Fracture-initiation pressure prediction for transversely isotropic formations

Fracture-initiation pressure (FIP) is one of the most important considerations for the exploitation of petroleum and geothermal energy as well as the storage of hydrogen energy, CO2 and hydrocarbon. Wellbore pressures above the FIP maybe lead to formation fracturing, gas leakage and mud lost circulation. The effect of rock anisotropy on FIP is significant, but has not received enough attention. In this paper, the rock mechanical properties were investigated for transverse isotropic rock. Taking into account the anisotropic deformation, modulus, tensile strength and in-situ stress, a FIP prediction model was developed. The FIP models with isotropic situation, anisotropic modulus, anisotropic strength, and fully anisotropic situation were compared. The influence of anisotropic mechanical parameters, horizontal stress ratio (HSR) and pore pressure (PP) on the FIP were also analyzed. The results suggested that the transversely isotropic rock exhibits a distinct anisotropic characteristic. When the anisotropy of rock mechanical parameters was taken into account, the FIP may be higher or lower than that of isotropic model, and the amplitude of variation was about +/- 10%. Poisson's ratio anisotropy had a slight impact on the maximum and minimum FIPs. While the more significant the tensile strength anisotropy means the lower FIP. In addition, the influence of anisotropy on the FIP becomes increasingly pronounced with larger HSR and higher PP. The present paper can provide effective guidance to improve the drilling and hydraulic fracturing.