Analysis of Non-Linear Formation Fracture Resistance Tests Obtained During Oil Well Drilling Operations.

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, cement, and formation below the casing seat can withstand the wellbore pressure required to drill safely 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 mud weights, casing setting depths, and well control techniques are based upon the result of a LOT. Accurate formation fracture gradient determination is critically important. Although the LOT is a simple and inexpensive test, its interpretation is not always easy or straightforward, particularly in formations that give non-linear relationships between the pumped volume and the observed pump pressure. 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. In this study, a mathematical model using a well-known compressibility equation together with material balance concept is derived for the total system compressibility to fully analyze non-linear LOT behavior. The system compressibility during a LOT was analyzed from five distinct phases: (1) drilling fluid compressibility, (2) casing string expansion, (3) borehole expansion, (4) leak (filtration) into the exposed formation, and (5) general solution, a combination of the first four phases. A simple and easy to use real time spreadsheet program using MS(c) Excel was prepared to analyze and plot the results of a LOT for all phases. An example showing how the spreadsheet program can be used for analyzing a non-linear LOT was presented. The developed model was verified using field data furnished by Unocal and Amoco petroleum companies and Amerada Hess Corporation. The model accurately predicts the observed non-linear behavior of field examples. In the case of a non-linear LOT, the model also predicts the maximum fracture pressure of the formation that would be observed for the given test conditions without running a test until formation fracture.