Application of a new model to analyze leak-off tests

A leak-off test (LOT), commonly known as a formation-pressure-integrity test, is a verification method to estimate the fracture pressure of exposed formations. After cementing each casing string, a LOT is run to verify that the casing seat can withstand the wellbore pressure required to drill safely to the next casing setting depth. Fracture pressure determined from this test is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for casing setting depths are based on LOT results. Although a LOT is a simple and inexpensive test, its interpretation is sometimes difficult, particularly in formations that give non-linear relationships between the pumped volume and the observed pump pressure. Ideally, a straight line is obtained that reflects the total system compressibility (i.e., the drilling fluid, the casing expansion, and the wellbore expansion.) Nonlinear LOT behavior is thought to be caused by gas in the system, by borehole failure, or by leakage of drilling fluid into the cemented casing/borehole annulus. There is, however, no mathematical model explaining nonlinear LOT behavior. In this study, a mathematical model is derived to assist in analyzing nonlinear LOT behavior. The model has been used to predict the observed nonlinear behavior of field examples. In some cases of a nonlinear LOT, the model can be used to predict the maximum fracture pressure of the formation.