地层水物化作用下的泥页岩破裂压力计算

Calculation of shale fracturing pressure under physicochemical effect of formation water

钻井过程中,钻井液与井壁围岩的接触产生水化作用会导致井壁围岩变形,引发井壁缩颈坍塌、破裂等事故。根据弹塑性力学和岩石力学相关理论,应用最大张应力准则,在黄氏模型的基础上考虑了钻井液在岩石孔隙中的渗流而在井壁围岩所产生的附加应力场、岩石的孔隙度和钻井液水化作用的影响,建立了泥页岩破裂压力模型,结合现场压裂实验数据和不同含水率泥页岩岩心三轴压缩实验结果,计算得到了泥页岩破裂压力的预测值、泥页岩含水率与抗张强度和破裂压力的关系曲线。结果表明:本文模型预测值和实测值相比,误差为3.65%,更加接近实测地层破裂压力,破裂压力和抗张强度均随着含水率的升高而降低,说明水软化了泥页岩,降低了它的力学性能。

In the process of drilling,hydration occurs when the drilling fluid contacts with the surrounding rock of the wellbore,which will lead to the deformation of shaft wall and cause the occurrence of accidents such as sidewall necking,collapse and cracking. According to the theory of elastic-plastic mechanics,rock mechanics and the maximum tensile stress criterion,the shale fracture pressure model was established on the basis of Huang’s model,considering the effects of additional stress field caused by the percolation of drilling fluids in rock pores of the well wall rock,rock porosity and the fluid hydration. According to field fracturing experiment data and triaxial compression test results of shale cores with different water contents,the prediction value of shale fracture pressure,and the relation curves of shale water contents with tensile strength and the fracture pressure were obtained. The results show that the error of this model is 3.65% compared with the measured value,which means that the predicted value is closer to the measured fracture pressure,and both the fracture pressure and tensile strength decrease with the increase of water contents. It shows that the water softens the shale and reduces its mechanical properties.