裸眼测试井壁失稳概率及参数敏感性分析

Analysis of wellbore instability probability and parameter sensitivity of openhole testing

为保证南海高温高压区块裸眼测试期间的井壁稳定性,基于裸眼层段高温和渗流特点,结合温度应力理论和多孔渗流效应,建立了裸眼井壁渗流-温度耦合应力模型。对比分析Mohr-Coulomb准则、Drucker-Prager准则、Hoek-Brown经验准则和能量破坏准则下的井壁失稳临界测试压差,并利用蒙特卡罗模拟,用正态分布和三角分布模拟岩石力学参数计算井壁失稳概率,采用Sobol方法对临界测试压差参数进行全局敏感性分析。研究结果表明,Drucker-Prager准则更适合作为裸眼测试井壁失稳临界测试压差计算准则,小于临界测试压差3~5 MPa时已经存在井壁失稳的可能,最大水平主应力对临界测试压差起到决定性作用。裸眼测试过程中,临界测试压差与实际测试压差的差值应保持在5 MPa以上,对于最大水平主应力变化较大的地层减小测试压差,以防止发生井壁失稳。

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.