Effect of parameter correlation on risk analysis of wellbore instability in deep igneous formations

Abstract

A Permian igneous formation in the Sichuan Basin typically exhibits complex lithology, strong heterogeneity, and strong uncertainty; consequently, there is a very high risk of wellbore collapse during drilling and completion, which would seriously affect the safety, efficiency, and cost of drilling operations. Therefore, to prevent wellbore collapse in Permian igneous rock formations, a risk assessment method of wellbore instability based on reliability theory was proposed, and the effects of parameter correlation on the reliability of wellbore stability were considered. First, profiles of the rock mechanical parameters, formation pore pressure, and in-situ stresses were established by logging interpretation. Second, the optimal probability distribution of each parameter was determined for three typical igneous formations, and the correlation coefficient matrix was obtained using correlation analysis. Third, the effect of parameter correlation was involved to propose a risk analysis method of wellbore instability, where the Monte Carlo method was used for random sampling. Finally, the safe mud weight window of each igneous formation was determined, and the sensitivity of wellbore stability to the parameter mean values and correlation coefficients was analyzed. The results indicate that significant uncertainty and parameter correlation exist for the igneous formations, which clearly affect wellbore stability. The effect is greatest for the breccia formation, followed by the tuff and basalt formations. The correlations between cohesion and internal friction angle and between cohesion and maximum horizontal principal stress are the most important factors affecting collapse pressure, whereas the correlation between the two horizontal stresses is most important for fracture pressure. The results suggest that the effect of parameter correlation on the reliability of wellbore stability in strongly heterogeneous settings cannot be ignored.