Optimization study of coal system multi-layer combined pressure process scheme in Linxing area
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摘要: 【目的】鄂尔多斯盆地东缘临兴区块含煤地层具有多层系交互赋存，纵向薄层(夹层)发育的特点，多层合压是实现煤系气经济高效开发的必然要求。多储层合压能否成功的关键取决于裂缝高度能否达到预期效果。当前裂缝垂向伸展距离和裂缝形态不明，裂缝高度预测与实际高度存在较大偏差，对增产储层体积有很大影响。【方法】通过室内实验测量煤层顶底板地应力，以上下部地层破裂压力为基础，结合现场测试数据与临界缝长公式所反映的裂缝形态，对一层煤层、上部煤层/下部砂岩、上部砂岩/下部煤层、煤层/砂岩/煤层4种模式下的合层压裂方案进行分析，提出改善裂缝形态和压裂效果的工艺优化措施。【结果和结论】结果表明：多储层合压时，以煤层起裂可利用上下部地层应力遮挡的优势，通过控制裂缝高度，增加煤层中裂缝长度；当砂岩为主要改造层位时，压裂目标为在砂岩层中造长缝、提高裂缝导流能力为主。压裂参数计算结果与现场压裂实例均表明，裂缝在缝高方向扩展突出，有必要对压裂工艺及参数进行调整，在压裂设计时根据多地层合压的需要，进行压裂液量的优化，保证缝长增量大于缝高增量。研究成果可为多层合压时各层应力计算、合压的判断依据与条件、起裂层位的选择以及不同组合地层压裂液量的设计提供一定理论依据。Abstract: 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.
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