Study of characteristics and formation stages of macroscopic natural fractures in coal seam #3 for CBM development in the east Qinnan block, Southern Quishui Basin, China

摘要

Natural fractures including the cleat systems play a key role in the coalbed methane production. Therefore, understanding the natural fracture growth and distribution in coal seams is of great importance for modeling gas flow in coal reservoirs. It is also important for the understanding of hydraulic fracture growth during well stimulation. In this paper, an integrated approach using underground coal wall observations, core log analyses, and image logging interpretation was utilized to study the characteristics of the natural fractures generated under different stages of paleotectonic stress field. As a major coalbed methane production and coal mining seam in China, the coal seam #3 in the east Qinnan block, Southern Quishui Basin is studied for its natural fracture characteristics. The results show that macroscopic natural fractures are well developed in coal seam #3 in east Qinnan block and the nearby areas. Importantly, the fracture scale, densities, in-fillings, and connectivity display marked variations among the fractures developed in different geological age. It is found that there are three groups of exogenous fractures formed during the Yanshanian period in the study area, which are oriented at NE10-19 degrees, NE60-79 degrees, and NE40-49 degrees, respectively. Meanwhile, three groups of endogenous fractures, which are orthogonal to the exogenous fractures, were formed during the same period. They are oriented at NW60-7 degrees 9, NW10-29 degrees and NW40-49 degrees, respectively. These three sets of exogenous and endogenous fractures can be related to the three main episodes of the Yanshanian tectonic movement. Moreover, there are also three groups of exogenous fractures formed during the Himalayan period in the study area, which are oriented at NW60-79 degrees, NWO-29 degrees and NW40-49 degrees, respectively. Meanwhile, three corresponding groups of endogenous fractures were formed from the Himalayan period, which are NE10-19 degrees, NE60-79 degrees and NE40-49 degrees, respectively. These three sets of exogenous and endogenous fractures are also correspondent to the three main episodes in the Himalayan period. The main macroscopic fractures formed in this period were during the second episode, which had the strongest tectonic movement in the Himalayan period. Influenced by the multiple stages of tectonic movements, macroscopic natural fractures are complex and polydirectional in strike in the study area. The results of this study are of great significance for the understanding of coalbed methane production behavior in the east Qinnan block. (C) 2016 Elsevier B.V. All rights reserved.