临兴地区煤系多层合压工艺方案优化研究

Optimization of schemes for multi-reservoir commingled fracturing of coal measures in the Linxing block

【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,压裂目标以在砂岩层中造长缝、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明显,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。

[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 in-creased more significantly than the fracture length,making it necessary to adjust the fracturing techniques and paramet-ers.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.