Coalbed Methane System In The Permo-Carboniferous Strata From Gujiao Block, Shanxi, China

There are multiple sets of CBM-bearing systems in Gujiao block. Differences and joint production compatibility among fluid pressure systems are the important restrictive factors for CBM co-mining production. Based on the data of CBM exploration and development in the block, the spatial distribution of the fluid pressure systems and their gas-bearing properties were investigated. Using the experiments of coal and rock samples from typical drillings, the physical properties cyclicity and sedimentary control mechanism of coal bearing strata were analyzed. It was considered that most of the coal reservoirs in the block are under pressue, and the reservoir pressure gradient difference of Shanxi and Taiyuan formations is smaller in northern fracture area. It was found that the permeability of the rocks near surface of maximum flooding is lower, which form the water- and gas-resisting key strata. CBM system has good corresponding relationship with stratigraphic sequence units. According to the occurrence of stratigraphic sequence and CBM-bearing units, it was considered that different gas bearing systems could be divided when the accumulated thickness of mudstone between upper and lower coal seams is greater than 2 meters. Accordingly, the forming process and spatial distribution framework of key strata within the block were discussed. Superposed relationship among CBM systems was indentified, combined with the occurrence pattern of open faults in the area. Production effect and major geological controls of previous CBM wells under both single- and co-mining conditions were analyzed based on the gas and water production history. Through the numerical simulation of CBM productivity, the compatibility of co-mining production for multiple coal reservoies in the block was defined and the evaluation method of geological compatibility was established. Then, the geological units for CBM drinage were divided.