WELLBORE INSTABILITY EVALUATION BY USING MOHR-COULOMB FAILURE CRITERION

Abstract

Wellbore instability represent one of the serious drilling problems encountered in shale formations due to interaction between drilling fluids and shale formations. The investigated Shale samples was taken from Well (K-246) located in Baba Dome in Kirkuk oil field.Pore pressure transmission experiments were performed to study the effect of variations in wellbore pressure (Pw) with different salt types and concentrations on the wellbore pressure(Pnw) which affect or disturb the state of stresses around the wellbore.The Invariant Shear Stress (√(J_2 )) profile are calculated to determine wellbore instability state as a function of salt types and concentrations depending on the Mohr-Coulomb failure criterion which is experimentally determined for the shale sample by Tri-axial Test. The results shows that well (K-246) is unstable(Collapse Failure) at (Pw = 2400 psi and 2440 psi) at salt concentration (14% and 16%) for NaCl, KCl and CaCl2 salt solutions but stable for other salt concentrations employed in the research.