裂缝性页岩储层水力裂缝非平面扩展实验

开发页岩气藏通常需要采用大规模的水力压裂工艺技术,而页岩储层中的天然裂缝、层理面对水力裂缝的扩展路径又有着非常重要的影响。研究天然裂缝对水力裂缝扩展的影响可为现场预测水力裂缝扩展方向以及实施缝网压裂提供技术支撑。为此,选取4块尺寸为400mm×400mm×400mm的下志留统龙马溪组页岩露头标本,来进行真三轴水力压裂实验和声发射监测,以便研究水力裂缝与天然裂缝的沟通行为。实验结果表明:水力裂缝遇到天然裂缝时可发生转向或者穿透天然裂缝,形成一种空间非平面裂缝网络;大开度、低胶结强度的天然裂缝容易导致水力裂缝转向,难以形成新的主水力裂缝面;水力裂缝穿透层理面时,流入到层理面上的压裂液呈椭圆状分布;水力裂缝从岩石本体起裂的方向上声发射点较集中,沿着天然裂缝扩展的方向上声发射点少。结论认为:1水力裂缝能否穿透天然裂缝与天然裂缝的开度、胶结强度有关;2裂缝性页岩储层水力压裂易形成空间非平面网状裂缝;3与主裂缝面相比,压裂液进入到层理面的体积较少。

Experiments on the non-planar extension of hydraulic fractures in fractured shale gas reservoirs

A large-scale hydraulic fracturing technology is commonly used in the development of shale gas reservoirs. Natural fractures and bedding planes have significant influences on the extension of hydraulic fractures. A series of true tri-axial hydraulic fracturing tests associated with acoustic emission monitoring were conducted on the four shale outcrops (dimensions: 400 mm × 400 mm × 400 mm), collected from the Lower Silurian Longmaxi Shale Fm to investigate the interactions between hydraulic and natural fractures. Experimental results indicate that the hydraulic fractures will deflect or penetrate through when encountering natural fractures, resulting in the formation of a spatially non-planar fracture network. The natural fractures with high apertures or low bonding strength often make hydraulic fractures deflect, which results in some difficulties in creating a new main hydraulic fracture. The geometry of the fracturing fluid flowing into the bedding plane is distributed elliptically after a hydraulic fracture penetrates a bedding plane. Acoustic emission signals concentrate on the direction of a crack initiation along the rock mass, while fewer signals on the direction along the natural fracture extension. Therefore, whether hydraulic fractures penetrate natural fractures is predominated by the apertures and bonding strength of natural fractures. The volume of fracturing fluid leaking off into the bedding plane is less than that in the main hydraulic fractures. This study on the impact of the natural fractures on the hydraulic fracture extension provides a technical support for the prediction of a hydraulic fracture extension direction and the construction of a network fracturing.