THE ANALYSIS OF WELLBORE INSTABILITY BASED ON THE HOEK-BROWN FAILURE CRITERION USING GEOMECHANICAL UNITS AND DISCONTINUITIES EVALUATION

The accuracy of wellbore instability assessment becomes reliable when sufficient geomechanical model data and appropriate failure criteria parameters are used. The Hoek-Brown failure criterion is widely used in wellbore instability analysis; however, limited focus has been given to parameter determination and the specific conditions for its application (intact rock or rock mass) within wells. In this study, theoretical wellbore analysis was conducted using various failure criteria, with a primary focus on comparing the Hoek-Brown criterion for both intact rock and rock mass conditions. A geomechanical model was developed using petrophysical logs, laboratory tests, and downhole measurements. The geomechanical units of the wellbore were determined through clustering algorithms, supported by downhole plugs. Natural discontinuities within the wellbore were identified using image logs, and their distribution was mapped for each geomechanical unit. The parameters for intact rock in the Hoek-Brown criterion were derived from laboratory test results, while those for the rock mass condition were determined using the rock mass rating (RMR) and geological strength index (GSI). The results indicate that applying the Hoek-Brown criterion assuming intact rock does not provide a reliable estimation of wellbore instability. However, by using the criterion under rock mass conditions, wellbore failures can be accurately predicted according to the wellbore conditions. Additionally, evaluating the wellbore conditions based on geomechanical units improves the theoretical analysis’s accuracy and identifies stable and unstable zones.