基于钻孔局部壁面应力解除法的深部页岩三维地应力计算方法

A calculation model for 3D in-situ stress tensor of deep shale based on borehole wall stress relief method

页岩气储层的地应力状态是评价页岩气可采性的重要指标之一。为了满足页岩气勘察的实际需要,在钻孔局部壁面应力解除法(BWSRM)的基础上,充分考虑各向异性、孔壁温度变化以及孔壁压力的影响,推导出适用于深部页岩的三维地应力计算方法,并通过算例分析上述因素对孔壁附近应力分布的影响。得到如下结论:(1)孔壁处应变和远场地应力之间的关系与多个力学参数以及热学参数有关,理论上需要得到不少于9个不同方向上的正应变数据并结合室内试验来确定测点附近的三维地应力状态;(2)孔壁压力以及孔壁温度改变会影响孔壁附近应力分布,影响范围为3~5倍钻孔半径。深部岩体力学以及热学上的各向异性也会影响深部地应力分布;(3)在使用BWSRM方法测量并计算深部岩体三维地应力过程中,必须考虑测试环境以及岩性条件的特点,否则无法得到正确的三维地应力场状态。

The in-situ stress condition of shale gas reservoir is a very important index to evaluate the gas extraction. A calculation model of 3 D in-situ stress tensor of deep shale based on borehole wall stress relief method is presented considering the anisotropy,temperature changes and pressure at borehole wall. The distributions of stress around borehole wall are studied through numerical examples. The results show that the formulas between the borehole strains and far-field in-situ stress tensor are related to the mechanical and thermal parameters. At least 9 normal strains at different directions and combined laboratory test are needed to solve the 3 D stress tensor around the measured area. The borehole pressure and temperature changes affect the distributions of stress within the range about twice of the borehole radii and the different degrees of mechanical and thermal anisotropy in deep rock also influence the results of stress measurements. When the 3 D stress tensor of deep rock is calculated using the method of stress relief in borehole wall,the measurement environment and lithological conditions must be considered.