Analysis of wellbore instability probability and parameter sensitivity of openhole testing

In view of the high temperature and flow features in openhole intervals, an openhole fluid flow-thermal coupling stress model was established based on the thermal stress theory and porous medium seepage effect, in order to ensure the wellbore stability during openhole testing in high-temperature high-pressure blocks of the South China Sea. The critical testing drawdown pressure of wellbore instability under the Mohr-Coulomb, Drucker-Prager, Hoek-Brown empirical criterion and energy failure criteria was analyzed. The Monte Carlo simulation was used to calculate the wellbore instability probability, in which the rock mechanics parameters were subjected to the normal distribution and triangular distribution. The Sobol method was used to perform global parameter sensitivity analysis of critical testing drawdown pressure. The results showed that Drucker-Prager criterion is more suitable for calculating the critical testing drawdown pressure of wellbore instability during openhole testing. Once the testing drawdown pressure reaches a value that is 3–5 MPa below the critical value, the wellbore instability becomes possible. The maximum horizontal principal stress plays a decisive role in the critical testing drawdown pressure. During openhole testing, the difference between the critical testing drawdown pressure and the actual value shall be kept above 5 MPa, and the testing drawdown pressure shall be reduced for formations with variable maximum horizontal principal stress to prevent the wellbore instability.