The Research On Mechanism Of Wellbore Stability In Fractured Formation Considering Multi-field Coupling

Ultra-deep fractured formation in western China can be treated as a typical dualporosity/dual-permeability medium.Drilling fluid interacts with chemically active formation in non-isothermal condition in drilling process,where heat transfer,mass transfer and flow transfer process of the surrounding rock matrix and fracture network are extremely complicated.The traditional deep formation elastic mechanics model only consider the long term wellbore stability,which neglects the process of dynamic response after excavation,local thermal non-equilibrium and mechanical-hydro-thermo-chemo coupling,thus it has limitations in prediction of time-dependent wellbore instability.Based on the sequence of surrounding rock interacting with drilling fluid,this dissertation divides wellbore instability into three kinds of state considering transient dynamic model after excavation,local thermal non-equilibrium and mechanical-hydro-thermo-chemo coupling,and establish corresponding model,analyze mechanism of wellbore instability,and provide basic theory for safe drilling in fractured formation.The specific innovation work of this paper is as follows:1.Study on dynamic model of wellbore stability in fractured formations with wellbore excavationTo clarify the poroelastodynamic response of fractured formation after sudden excavation,the dynamic model for dual-porosity/dual-permeability medium considering mechanical-hydro coupling is established,which overcomes the problem that the quasistatic model assumes the instantaneous stress balance and ignores the finiteness of stress wave propagation velocity.The abnormal pore fluid pressure response mechanism after borehole excavation is revealed,and model results show that the low permeability formation is more prone to wellbore instability than the high permeability formation,and excavation time of orders of millisecond level also has a significant impact on the wellbore stability.2.Study on wellbore stability model of fractured formation considering local thermal non-equilibriumTo clarify the porothermoelastic response of fractured formation in the early stage of excavation,the mechanical-hydro-thermo coupling model considering local thermal non-equilibrium is used.Different from the previous model assuming the skeleton and fluid's temperatures are instantaneously balanced,model result indicate that inconsisty of temperatures of fracture fluid with skeleton-pore-fluid are found in the initial stage of excavation,due to difference of heat diffusivity coefficient of solid and fluid and insufficient heat transfer.And local thermal non-equilibrium induces different profiles of fluid pressures compared with Local thermal equilibrium model,thus may increase failure potential of wellbore.Besides,calibration method for heat transfer coefficient of solid skeleton and pore fluid is established,and then provides a theoretical basis for accurately predicting the formation temperature field and the well stress field.3.Wellbore stability model considering Multi-field coupling in fractured formationTo clarify the poroelastic response mechanism of fractured formation under theralchemical-mechanical-hydro perturbation,the wellbore stability model of dual-porosity medium considering mechanical-hydro-thermo-chemo coupling is constructed for the first time,overcoming the problem that previous model only considers the single factor of temperature or chemical effect.The time-dependent stress field under the combined perturbation of temperature,chemical,hydraulic and stress factors is obtained,and then analyze the influence of drilling fluid solute concentration,temperature and liquid column pressure on wellbore stability,and establish prediction model for fractured formation under multi-field perturbation in the end.