TM耦合下科钻深井结晶岩井壁失稳力学响应规律

Mechanical responses to wellbore instability of crystalline rocks in scientific deep well during thermo-mechano coupling

依据热力(thermo-mechano,TM)耦合影响下坍塌和破裂压力计算模型,探讨8000 m科钻深井处结晶岩井壁失稳规律。在温度为260℃、应力为200 MPa时,采用Mohr-Coulomb破坏准则和最大拉应力理论,通过力学推导和MATLAB编程运算求解坍塌、破裂压力演化模型;揭示TM耦合下井壁失稳的力学响应规律,得出钻井液安全密度窗口,以便为井眼轨迹设计和钻井液优选提供指导。研究结果表明:当井眼轨迹沿最大或最小水平地应力方向钻井时,井壁在最小水平地应力方向最易坍塌,在最大水平地应力方向最易破裂;坍塌、破裂压力在一定深度的井壁上呈周期性规律分布,最易破坏点随着井斜角、方位角的变化而变化;热效应对井壁破裂压力的影响明显,钻井液循环改变了安全密度窗口大小;钻井液安全密度窗口可为井眼轨迹设计和钻井液优选提供指导。

Based on the model calculation of the collapse and fracture pressure under the thermo-mechano coupling,the wellbore instability of crystalline rock in a scientific depth of 8000 m was discussed.At 260℃and 200 MPa,the evolution models of collapse and fracture pressure were developed by mechanical calculations on MATLAB based on Mohr-Coulomb failure criterion and the maximum tensile stress theory.The mechanical responses to the wellbore instability of crystalline rock formation during the thermo-mechano coupling were revealed,and the safety density window of drilling fluid was obtained.These would provide guidance for the well structure design and drilling fluid optimization for the scientific deep wells.The results show that,when the wellbore is drilled along the direction of the maximum or the minimum horizontal ground stress,wellbore inclines to collapse and fracture in the minimum and the maximum horizontal ground stress directions,respectively.The collapse and fracture pressures are periodically distributed on the wellbore with depth,and the most vulnerable points change with the azimuth and oblique angles.The thermal effect is significant under the wellbore fracture pressure,and the drilling fluid circulation changes the safety density window size in the scientific deep well.The safety density window of drilling fluid can provide guidance for the well structure design and drilling fluid optimization for the scientific deep wells.