We present a comprehensive overview of the influence of water on coalbed methane production. The influence of water on coalbed methane production is associated with adsorption of water in pores, capillary condensation, micropore filling, adsorption hysteresis and its effect on methane diffusivity, coal swelling due to adsorption of water and a corresponding change in methane permeability, pore saturation by water (water imbibition into pores), and gas-water two-phase flow in the cleats. We consider methods for reducing the negative impact of water on coalbed methane production: carbon dioxide injection and microwave technology. We identify promising areas of study. [ABSTRACT FROM AUTHOR]