Application of a new model to analyze leak-off tests

A leak-off test (LOT); commonly known as a formation-pressure-integrity les t, is a verification method to estimate the fracture pressure of exposed fo rmations. After cementing each casing string, a LOT is run to verify that t he casing seat can withstand the wellbore pressure required to drill safely to the next casing setting depth. Fracture pressure determined from this t est is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for subsequent casing setting depths are based on LOT results. Although a LOT is a simple and inexpensive test, its interpretation is some times difficult, particularly in formations that give nonlinear relationshi ps between the pumped volume and the observed pump pressure, ideally, a str aight line is obtained that reflects the total system compressibility (i.e. , the drilling fluid, the casing expansion: and the,wellbore expansion.) No nlinear LOT behavior is thought to be caused by gas in the system, by boreh ole failure, or by leakage of drilling fluid into the cemented casing/boreh ole annulus. There is, however, no mathematical model explaining nonlinear LOT behavior. In this study, a mathematical model is derived to assist in analyzing nonli near LOT behavior. The model has been used to predict the observed nonlinea r behavior of field examples. In some cases of a nonlinear LOT, the,model c an be used to predict the maximum fracture pressure, of the formation.