Research On Fracture Network Formation And Propagation Mechanism Induced By Weak Plane Shear

Shale gas is an important unconventional gas resource to realize the replace of natural gas production, the effective development of shale gas cannot be achieved without hydraulic fracturing. Based on the high fragility of shale reservoir and weak plane development, an effect analysis of the plane shear for the hydraulic fracture, reveals the fracture network formation mechanism and propagation rules. This has great significance for the effective development of shale reservoirs.According to the equilibrium equation of porous media and fluid flow continuity equation, this paper firstly sets up fluid-solid coupling mathematical model, derived the finite element equation to obtain node displacement and pore pressure. Analyze crack initiation criterion in shale rock and judgment criterion of weak plane propagation induced by shear slip, introduce cohesive element used to describe weak plane, set up mathematical model to analyze fracture network formation mechanism.Secondly, this paper sets up the test bed of mode I crack extension with confining load function. Digital speckle correlation method and high-speed cameras are used to capture the specimen surface speckle field changes, obtain the specimen surface displacement and strain field changes when the crack extends through the weak plane, and obtain the variation of the crack tip stress intensity factor by the displacement extrapolation technique, to find the control mechanism of the crack deflection.Thirdly, through the establish of a hydraulic fractures and weak plane intersection mechanical model, by using numerical simulation method, use a second order damage tensor to descript disorderly distribution of weak planes, a plane fracture network model that groups bedrock-hydraulic fractures-weak plane is built, use traction-separation criterion of stiffness degradation to simulate crack initiation and expansion, form fracture network numerical simulation method for the shale reservoir, verify the validity of the method by calculating the on-site well.Fourthly, based on the fracture network numerical simulation method, when the hydraulic fractures and weak plane intersect, under the influence of hydraulic fracture and in-situ stress, the weak plane shear failure forms a branch fracture. Comprehensive analysis the contribution to the shear failure and influence on crack propagation speed of various factors, in the case that the horizontal principal stress difference is smaller, the injection flow rate is greater, the reservoir rock elastic modulus is larger and the suitable fracturing fluid viscosity can activate the weak bedding formation fracture network effectively and the stimulated reservoir volume can be greater.Based on the characteristics of weak plane development in unconventional shale reservoirs, this paper reveals the fracture network formation mechanism induced by weak plane shear slip, and recognizes the influence the rule on fracture network form, can provide technical support for fracturing design and then improving the fracturing effect.