In-situ stress, stress-dependent permeability, pore pressure and gas-bearing system in multiple coal seams in the Panguan area, western Guizhou, China

Independent superposed coalbed methane (CBM)-bearing systems are widely distributed in coal-bearing strata with multiple coal seams in western Guizhou province, which is liable to cause interference among systems in the multilayer combination production process. Therefore, precisely defining the independent superposed CBM-bearing systems is important for CBM development in this area. However, the recognition of gas-bearing system at present was mostly based on the sedimentary cycle and the sequence stratigraphic framework, of which ignored the control of in-situ stress on physical properties of coal-bearing strata. In this work, the distribution characteristics of in-situ stress and its control on coal permeability was analyzed systematically based on well test parameters of 22-layer coal seams measured within depths from 352 to 1245 m in the Panguan area. Within depths shallower than 500 m, sigma(H) and sigma(h) tend to decrease with an increasing depth, and the stress field gradually transforms from sigma(H) > sigma(h) > sigma(v) to sigma(v) > sigma(H) > sigma(h). For immediately coal seams (500-750 m), the sigma(v) > sigma(H) > sigma(h) type is dominant, revealing a normal faulting stress type. For coal seams with depth > 750 m, sH and sh increases rapidly instead of continuously decreasing, and the stress field transformed into the type of sigma(H) > sigma(v)> sigma(h) when being deeper than 1000 m. Coal permeability shows a low -> high -> low -> extremely low trend with the increase of depth, and it was verified with well test permeability measured in other areas of western Guizhou, the essence of which is the compression and deformation of the pore-fracture system under the control of in-situ stress. The correlation between pore pressure and depth was also illustrated, which showed five linear relationships in depth of < 200, 200-500, 500-750 m, 750-1000 m and > 1000 m, separately. Those five depth ranges represent five independent pressure systems, and the depth boundary point between different pressure systems is precisely the conversion depth of stress fields and coal permeability. Within the same pressure system, the gas content increases as the buried depth increases. For CBM development in western Guizhou province, commingling production need carry out in the same pressure system to avoid interlayer interference, and it is recommended that middle CBM systems (500-750 m) with lower stress and higher permeability should be a priority for CBM development. At depths > 750 m, CBM development conditions tend to deteriorate.