Application of a New Model to Analyze Leakoff Tests

Abstract

Abstract A leak-off test (LOT), commonly known as formation pressure integrity test (PIT), 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 safely drill to the next casing setting depth. Fracture pressure determined from the test is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for subsequent casing setting depths are based upon the result of a LOT. Accurate formation fracture gradient determination is critically important. Although a LOT is a simple and inexpensive test, its interpretation is not always easy, particularly in formations that give non-linear relationships between the pumped volume and the observed pump pressure. Ideally, a straight-line relationship of pump pressure to volume pumped is obtained that reflects primarily the total system compressibility, i.e., the compressibility of the drilling fluid, casing expansion, and wellbore expansion. Non-linear 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 the non-linear LOT behavior. Disagreement on determining or interpreting actual leak-off pressure from the test data among the operators is common, especially for shallow formations. In this study, a mathematical model is derived to assist in the analysis of non-linear LOT behavior. The model has been used to predict the observed non-linear behavior of field examples. In the case of a non-linear LOT, the model has also been used to predict the maximum fracture pressure of the formation without running a test until formation fracture.