Study On Hydrodynamic Fracturing Cracking Model With Elastic - Plastic Characteristics

High clay reservoir such as mudstone, shale and sand-shale, and the deep oil and gas reservoir has obvious characteristics of plasticity. Traditional hydraulic fracturing initiation models are not applicable for reservoirs because they only consider elastic-brittle deformation of rock, so it is necessary to establish fracturing initiation model considering plasticity.At first, the nonlinear deformation characteristics of elastic-plastic rock were studied with the results of triaxial mechanics tests, and based on total theory of plasticity, a single segment power hardening constitutive equation and a further developed multistage power hardening constitutive equation were established.Secondly, based on the single segment and multistage power hardening constitutive equations, the elastic-plastic model of stress field around the well was established and the characteristics of the stress field around the well were discussed, then the initiation modes of the elastic-plastic rock were discussed and the initiation criterions were selected, and then the elastic-plastic initiation model was established and it were taken to parameter sensitivity analysis for initiation pressure and initiation modes.Thirdly, based on the established stress field model and initiation model and the superposition of leakoff effect, the initiation model was established with coupling leakoff effect with the multistage power hardening constitutive equation, and then the influence of the leakoff effect on the stress field model and initiation pressure. At last, two application cases with the finally initiation model the had been carried out.This paper studies the elastic-plastic constitutive equation of formation, the stress field around wellbore and initiation model and the following conclusions have been maked:(1) Nonlinear constitutive equation is the physical basis of elastic-plastic formation stress field research. Based on the total plastic theory, the single segment multistage power hardening constitutive equations can accurately describe the nonlinear of rock mechanics.(2) Whether the rock is yield or not is determined by in-situ stress and initial yield stress, the greater the in-situ stress, the smaller the yield stress, the rock more likely to be yield. Plastic yield has a little influence on radial stress, but it increases the circumferential stress, reduces the borehole stress concentration.The greater in-situ stress, the smaller yield stress,the smaller power hardening index, the smaller pore pressure, the smaller Biot coefficient, the smaller poisson’s ratio, the yield degree is higher, the more difficult to take place for tensile stress.(3) By the combined effect of rock mechanical parameters and in-situ stress, tensile failure will take place when formation is in the elastic state, while initiation modes include both plastic tensile failure and plastic shear failure.①The initiation orientation of the elastic tension failure, plastic tensile failure and plastic shear failure always is maximum horizontal principal stress direction, but there is angle between plastic shear crack failure surface and maximum horizontal principal stress direction.②Then the rock is yield, only plastic shear failure can take place when power hardening index is less than or equal to0.5. Both plastic tensile failure and plastic shear failure can take place when power hardening index is greater than0.5.③The greater in-situ stress, the smaller yield stress, the smaller power hardening index, the smaller Angle of internal friction and cohesion, the smaller pore pressure, the smaller Biot coefficient, the smaller poisson’s ratio, shear failure is prone to take place, conversely, the tensile failure is prone to take place.(4) The initiation pressure is higher than predicted pressure using elastic theory when the rock is yield. The greater in-situ stress, the smaller yield stress, the smaller power hardening index, the smaller Angle of internal friction and cohesion, the smaller pore pressure, the smaller Biot coefficient, the smaller poisson’s ratio, the initiation pressure is greater, conversely, the initiation pressure is greater is smaller.(5) The initiation model based on nonlinear constitutive equation ffectively predict the elastoplastic initiation mode and initiation pressure,and it is significant for research and field application.