安全钻井液密度上限的确定方法

当钻井液密度达到安全钻井液密度窗口的上限时，井壁并不只是发生拉伸破裂，还有可能发生剪切破裂。为此，根据井壁围岩的应力分布规律，分析了井壁破裂时可能发生的各种破坏模式，并给出了相应的3个破裂压力计算公式。在此基础上，定量分析了地应力非均匀性、地应力大小、地层强度和孔隙压力对井壁破裂时破坏模式的影响规律。结果认为：地应力非均匀性较小时，井壁易发生剪切破裂，发生拉伸破裂的可能性随地应力非均匀性的增强而增大；当水平地应力较小时，剪切破裂和拉伸破裂均有可能发生，随地应力增大，发生拉伸破裂的可能性减小；地层强度和孔隙压力越大，井壁发生拉伸破裂的可能性越大。最后，通过东方13 1气田的1口井实例分析说明了在确定安全钻井液密度窗口上限时，必须考虑剪切破裂发生的可能性，应以发生拉伸破裂和剪切破裂所需钻井液密度的最低值作为安全钻井液密度窗口的上限。

A method of determining the safe upper limit of drilling fluid density

When the drilling fluid density reaches the upper limit of the safe window, not only tensile but shear fractures would occur in the well wall. In view of this, we analyzed multiple failure modes in the wellbore wall fracturing according to the stress distribution rule of the wall rocks, and provided 3 fracture pressure calculation formulas correspondingly. On this basis, we performed a quantitative analysis of how the heterogeneity and degree of in situ stress, formation strength and pore pressure have impact on the failure modes in the wellbore wall fracturing. The following findings were concluded. (1) When the heterogeneity of in situ stress is weak, shear fracture is likely to occur; but when it becomes stronger, tensile fracture is more and more likely to happen. (2) When the horizontal in situ stress is small, both shear and tensile fractures are likely to occur, but if it increases, tensile fracture becomes less likely to occur. (3) The higher the formation strength and pore pressure are, the greater the possibility of tensile fracture is on the well wall. Through the case analysis of a well in the Dongfang 13 1 Gas Field, western South China Sea, it is concluded that any possibility of shear fracture must be considered in determining the upper limit of the safe drilling fluid density window, as which the minimum density of drilling fluid causing tensile and shear fractures should be taken.