Objective The coal-bearing strata in the Linxing block on the eastern margin of the Ordos Basin exhibit the alternating occurrence of multiple sequences and vertical thin layers (interlayers). Therefore, multi-reservoir commingled fracturing is essential for the cost-effective, efficient exploitation of coal-measure gas from these coal-bearing strata. The key to successful multi-reservoir commingled fracturing is the achievement of expected fracture heights. The current challenges include the unclear vertically extended distance and morphologies of fractures, along with great deviations between the predicted and actual fracture heights, significantly affecting the volume of reservoirs for production growth.
Methods Based on the in-situ stresses on the roofs and floors of coal seams determined using indoor experiments, the fracturing pressures of their overlying and underlying strata, and fracture morphologies reflected by the field test data and the formula for the critical seam length, we analyzed the multi-reservoir commingled fracturing schemes for four modes: Single coal seam, an upper coal seam and lower sandstone, upper sandstone and a lower coal seam, and an upper coal seam, middle sandstones, and a lower coal seam. Consequently, we proposed technique optimization measures to improve fracture morphologies and fracturing effects.
Results and Conclusion The results indicate that, by taking advantage of the upper and lower strata as barriers in initial fracturing, multi-reservoir commingled fracturing can increase the fracture lengths in coal seams by limiting the fracture heights. In the case where sandstones are primarily to be simulated, the main goal of fracturing is to create long fractures in the sandstone layer to improve the conductivity of fractures. Both the calculated fracturing parameters and field fracturing examples indicate that the fracture height increased more significantly than the fracture length, making it necessary to adjust the fracturing techniques and parameters. During fracturing design, the fracturing fluid volume should be optimized based on the needs of multi-reservoir commingled fracturing to ensure a greater increase in the fracture length than in the fracture height. The results of this study can provide a certain theoretical basis for the applicability of the multi-reservoir commingled fracturing, as well as the stress calculation of various layers, the selection of the initial fracturing horizon, and the design of fracturing fluid volumes for various stratigraphic assemblages in the process of multi-reservoir commingled fracturing.
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