Numerical modeling of logged wellbore breakouts using cohesion-weakening frictional-strengthening models

Abstract

The wellbore stability analysis based on the simplified models such as the elastic-perfect plastic or the elastic-brittle models results in the unrealistic prediction of the wellbore damage zone characteristics and consequently underestimate or overestimate the required mud pressure respectively. To overcome this weakness, the advanced elastic-plastic models considering the full rock failure mechanisms in the post-peak region are reviewed briefly in this paper. The damage zone surrounding a wellbore located in a carbonate oil field in a southwest of Iran is analyzed based on the different advanced models. The objective is to study how well the numerical simulation results based on the advanced models agree with the logged fallouts with respect to location, depth, and shape of damage zone around the wellbore. An evaluation of which advanced model can best capture the actual rock behavior especially in the post-peak region is carried out by comparing the simulation results with the logged fallouts around the wellbore.The depth of the damage zone and transverse extension angle around the studied wellbore predicted well based on a kind of the cohesion-weakening frictional-strengthening (CWFS) model are 3 cm and 84.3° respectively. The simulation results of this model derived from the tests on the carbonate rocks are in more agreement with the logged breakouts around the studied wellbore drilled in the carbonate rocks like limestone. Moreover, according to the key input parameters sensitivity study carried out in this paper, decrease in the residual cohesion and critical plastic strain parameters result in the larger simulated damage zone. On the contrary, increment of the initial friction angle and drilling mud pressure leads to the smaller simulated breakout around the wellbore. As higher dilation angle is utilized in the numerical model as input parameter, the larger simulated breakout is created around the wellbore.