New model to analyze nonlinear leak-off test behavior

A leak-off test (LOT) is a verification method to estimate fracture pressur e of exposed formations. After cementing each casing string, LOT is run to verify that the casing, cement and formation below the casing seat can with stand the wellbore pressure required to drill for the next casing string sa fely. Estimated fracture pressure from the test is used as the maximum pres sure that may be imposed on that formation. Critical drilling decisions for mild weights, casing setting depths, and well control techniques are based upon the result of a LOT. Although LOT is a simple and inexpensive test, i ts interpretation is not always easy, particularly in formations that give nonlinear relationships between pumped volume and injection pressure. The o bserved shape of the LOT is primarily controlled by the local stresses. How ever, there are other factors that can affect and distort LOT results. Phys ically the LOT, indeed, reflects the total system compressibility, i.e., th e compressibility of the drilling fluid, wellbore expansion, or so-called b orehole ballooning, and leak (filtration) of drilling fluids into the forma tion. There is, however, no mathematical model explaining the nonlinear beh aviour. Disagreement on determining or interpreting actual leak-off pressur e from the test data among the operators is common. In this paper, a mathem atical model using a well-known compressibility equation is derived for tot al system compressibility to fully analyze nonlinear LOT behavior. This mod el accurately predicts the observed nonlinear behavior in a field example. The model also predicts the fracture pressure of the formation without runn ing a test until formation fracture.