泥页岩地层破裂压力计算模型研究

Calculation model of breakdown pressure in shale formation

准确预测压裂井的破裂压力对水力压裂的成功实施具有重要作用。由于存在水化现象,使得泥页岩的岩石力学性质发生变化,从而使泥页岩地层的破裂压力计算与常规地层的破裂压力计算存在一定的差异,因此,不能简单地把常规地层的破裂压力计算方法应用到泥页岩地层中,需要对泥页岩地层破裂压力计算方法进行深入研究。为此,基于弹性力学和岩石力学相关理论,并考虑水化应力对井筒周围应力场的影响,应用最大张应力准则,建立了考虑水化应力计算破裂压力的简单模型,并用此模型结合2种泥页岩水化应力的方法进行实例计算。结果表明,考虑水化应力后比不考虑这一情况的泥页岩地层的破裂压力更接近实际的地层破裂压力。该计算模型为快速预测地层破裂压力提供了依据。

It is very important to predi ct formation fracture pressure accurately for successful hydraulic fracturing.Calculating the fracture pressure in shale formation is different from that in common formations as a result of changes of mechanical properties due to hydration, so the calculation method of common formations cannot be applied to mud shale formation and an in-depth research should be made on the calculation method of fracture pressure in shale formation. Therefore, based on theories of elastic mechanics and rock mechanics, considering the effect of hydration on the wellbore stress field, this paper applied maximum tensile stress criterion to establish a new model for calculating fracture pressure by taking hydration stress into account. Besides, this paper carried out calculation examples by using the established model combined with two ways to calculate hydration stress of shale. The result shows that the fracture pressure in shale formation with hydration stress considered is much closer to the fracture pressure in actual formation than that not considered. Hence, this calculation model provides a reference basis for fast predicting the formation fracture pressure.