AN IMPROVED METHODOLOGY ON WELLBORE STABILITY PREDICTION USING GEOMECHANICAL ANALYSIS

The result of geo-mechanical analysis of wellbore stability prediction is the reduction in non-productive time caused by wellbore stability problems such as mud loss, loss circulation, breakouts, collapse, tight hole, etc. Wellbore failures are grouped into two modes, tensile failure occurs due to very high mud weight greater than fracture gradient and compressive failure occurs due to low mud less than the shear failure gradient. The main parameters which are used to predict the wellbore stability includes the in-situ maximum and minimum horizontal stresses, the in-situ overburden stress, pore pressure, and the rock strength and elastic parameters(Unconfined compressive strength, friction angle, cohesion, Poisson ratio and Young’s Modulus) coupled with a suitable failure criteria which is used in estimating the upper limit of the mud weight window(fracture gradient) and lower limit of the mud weight window (shear failure gradient). The critical or optimum mud weight is obtained within mud weight window. The main objective of this project is to apply a geo-mechanical model based on a well data that will numerically represent the states of the in-situ stresses, pore pressure and the rock strength at subsurface coupled with a suitable failure criteria in order to obtain a safe mud weight window and recommend a safe drilling direction for wells which will be developed in the future. The Mohr-Coulomb failure criteriais used in this project for deriving the failure equations for cases of tensile and compressive failure modes. The tangential, radial and the axial stresses (wellbore stresses) are determined as a function of change in dimensional radial distance from (r = R) at the wellbore wall to (r = 5R) at angle of orientation() and (. The Mat lab software isused in this project for writing programming language for the stress equations, obtain results and perform analysis.The results from the use of the Mat lab software analysis are used to conclude the effect of mud weight, well bore inclination, and well bore orientation on effective wellbore stress distribution.Variation of wellbore inclination and azimuth are also be used to recommend a safe drilling direction and mud weight window. Evaluation of compressive and tensile failure from plot of predicted mud weight, pore pressure, shear failure gradient, fracture gradient and overburden stress profilesas a function of depth is considered in this analysis.