The Stability Study Of Wellbore In Soft Shale Formation Based On Plastic Damage Theory

The control of wellbore stability during drilling or cementing has always been a major technical topic in the field of oil and gas well engineering.Under the influence of wellbore structure,reservoir lithology,in-suit stress pattern and pore fluid coupling,the deformation and stress distribution in the near wellbore area is very complex,and showing the nonlinear mechanical characteristics which various with time and space.As well as the progressive damage and fracture evolution of cement sheath and wellbore surrounding rock.A series of mechanical and technical problems related to wellbore stability control have been formed.This paper has carried out in the following aspects:(1)Based on the plastic strain softening theory,the strength parameter softening model is introduced into the Drucker-Prager yield function,and the nonlinear constitutive model of surrounding rock in soft shale formation is established.The Updated Lagrangian format is used to describe the solid deformation,and a subroutine is written to modify the permeability coefficient during the seepage process.The influence of fluid seepage on wellbore stability is discussed.(2)Through some engineering cases study,the problems of wellbore plastic damage and wellbore stability in soft shale formation during drilling are studied.A numerical evaluation model of wellbore stability is established considering the effects of seepage-stress coupling,plastic damage and permeability variation.The influences of fluid seepage,mud pressure,horizontal stress difference and wellbore radius on wellbore stability are studied.The results show that seepage-stress coupling effect promotes the plastic damage of wellbore surrounding rock and makes the wellbore become unstable.For overbalanced drilling,when the mud pressure exceeds ten percent of the formation pore pressure,the seepage-stress coupling effect leads to an expansion of the plastic damage area,at least sixty-five percent.Moreover,the increase in the horizontal stress difference promotes the plastic damage zone to concentrate in the direction of the minimum horizontal principal stress,and the damage zone becomes deeper.With the increase of wellbore radius,the wellbore tends to be unstable.(3)Further research of cement sheath damage evolution and mechanical failure mechanism during cementing.The plastic damage constitutive model of concrete is used to simulate the damage evolution process of cement sheath during cementing process.Considering the cement hardening process,the plastic damage constitutive relation of cement sheath and seepage-stress coupling,the stress distribution in the near wellbore and the damage evolution law of the cement sheath are studied.And the effect of hardening process of the cement,the change of the internal pressure of the casing and casing eccentricity on damage of cement sheath is discussed.The results show that at the stage of cement hardening,the cement sheath is less likely to damage at the first interface when the increment of elastic modulus is low.The increase of the inner casing pressure causes radial cracks in the cement sheath.Moreover,casing eccentricity leads to the increase of tangential stress and the concentration of radial damage in the thin cement sheath area,which forms the damage fracture zone.