破碎性地层坍塌压力计算模型

A MATHEMATICAL MODEL OF CALCULATING COLLAPSE PRESSURE FOR FRACTURE FORMATION

破碎性地层裂缝、微裂缝及层理十分发育,地层各向异性明显,地层坍塌压力高,安全钻井液密度窗口窄,在低井筒液柱压力下井壁易垮塌,在高井筒液柱压力下又容易发生井漏。不同于均质各向同性地层,这类地层的井壁围岩力学稳定性主要取决于这些单元体结构面的强度(抗滑能力)。当单元体所受应力超过了其结构面抗滑能力时,井壁围岩即出现滑动失稳。由均质各向同性地层井壁力学稳定性分析理论出发,引入地层岩石破碎程度指数Bd来定量的描述地层岩石破碎程度对井壁围岩等效应力状态的影响,并根据Mohr-Coulomb准则建立了破碎性地层坍塌压力计算式,Bd值越大,井壁坍塌压力越大,井壁越易失稳,实例计算表明所建立的模型计算结果与工程实际吻合较好。

Fracture formation has many characters,such as well-developed fracture,micro-fracture and bedding,obvious aeolotropism,high collapse pressure and narrow safe mud density window,the borehole is easily to fall in with low wellbore pressure,and lost circulation with high wellbore pressure.Distinguished with homogeneous and isotropic rock formations,the mechanics stability of these boreholes in such formations mainly depends on the unit body structural plane intensity(antiskid capacity).The wall rock would experience destabilizing when the stress acted on unit body exceeded their antiskid capacity.On the basis of the theory of the mechanics stability of wall rock in isotropic homogeneity formations,a new mathematical model based on Mohr-Coulomb Criterion was established to calculate the collapse pressure of fracture formations,and break degree(Bd) of rock formation was introduced into the new formula to quantitatively describe the influence of the rock fragmentation degree on in-situ stress.Example showed that the calculation results of the new model were consistent with practice.