页岩水力裂缝网络形态及激活机制研究

针对页岩体积压裂后量化复杂裂缝网络构成及裂缝激活程度的问题，基于有限元、全局嵌入黏聚区域模型以及真实页岩露头，建立了预制结构弱面的复杂水力缝网模型。在考虑全耦合应力-流体影响下，研究了弱面逼近角、水平应力差、压裂液黏度以及排量对裂缝网络组成、几何形态以及SRV的影响，提出能够量化分析裂缝网络构成以及裂缝激活程度的裂缝相对激活程度概念。研究结果表明，页岩压后裂缝几何形态受力学性质最弱的弱面控制，分别呈轴对称与中心对称网络状分布；裂缝网络由主导游离气传输的弱面型裂缝与主导吸附气传输的基质型微裂缝共同构成；弱面逼近角、水平应力差以及压裂液黏度对SRV长轴的影响并非呈现单调性变化，逼近角的增加、水平应力差、压裂液黏度以及适当排量的降低会导致SRV短轴增加；水平应力差对裂缝相对激活程度的影响也并非呈现单调性变化，弱面逼近角、压裂液黏度以及排量的增加会引起基质型微裂缝相对激活程度、激活裂缝总长度的增加以及弱面型裂缝相对激活程度的降低。

Investigation on the Morphology and Activation Mechanism of Hydraulic Fracture Network in Shale

Aiming at the problem of quantifying the composition of complex fracture network and fracture activation after shale volume fracturing, a complex fracture network model of weak surface of prefabricated structure is established based on finite element method, global embedded cohesive zone model and real shale outcrop. Considering the influence of fully coupled stress/fluid, the effects of weak plane azimuth, horizontal stress difference, fracturing fluid viscosity and displacement on fracture network composition, geometry and SRV are studied. The concept of fracture relative activation, which can quantitatively analyze fracture network composition and fracture activation, is proposed. The results show that the fracture geometry of two kinds of conjugated shale after compaction is controlled by the most weakly mechanical weak-plane, and the fracture network is respectively axisymmetric and centrosymmetric. The fracture network is composed of the weak-plane fractures with the dominant free gas transport and the matrix microfractures with the dominant adsorbed gas transport; the effects of weak-plane azimuth, horizontal stress difference and fracturing fluid viscosity on SRV length are not monotonous, while the increase of the azimuth, horizontal stress difference, fracturing fluid viscosity and the reduction of appropriate displacement will lead to the increase of SRV width; the effect of horizontal stress difference on the relative activation of fractures does not change monotonically, while the increase of weak-plane azimuth, fracturing fluid viscosity and displacement will lead to the increase of the relative activation of matrix microfractures, the increase of the total length of activated fractures and the decrease of the relative activation of weak-plane fractures.