煤系“三气”共采产层组压裂裂缝扩展物模试验研究

硬脆性砂岩、页岩储层与塑性的煤层组合开发时,层间应力差、岩性差异、界面性质等因素会对水力裂缝扩展产生较大影响,复合储层人工裂缝扩展规律存在认识不清的难点,给储层改造方案的合理设计带来了难度。基于此,选取鄂尔多斯盆地东缘临兴地区天然露头岩石进行不同岩性组合,开展室内大尺寸真三轴水力压裂物理模拟试验,研究不同地应力差、弹性模量差异、煤岩割理等参数对水力裂缝起裂及扩展的影响。结果表明:水力裂缝起裂方向受地应力条件及近井筒天然弱面共同控制;当应力差在4~6 MPa时,既能保证裂缝穿透界面,又能保证高效沟通煤岩中天然弱面形成网络裂缝;层间弹性模量差异越大,缝内流体压力越高,穿透界面时释放的压力脉冲有助于激活煤岩中的微裂缝形成复杂的裂缝网络。水力裂缝在煤岩中的扩展路径受割理影响较大,易发生转向或者分叉扩展;水力裂缝穿透界面时压裂曲线存在明显的2次憋压,沟通煤岩割理过程中压裂曲线上下波动明显。研究复合储层水力裂缝起裂及扩展规律可为油田现场预测裂缝形态以及优化泵注程序提供参考。

Experimental research on hydraulic fracture propagation through mixed layers of shale,tight sand and coal seam

When multi-layers strata with shale gas,tight gas and coal bed gas are developed together,differential stress between interlayer,lithology difference,interfacial property and other factors have great impacts on hydraulic fracture geometry.So it is difficult to understand the fracture propagation rule in composite reservoir,which brings great challenges in planning for reservoir stimulation.In this paper,the natural outcrop rocks from the eastern margin of Ordos Basin are excavated and mixed in a certain way to carry out the physical simulation experiments with a true tri-axial fracture system.The effects of differential stress,differential elastic modulus and cleats on the hydraulic fracture initiation and propagation are studied.The result shows that the stress condition and natural weak plane commonly control the propagation direction of hydraulic fracture.When the stress difference ranges from 4 MPa to 6 MPa,the hydraulic fracture can not only penetrate the interface in vertical direction but also activate the natural fractures in coal to form a complex fracture network.The higher elastic modulus difference between layers brings a higher pressure in fracture,when the hydraulic fracture reaches interface pressure releases to generate pressure wave which can contribute to activate the micro fracture in coal.The expansion path of hydraulic fracture in coal rock is greatly influenced by cutting principle,and could easily deflect to form branches.There are obvious hold pressure for two times when hydraulic fracture penetrates interface,and the fluctuation of fracturing curve is obvious when the weak planes in coal is activated.The study of hydraulic fracture initiation and propagation in multi-layers reservoir can provide a reference for the field prediction of fracture morphology and optimize pump injection program.