Distinct Element Analysis of Borehole Instability in Fractured Petroleum Reservoir Seal Formation

Abstract

Abstract This work presents the analysis of a borehole instability case at 2km in a seal formation within the oil field environment. Field observations of severe borehole instabilities are given. The factors causing this instability are observed as being due to cyclic mud pressure loading, drilling induced damage, wellbore orientation and fault set intersection at the wellbore. Borehole breakout data both from image logs and calliper data has been used to determine the in-situ stress field. The minimum principal stress magnitude has been obtained using leak off testing, extended leak off testing through the analysis of borehole breathing during drilling with losses. The data has been analysed using non-parametric statistical techniques to determine the complete stress tensor. Fault property and in-situ rock mass data has been obtained from triaxial and shear box testing. For this study the distinct element method (UDEC) has been used in 2D to study the wellbore stability under dynamic (fluid flow) conditions. Factors taken into account include fault, fracture and bedding surface orientation, fracture parameters, in-situ stress conditions, cyclic mud pressure loading, initial drilling induced damage and well bore orientation. The study has investigated the behaviour of a synthetic rock mass containing low permeability rock blocks separated by low to very low permeability joints. It has been observed that all parameters influence the level of stability but using a prolonged delay in mud pressure reduction, stability can be maintained, also it is possible to select optimum wellbore orientation in advance using the numerical procedures.