Application Of Single-Parameter Parabolic Strength Criterion And Hoek-Brown Strength Criterion In Wellbore Stability Analysis

Though Mohr-Coulomb strength criterion is the most widely used wellbore stability analysis theory,the linear strength criterion is difficult to accurately describe the nonlinear characteristics of the rock strength under confining pressure,especially for those rock masses buried in deep formation with high confining pressure in oil&gas well engineering.Thus,adopting a better strength criterion will greatly improve the accuracy of wellbore stability prediction.Through the research of this paper,the following achievements and understanding are obtained:(1)Two strength criteria with parabolic failure envelope are deduced.Based on the parabolic Mohr strength criterion(Fairhurst criterion)and the characteristics of rock strength data presented in numerous literatures,we deduced the single-parameter parabolic criterion and Hoek-Brown strength criterion,and analyzed their geometrical characteristics in principal stress space,as well as their application advantages in wellbore stability analysis in deep formation.(2)The fitting precision and application advantages of the presented two parabolic strength criteria are analyzed.The fitting accuracy of the Mohr-Coulomb criterion,the Hoek-Brown criterion and the single-parameter parabolic criterion is analyzed using mathematical statistics and numerical fitting method to illustrate the fitting advantages of parabolic strength criterion when predict the non-linear destruction rock strength data.Two field cases are studied where the optimization of wellbore trajectory and configuration of drilling fluid density is carried out to show the superiority of the parabolic strength criterion to Mohr-Coulomb criterion.(3)The criterion parameters of two parabolic strength criteria are determined and the application conditions are analyzed.The high precision and parameters of the strength criterion under high confining pressure is obtained by fitting the rock strength data with confining pressure greater than 15MPa using single parameter parabolic criterion.The equivalent Hoek-Brown and the Mohr-Coulomb method are used to determine the rock mass parameters m and s in Hoek-Brown criterion,thus to establish the corresponding transformation relations between rock strength parameters(cohesion and internal friction angle)and the criterion parameters m and s.(4)A prediction model of rock uniaxial compressive strength considering porosity and water content was established.Uniaxial compressive strength is one of the most important mechanical and strength properties of rock,and it is also the criterion parameter of single-parameter parabolic strength criterion.Based on the binary analysis method,we established the rock uniaxial compressive strength prediction model with consideration of rock porosity and water content,and analyzed model parameters with their physical meanings,which provides a reference method for laboratory results to be extrapolated in engineering practice.(5)The wellbore collapse model based on the two parabolic strength criteria are respectively established.Based on the elastic solution of the stress distribution,the wellbore stability models respectively based on Hoek-Brown criterion and single-parameter parabolic criterion are established,and the collapse pressure considering the seepage and the non-seepage condition is given to evaluate the wellbore stability.