Investigation On The Interlaminar Deformation And Fracture Mechanism Of Shale

China is rich in shale oil and gas resources,and promoting shale exploitation can help to ensure national energy security.Shale reservoirs are usually developed with bedding interfaces,affecting the fracturing during the hydraulic stimulation.Hydraulic fractures will induce interlaminar deformation,and the bedding interfaces could affect fracture propagation.Thus,the interactions between hydraulic fractures and bedding planes are the research hots.In this paper,theoretical and experimental studies were conducted on shale to investigate the interlaminar deformation and fracture mechanism,including the fracture mechanism of the cross-layer and along-layer fractures,the interlaminar deformation during hydraulic fracturing,and the criteria of the fracture propagation crossing layers,aiming to provide understanding and guidance for the filed fracturing.From the fracture experiment,the fracture propagation modes of layered shale were obtained,and the geometrical characteristics of cross-layer fractures were investigated.Based on the cohesive zone model,the fracture process zone(FPZ)of cross-layer fractures was studied,and the development process was detected using digital image correlation(DIC).The characteristics of cross-layer fracture process zone and the corresponding fracture mechanism were found,from which the critical parameters affecting the dislocation of FPZ were investigated.The cohesive zone model considering the difference of bedding media was discussed to describe the constitutive relationship and fracture energy during the FPZ development.It is found that the FPZ softening curve is accompanied with discontinuous characteristics at the bedding plane.The fracture process of interlaminar slip was studied.Based on the slip weakening model,a method for identifying the slip weakening zone(SWZ)by DIC was proposed.The development of SWZ when the Mode-I fracture turns to the interface was studied.The dominant path of slip could be predicted from the size of the conjugated SWZs.The slip weakening and energy dissipation considering the opening displacement of the Mode-I fracture tip were discussed.The slip difference of cementation and non-cementation beddings was studied,where the slip mode and acoustic emission characteristics was clarified.The opening characteristics of cohesionless slipping fracture were studied,and the relationship between interlaminar opening and slip was obtained.The hydraulic fracturing experiment was carried out on the shale specimen using the visualization fracturing equipment.The influence of the wellbore pressure on the interlaminar deformation before fracture initiation was studied,and the mechanical condition for interlaminar slip nucleation around the wellbore was obtained.The interlaminar deformation of hydraulic fracture propagation along the interface was monitored,where the residual slip zone was defined,and the influence of fracturing engineering parameters on interlaminar deformation was explored.The fracture process and interlaminar deformation were studied when the fracture propagated through the interface.It is found that slip bands will be generated along the interlamination on both sides of the fracture after penetration.The residual slip zone and the bilateral slip bands were defined as the potential components of the hydraulic fracture systems.The criterion considering the influence of FPZ was established for estimating the fracture propagation across the interface,which considers a more accurate application scope of elastic fracture mechanics in front of the fracture.The influence of fracture growth rate on FPZ was studied for the rate-dependence of reservoir rocks,and the cohesive zone model considering fracture growth rate was established.Taking FPZ as a bridge,the criterion for estimating fracture penetration considering the fracture growth rate was proposed.The influence of fracture growth rate on the penetration was explained from the fracture mechanism of FPZ.