Optimization study of coal system multi-layer combined pressure process scheme in Linxing area

Coal-bearing strata in the eastern edge of Ordos Basin are characterized by the interaction of multi-layer system and the development of longitudinal thin layer (interlayer), and multi-layer compression is an inevitable requirement to realize the economic and efficient development of coal gas. The key to the success of multi-layer compression depends on whether the fracture height can reach the expected effect. Currently, the vertical extension distance and fracture morphology are unknown, and there is a large deviation between the fracture height prediction and the actual height, which has a great impact on the volume of the production reservoir. Through indoor experiments to measure the top and bottom ground stresses of coal seams, based on the rupture pressure of the upper and lower strata, combined with the field test data and the fracture morphology reflected by the critical seam length formula, we analyze the combined fracturing schemes in four modes: one coal seam, upper coal seam/lower sandstone, upper sandstone/lower coal seam, and coal seam/sandstone/coal seam, and put forward the optimization measures for the process of improving the fracture morphology and the fracturing effect. The results show that, in multi-reservoir combined fracturing, starting fracturing with coal seam can take advantage of the upper and lower strata stress shielding, and increase the fracture length in coal seam by controlling the fracture height; when sandstone is the main reforming stratum, the fracturing target is to create long fractures in sandstone layer and improve the fracture inflow capacity. The results of fracturing parameter calculations and on-site fracturing examples show that the fracture extension is prominent in the direction of the seam height, which makes it necessary to adjust the fracturing process and parameters, and optimize the fracturing fluid volume according to the needs of multi-layer combined fracturing in the fracturing design, so as to ensure that the increment of the seam length is greater than the increment of the seam height. The research results can provide some theoretical basis for the calculation of the stress in each layer, the judgment basis and conditions of combined fracturing, the selection of fracture initiating layer and the design of fracturing fluid volume in different combinations of formations.