ANALYSIS OF INTERMEDIATE PRINCIPAL STREE ON OIL AND GAS WELLBORE COLLAPSE PRESSURE

In order to optimize wellbore shear failure criteria, this article, comprehensively, evaluated the application characteristics of Mohr Coulomb criterion, Drucker-Prager criterion and modified Lade criterion. At present, the research on the stability of the wellbore is mainly based on the classic Mohr-coulomb criterion, but lots of experiments show that the intermediate principal stress still has a huge impact on the rock damage. In order to accurately describe the influence of intermediate principal stress on the borehole wall stability, this paper applies the modified Lade criterion to wellbore stability analysis for isotropic stratum. Results indicated that Mohr-Coulomb criterion, which is linear and neglects the effect of intermediate stress, underestimates the strength and estimates the highest minimum mud pressure required to stabilize the wellbore. The Drucker-Prager criterion can provide a better estimation compared with Mohr-Coulomb. However, the modified Lade criterion that considering the effect of intermediate stress is the best failure criterion for determination of safe mud weight window. The in-situ stress distribution and the actual space state of the borehole also largely determine the stability of the borehole. The wellbore collapse pressure cloud pictures showed that the changing regularity and sensitivity of well bore collapse pressure with deviation angle and azimuth angle is different. The research can be used to optimize borehole trajectory, avoid drilling accidentprone areas and improve drilling efficiency.