ANALYSIS AND APPLICATION OF EXPONENTIAL STRENGTH CRITERION IN WELLBORE STABILITY INVESTIGATION DURING OIL ANDGAS DEVELOPMENT

摘要

In engineering practice,a linear poroelasticity stress model in combination with a rock strength criterion is commonly used to determine the minimum and pressure required for ensuring wellbore stability.Therefore,a main aspect of wellbore stability analysis is the selection of an appropriate rock strength criterion.The Mohr-Coulomb criterion is the most widely used strength criterion in rock engineering problems.However,the nonlinear strength characteristics of rocks under high in-situ stress can no longer be accurately described and characterized by traditional strength criteria.Bottom-hole rock strength assessment of deep hole is important for optimization of drilling method,drilling parameters and design of bit-tool.In this paper,the distribution characteristics of rock true triaxial strength test data in the main stress space are studied.Then the exponential strength is used to predict the collapse pressure of wellbore based on the linear elastic wellbore stress model.The research results prove that,the present form of Mohr-Coulomb criterion suffers from two major limitations.Firstly,it represents the strength of rock as a linear function of confining pressure.Secondly,the effect of intermediate principal stress is not considered by this criterion.As a result,the collapse pressure predicted by Mohr-Coulomb criterion is conservative.The effect of intermediate principal stress on rock strength is restricted by o3.Besides,in normal faulting stress regime,horizontal wellbore is most prone to collapse.In reverse faulting regime,the wellbore collapse pressure is smaller when drilling along the direction of horizontal maximum In-situ stress.In strike-slip stress regime,the risk of wellbore instability is highest in vertical wells,while the horizontal borehole is more stable.The quantitative study of drilling fluid density to ensure that the stability of wellbore provides basis and suggestions for well structure,optimization of hole trajectory and reasonable determination of drilling fluid density.