Fracture Pressure
The pressure at which the rock breaks down is known as Fracture Pressure. Fracture pressure is higher than pore pressure and less than overburden pressure. Pore pressure and Fracture pressures are very important for designing a well. The specific gravity of mud is computed in such a manner that the hydrostatic pressure created by the column of mud is more than pore pressure but less than Fracture Pressure at any point in the well. In a deep water environment, the window between pore pressure and fracture pressure becomes narrow.
Casing points are carefully selected to ensure that the open hole is cased off before the Hydrostatic Pressure of the mud exceeds the estimated fracture pressure. Since Fracture Pressure while planning a well is an estimated value based on offset well data, ‘Leak Off Test (LOT)’ or ‘Formation Integrity Test (FIT)’ is carried out below critical casing shoes to calibrate Fracture Pressure at that point in the well. Actual values help extrapolation for the remaining part of the well and provide baseline value for future wells in the area as well.
Stress Caging - An effective wellbore-strengthening approach