采用孔隙热弹性模型分析页岩井眼失稳规律

基于多孔介质连续力学,考虑各向异性页岩的热流固耦合作用,建立了各向异性多孔介质孔隙热弹性力学基本方程和页岩地层井眼稳定分析数学模型,采用MATLAB软件编制了数值求解程序,分析了流固耦合、热固耦合、热流固耦合三种不同耦合模式下井周孔隙压力、地层温度、应力分布和井眼稳定性随时间变化的特征。结果表明:数值解与解析解计算结果吻合较好(误差仅0.028%),实例井井眼扩径率计算结果与实际情况基本吻合,验证了本文模型和方法的正确性;温度和孔隙压力的改变对井周应力分布影响较大,地层温度降低、井内压力增加有利于井眼稳定,孔隙压力升高不利于井眼稳定;页岩地层井眼稳定性具有显著的时变效应,随着钻开地层时间的增加,井眼不稳定区域和井眼扩径率逐级变大。孔隙热弹性力学分析方法可用于分析不同时间的井眼失稳规律,可为解决页岩井眼失稳问题提供更加科学的计算手段和方法。

Analysis of Wellbore Instability Law of Shale Formations by Porothermoelastic Mechanics Model

Based on the continuous mechanics of porous media and considering the thermo-hydro-mechanical (THM)coupling of anisotropy shale, the porothermoelastic mechanics basic equations of anisotropic porous media and the mathematical model for the borehole stability analysis of shale formation are established. The numerical procedures to solve the problems have been compiled by MATLAB, and analyzing the characteristic of time-varying pore pressure, formation temperature, stress distribution and borehole stability around borehole under three different coupled modes, that is hydro-mechanical (HM)coupling, thermo-mechanical (TM)coupling and THM coupling. The results indicate that the calculation results of numerical solution and analytic solution are in good agreement (the error is 0.028%), and the enlargement ratio of borehole in example well are in consistent with the actual situation, the correctness of the model and method in this paper are verified. The changes of temperature and pore pressure have a larger influence on stress distribution around borehole, formation temperature decrease and pressure build up in borehole are in favor of borehole stability, and the rising of pore pressure is adverse to borehole stability. The borehole stability of shale formation has a significant time-varying effect, with the time increasing of drilling formation, the area of borehole instability and the hole-size elongation ratio increase gradually. The analysis method of porothermoelastic mechanics can be available for analyzing the law of borehole instability in different time, and it also can provide a scientific calculated means and methods for solving the shale borehole instability.