Risk and Uncertainties in Determining Fracture Gradient and Closure Pressure

摘要

The fracture gradient is considered as the upper bound of the mud weight window in drilling. Drilling engineers usually treat the leakoff pressure (LOP) from the leakoff test (LOT) as the fracture gradient. Geomechanical engineers argue, however, that the minimum horizontal stress (or closure pressure) should be treated as the fracture gradient. Because the closure pressure is the theoretically correct fracture gradient, this paper investigates the relationship between the LOP and the closure pressure (CP) to assess the error caused by using LOP as the fracture gradient. The study focuses on the fracture mechanics approach in which the correlation between LOP and CP is related to the fracture toughness and rock tensile strength. Deviating from the conventional approach in which the LOP is considered as the initial fracturing pressure, this paper presents the model based on the brittle and ductile rock failure mechanisms and claims that the slope change at the LOP may indicate the transition between the brittle rock failure and ductile rock failure. In this circumstance, the LOP is equal to the breakdown pressure. Various relationships between the LOP and the CP are proposed for several physical circumstances.