# Computer-Aided Analysis of Formation Pressure Integrity Tests Used in Oil Well Drilling.

1986

Dissertation

## Degree Name

Doctor of Philosophy (PhD)

## Abstract

Formation pressure-integrity tests or leak-off tests have been increasingly performed throughout the world. The test is performed primarily to verify how much pressure can be applied to the formation below the bottom of casing, and to test the cement job sheath around the outside of the casing. The initial design of the various casing strings to be cemented in the well depends heavily on the predicted fracture-pressure curve. If this predicted curve is not verified by the test, the casing plan will have to be changed. Thus, verifying formation fracture-pressure during drilling is of great importance. However, the lack of a standard leak-off test procedure and standard interpretation technique for data analysis generally cause the results obtained by different well operators to vary significantly. In this study, the development of a computer simulation model for leak-off tests has been accomplished. This model is more realistic than the one currently used, but is sufficiently simple that it can be applied with data normally available during leak-off test operations in the field. The model includes the many factors that affect pressure behavior during the test, and can predict with reasonable accuracy what the pressure curve will look like. In addition, test interpretation using the computer model is easily achieved using a curve matching technique. The first step toward the development of the computer model was to subdivide the leak-off test into four phases: (1) pressure increase due to overall compressibility of the system, (2) fracture initiation, (3) fracture expansion, and (4) pressure decline and fracture closure after the pump is shut-in. The second step was the development of mathematical models for each phase separatedly. The mathematical model that predicts pressure increase before fracture initiation includes the most important variables affecting overall compressibility of the system. The modelling of fracture initiation is based on the classical elasticity theory. The modelling of fracture expansion and closure is based on the solution of the continuity equation for flow into a vertical-elliptical fracture with constant height. A computer program that predicts the pressure behavior during the leak-off test was written. This computer model was then verified using field data furnished by Tenneco Oil Company.

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