修正Hoek-Brown强度准则在钻孔稳定性分析中的应用

Application of a modified Hoek-Brown strength criterion to borehole stability analysis

钻孔稳定性分析是深部钻探工程的重要科学问题之一,选择合理的强度准则有助于提高钻孔稳定性预测的准确性,可有效地减少钻孔崩落事件的发生。由于广泛使用的Hoek-Brown准则忽略了中间主应力的影响,应用时无法充分发挥岩石强度潜能。引入一种有效克服诸多经典强度准则的奇异性与非外凸性缺陷的修正Hoek-Brown强度准则,将其应用于水平、垂直和倾斜钻孔稳定性分析中的最小泥浆压力计算,同时与Mohr-Coulomb、Drucker-Prager、Hoek-Brown、3D Hoek-Brown、Mogi-Coulomb和修正Lade准则等经典强度准则预测值进行对比。研究结果表明,修正Hoek-Brown准则与修正Lade、Mogi-Coulomb和3DHoek-Brown准则的预测结果相近,且修正Hoek-Brown预测值与实际工程所用泥浆的相对误差分别为2.1%和0.76%,证明修正Hoek-Brown准则应用于钻孔稳定性分析的适用性与准确性;最小泥浆压力随钻孔深度增加呈线性递增,且受钻孔方位角和倾斜角的影响;Mohr-Coulomb与Hoek-Brown准则所预测的最小泥浆压力值最大,增加了实际工程泥浆的用量,Drucker-Prager准则所预测的最小泥浆压力最小,但给出的预测结果偏于危险,易发生钻孔失稳;基于修正Hoek-Brown准则可得到理论上最佳的钻孔位置(即所需最小泥浆压力值达到最小),为钻井工程师进行钻孔位置优化提供有益参考。

Borehole stability analysis is one of the key scientific issues in the deep drilling engineering. Adoption of a reasonable strength criterion facilitates the prediction accuracy of borehole stability and reduces the chance of borehole breakout. Since the widely-used Hoek-Brown strength criterion neglects the effect of intermediate principal stress, the potential of rock strength cannot be fully realized. To solve the above problems, a newly proposed modified Hoek-Brown strength criterion that overcomes the shortcomings of singularity and non-convexity that exist in most classic strength criteria, is used for the stability analysis of horizontal, vertical, and inclined borehole, respectively. By calculating the minimum mud pressure required for ensuring borehole stability, comparisons with several classic strength criteria such as Mohr-Coulomb, Drucker-Prager, Hoek-Brown, 3 D Hoek-Brown, Mogi-Coulomb, and modified Lade strength criterion are conducted, coming to the following conclusions: the predicted results of the adopted modified Hoek-Brown strength criterion are close to those predicted by 3 D Hoek-Brown, Mogi-Coulomb, and the modified Lade strength criterion;the relative error between the predicted value and the mud value used in two actual projects are 2.1% and 0.76%, respectively;those results prove the accuracy of the modified Hoek-Brown criterion for borehole stability analysis;the minimum mud pressure increase linearly with increasing borehole depth;meanwhile, it is affected by the inclination angle and the azimuth angle of the borehole;both Mohr-Coulomb and Hoek-Brown criteria are conservative in predicting the minimum mud pressure, increasing the amount of mud that actually used;and the Drucker-Prager criterion over-predicts the rock strength, bringing about an unsafe predictive value of minimum mud pressure;base on the modified Hoek-Brown criterion, the theoretically optimal drilling position can be obtained(i.e. the minimum required mud pressure value), which provides a useful reference for the drilling engineer to optimize the position of borehole.