Research On Elastoplastic Mechanical Characterization And Hydraulic Fracture Propagation Mechanism Of Deep Reservoir

With the increasing energy demand,the development of deeper oil and gas resources has become an important direction of scientific and technological innovation in the energy field.Deep resource reserves are abundant and exploration and development potential is huge.It has gradually become an important replacement field for oil and gas supply.Under the condition of deep reservoir with high temperature,high confining pressure and high in-situ stress difference,rock deformation and failure behavior has the mechanical characteristics of plastic enhancement,which leads to unclear hydraulic fracture propagation mechanism and difficult to achieve the expected effect of fracturing reconstruction.Starting with the characteristics of deep reservoir plasticity enhancement,this paper takes Longmaxi shale in southern Sichuan area as the research object,and studies the elastoplastic mechanical characterization of deep reservoir and the mechanism of hydraulic fracture propagation,in order to provide theoretical support for deep reservoir fracturing reform design.The main research work is as follows:First of all,based on the characteristics of deep reservoir environment,high temperature and high pressure rock compression experiments were carried out to obtain the deformation,strength and failure characteristics of rocks under the single and two-factor coupling effects of different temperature and confining pressure,and to define the influence law of temperature and confining pressure on rock mechanical properties.The results show that confining pressure is the main controlling factor of rock plasticity strengthening,and temperature is the influence factor of confining pressure.Under the condition of increasing confining pressure and temperature,the rock exhibits the characteristics of increasing strength and plastic strength.However,the coupling condition of the two factors will lead to thermal microcracks in the rock.Compared with the confining pressure acting alone,the strength deteriorates and the plastic deformation is more significant.Aiming at the elastic-plastic deformation characteristics of rocks with nonlinear plastic strengthening under high temperature and high pressure coupling conditions in deep reservoir,based on Drucker-Prager yield criterion,a strengthening function in the form of first-order exponential attenuation function was constructed,and an elastic-plastic constitutive model considering nonlinear strengthening was established.With the increase of buried depth,The joint increase of temperature and confining pressure has significant influence on the plastic yield and nonlinear strengthening.The elastoplastic mechanical properties of deep reservoir rocks can be effectively characterized by the secondary development of ABAQUS finite element software.Secondly,the visualization experiment system of rock fracture under high temperature and high pressure coupling environment was innovatively designed and built,and rock fracture experiment under different temperature single factor and temperature pressure double factor coupling conditions was carried out.Digital image correlation technology was used to monitor the evolution and development process of crack tip fracture process.According to the relationship between the overall response of rock fracture process and the local displacement field of crack tip,The influence law of temperature and confining pressure on fracture behavior of rock crack tip in deep reservoir is analyzed.The results show that the cohesion-fracture opening displacement relationship tends to be linear in the post-peak softening stage within 180℃ under normal pressure,with the characteristics of brittle fracture and local plastic enhancement of crack tip.The crack tip still yields in a small range under high temperature conditions without confining pressure.Under the coupling effect of high temperature and high pressure,the large range of plastic yield of the matrix material near the crack tip leads to the passivation of the crack tip.In the post-peak softening stage,the cohesion-crack opening displacement relationship shows an upward convex nonlinear trend.Based on the cohesive fracture model theory,the nonlinear cohesive fracture constitutive model is established by introducing exponential coefficients to control the post-peak nonlinearity.With the increase of burial depth,the nonlinear degree of post-peak softening relationship increases with the increase of temperature and confining pressure.Through the secondary development of ABAQUS finite element software,the nonlinear fracture behavior of deep reservoir rock is effectively characterized.Then,a hydraulic fracturing propagation model is established based on the coupled principle of seepage,stress and damage.The elastoplastic constitutive model considering nonlinear strengthening and nonlinear fracture constitutive model are introduced into reservoir matrix unit and fracture propagation unit respectively,forming a numerical simulation method for deep reservoir hydraulic fracture propagation and analyzing the characteristics of deep reservoir hydraulic fracture propagation.And the act of communicating natural fractures.The research results show that,compared with the linear elastic reservoir condition,the elastic plastic deformation around the hydraulic fracture in deep reservoir is obvious,which hinders the hydraulic fracture propagation,and the fracture propagation pressure increases,forming short and wide fractures.The plastic deformation leads to the fracture tip is blunt,the stress concentration effect is weakened,and the fracture propagation resistance is increased,which is the propagation mechanism of deep reservoir hydraulic fracture.High in-situ stress difference is the main controlling factor limiting the volume and complexity of deep reservoir fracture reconstruction.Reducing in-situ stress difference can reduce the obstruction of plastic deformation to hydraulic fracture propagation and promote the opening of natural fractures.Therefore,changing local in-situ stress difference by using the induced stress between multiple clusters of fractures is a favorable condition to improve the scale of elastoplastic hydraulic fracture and promote the formation of complex fractures.It provides a design idea for horizontal well fracturing in multiple stage clusters.Finally,the multi-cluster fracture competition and propagation model of horizontal Wells in deep reservoirs is established,considering the stress interference between multi-cluster fractures,the friction of perforating hole and the dynamic distribution of fluid transfer between wellbore and fracture,and analyzing the influence law of fracturing parameters and perforating parameters on the multi-cluster fracture competition and propagation.From the perspective of stress field reconstruction,the influences of different fracturing methods and interval distance on multi-stage fracture propagation are analyzed.The results show that appropriately reducing cluster spacing or increasing cluster number is beneficial to reducing local in-situ stress difference and reducing the obstruction of plastic deformation to hydraulic fracture propagation,but not conducive to uniform propagation.The relatively uniform propagation of multi-cluster fractures can be promoted by increasing injection rate and viscosity,reducing the number of perforating holes and adopting non-uniform perforating.In the process of stage fracturing,the stress transformation of the alternate fracturing method is more sufficient and the sweep range is large,which can effectively reduce the local stress difference of the subsequent fracturing section and weaken the plastic deformation obstruction.By reasonably reducing the distance between the sections,the elastic-plastic hydraulic fracture propagation of deep reservoir can be effectively promoted.Based on the analysis of elastoplastic hydraulic fracture propagation mechanism,the correlation between engineering and geological factors,plastic influence and fracture propagation is proposed to provide theoretical basis for the fracture reconstruction design of deep reservoir with multiple stages and clusters.