| The development characteristics of fracture systems in coal reservoirs, as one of the key factors that influence the exploration and development of coalbed methane(CBM) reservoirs, have a significant influence on the drilling, fracturing, and drainage of CBM wells. However, in the current exploration and development of CBM, characteristics of fracture systems are seldom considered. The causes of this situation encompass the following three aspects:firstly, there are fewer relevant researches on it. The existing researches mainly focus on fracture orientations, filling characteristics of minerals, descriptions of fracture features and development degree, and the influences on permeability and gas content. However, fewer researches have been conducted on the relations of the characteristics of fractures with the coal structure and engineering application. These characteristics include the anisotropic development characteristics, formation mechanism, and distribution of fracture systems in coal reservoirs. Secondly, many difficulties exist in the research. As generally occurring in the underground at a certain depth, coal reservoirs cannot be observed or measured directly, which makes it hard to obtain data of fractures in coal reservoirs. Thirdly, there are limitations in research methods. The existing methods for researching fracture systems are mainly carried out by collecting samples or cores to observe the pores and fractures or employing geophysical prospecting, numerical simulation, and fractal geometry. Nevertheless, due to the high heterogeneity of coal reservoirs, observation results and geophysical data obtained from several samples cannot represent the development characteristics of fractures in the reservoirs of the whole development block.This research aims to reveal that, under the same tectonic stress, there are corresponding relations between fractures formed in strata of different lithologies and thicknesses. To achieve this objective, CBM development block of Kubai coal field was studied, where the outcrops of surrounding rocks and coal reservoirs are intact. Meanwhile, technological means such as high-precision field mapping of structural fractures and fine dissection and comparison for underground coal reservoirs were adopted to reveal the relations between surrounding rocks and fractures in the coal reservoir. Based on the obtained relations, the corresponding quantitative relations between the characteristic parameters of coal reservoirs and fractures of surrounding rocks in the same structural layer were established. At the meantime, this research explored the geological controlling factors to explain the formation mechanism of fractures in the surrounding rocks and the coal reservoirs in the same structural layer from geomechanics. Besides, this research evaluated and predicted the distribution of fractures, coal structure and permeability of coal reservoirs. The method was then verified and applied in the spacing, drilling, fracturing and drainage of CBM wells.In terms of regional tectonic, Kubai coal field belongs to Kuqa depression. It is situated in the northern monoclinal zone of Kuqa depression and bears the same tectonic deformation as the whole Kuqa depression. The Cenozoic strata of Kubai coal field have received six stages of paleo-tectonic stress; while its coal-bearing strata, that is, the lower series of Jurassic system, have experienced five stages of paleo-tectonic stress. These stages include early and late Yanshanian, and early, middle and late Himalayan. Except for early Yanshanian, when the maximum principal stress was in north-west (NW) direction, other stages presented a maximum principal stress which is nearly north-south (NS) trended. Under the stress of several stages, a large number of similar and closely related fractures were generated around the surrounding rocks and the coal reservoirs, with favorable conditions for outcrop.By mapping the surface structural fractures in the CBM development block of Kubai coal field, macro-geometric parameters of joint fissures of the surrounding rocks were researched including: ①Dip angle;②Direction;③Cutting relation;④ Combination form;⑤Scale;⑥ Filling degree. The micro-morphology of the joints in the surrounding rocks is studied in the following aspects:(1) rock particles, (2) brittle deformation, (3) directions of themicro-fractures. As for the development directions of micro-fractures, they are basically the same as that of the macro ones. The comprehensive analysis demonstrates that the development density of joints fractures is influenced by lithology, thickness of strata and tectonic stress. In addition, the density changes greatly with the position in the structure.Fine observation and dissection of coal reservoirs in the surface and underground were performed. Based on this, three major characteristics most closely related to coal reservoirs were observed including ①spatial distribution characteristics of coal reservoir②Structural characteristics of coal; and③ physical characteristics of rocks. The development characteristics of fractures in the coal reservoirs were researched in both macroscopic and microscopic scales. The results validate that in the same structural layer, coal seams and surrounding rocks bear the same tectonic stress and therefore form similar joints. The fractures developed in coal seams are impacted by their physical characteristics such as coal rank, lithotype of coal and fluid filling.The formation of fracture systems in coal is caused by several mechanical factors. Under the microscope, micro-fissures show different shapes and good directionality. This observation result is consistent with the explanations for fracturing of micro-fissures according to Griffith criterion. The macro damage of coal includes fracturing and friction sliding, which can be determined based on Coulomb criterion and the sliding criterion of weak planes. The mechanical models of fracturing, sliding and fracture expansion of coal were verified through examples. Besides, the formation mode of fracture zones in coal reservoir was revealed. In addition, fractures in the coal reservoirs were investigated in micro scale, the underground dissection points, the stope face, and the development block of CBM, integrating the research of fractures from micro to macro scales.Various parameters can be obtained by conducting the high-precision field mapping of structural fractures and fine dissection and comparison of underground coal reservoirs, especially the measurement for multiple profiles. These parameters include the development characteristics, predominant directions, and densities of fractures in outcropped and underground coal reservoirs and surrounding rocks, respectively. Based on these parameters, to the relations between the fractures in the outcropped and the underground coal reservoirs and surrounding rocks were revealed for predicting the development degree of the fractures and coal structure of underground coal reservoirs. Thus, the corresponding quantitative relations between fractures in surface surrounding rocks and underground coal reservoirs were established. According to the quantitative relations, the development degree of fractures in underground coal reservoirs was quantitatively predicted to distinguish the fractured zones and zones with less developed fractures. Based on the measured data obtained inBaiyang mining area, Kubai coal field, Shiguai mining area, Zhijin mining area, Chengzhuang mining area of Qinnan, data relationship model and spatial relation model were established for the fracture systems in nearly upright, steeply inclined, inclined and nearly horizontal coal reservoirs.In Baiyang mining area and Kubai coal field, when this research was conducted,CBM wells had not been drilled. As the exploration of CBM deepened, the research was validated and supplemented with a mass of data regarding well drilling and capacity. The several fractured zones and zones with less developed fractures predicted in the coal reservoirs have been proven to be consistent with the actual conditions by a lot of data in the subsequent drilling, fracturing and drainage of CBM wells. The fractured zones are characterized by good permeability and high gas output. While, in the zones with less developed fractures, the yields of water and gas are low due to the low permeability. In addition, living examples verified the effectiveness and feasibility of the method for predicting the development characteristics of fractures in coal reservoirs by combining the mapping of surface structural fractures and the observation and dissection of coal reservoirs. The research results provide a new method for searching enrichment zones with high permeability in the exploration of CBM. |
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