页岩水力压裂裂缝形态的试验研究

Experimental study on hydraulic fracture geometry of shale

为深入认识页岩储层水力裂缝延伸规律及其空间形态,采用真三轴岩土工程模型试验机、压裂泵压伺服控制系统、Disp声发射定位系统和工业CT扫描技术,建立了一套室内页岩水力压裂大型物理模拟试验方法,并通过试验后页岩试样水力裂缝的延伸与空间展布规律,初步探讨了页岩水力压裂网状裂缝的形成机理。结果表明:裂缝延伸时泵压曲线典型的锯齿状波动与裂缝网络的形成密切相关,是页岩体积压裂的一个明显特征。页岩层理面的发育程度、泵压大小和地应力状态对裂缝形态有显著影响,水力裂缝在层理面内的分叉、转向进而沟通天然裂缝是形成裂缝网络的关键,而层理面过弱或过强都不利于网状裂缝的形成;对层理面胶结强度适中的地层,地应力对裂缝的延伸有较大影响;在低排量且维持较低泵压时,裂缝易于转向,且更易形成网状裂缝,而达到体积压裂。建立的页岩水力压裂物理模拟试验方法及试验结果可为页岩气压裂优化设计等提供技术支持。

For thorough understanding of the propagation and spatial form of hydraulic fracture of shale formations, a large-scale physical simulation test method for hydraulic fracture of shale is established by means of the large real triaxial simulation experiment system, servo control system of hydraulic fracturing, acoustic emission （AE） source orientation technique and CT scanning technology. The mechanism of the formation of the fi＇acture network is exposed according to the crack extension and special distribution of shale tested by this method. The results indicate that the typical jagged pump pressure-time curve at extending stage, which may be closely related to the formation of fracture network, is an obvious feature of the volume fracture of shale. The fracture morphology is strongly influenced by the development degree of bedding planes, pump pressures and stress conditions. Branching and re-orientation of hydraulic fractures in bedding planes and then interconnecting with natural fractures are the main factors of the formation of fracture network. The bedding plane which is too weak or too strong is not conducive to the formation of fracture mesh. The stress condition plays a great role in controlling the extension of fractures for intermediate formations. Hydraulic fractures are easy to change direction and propagate when the pump pressure is kept at a lower level. The physical simulation method for hydraulic fracture and the corresponding test results can provide references for fracture optimization design in exploiting shale gas.