Leak off tests are routinely performed by the drilling industry and have been used for many applications including the determination of cement integrity, mud weight limit for the next hole section, and the estimation of minimum horizontal stress. The potential of these tests to provide information that is useful in terms of its quantity and quality is often underutilized. Inadequate test design is often the culprit, but post job interpretation difficulties certainly contribute. The value of information obtained from a properly designed leak off test is much greater than the cost of the test. In some cases, an increase in the sampling of the pressure/time/volume measurement is all that is necessary to realize an improvement in quantity and quality of information.
This paper starts by describing test procedures needed to provide sufficient data to estimate in-situ stresses. Several examples are presented that illustrate different interpretation techniques. A mechanical earth model is shown to be necessary for understanding and interpreting leak off test data. The KGD fracture mechanics model is used to obtain information about the mechanicalproperties and stresses within the tested zone and these results can be used to update and refine the original mechanical earth model. The appropriate use and interpretation of leak-off test data provides valuable information that can be applied to subsequent drilling as well as future completion designs.
A recent study of wells drilled in the Gulf of Mexico1 showed that 24-27% of the total drilling cost was from non-productive time (NPT) events. A review of the NPT categories shows that almost 40% were geomechanics related (lost circulation, well bore instability, pack offs, etc). Knowledge of earth stresses, derived from geomechanical analysis, impacts decisions made in all phases of field development from well design to tertiary recovery. Accurate knowledge of these stresses, especiallyminimum horizontal stress (Sh), is necessary for predicting and mitigating drilling problems and applying efficient field completion methods. Quantifying the minimum horizontal stress has such a large impact on drilling related and completion issues that measuring Sh results in the realization of immediate benefits. Multiple methods exist for measuring Sh including, mini-frac tests, MDT stress tests, down hole pressure sensors or inversion from borehole images after mud losses, and casing shoe pressure integrity tests. Pressure integrity tests are required at most casing points, and yet these tests are often underutilized or poorly acquired, missing the opportunity to obtain accurate in-situ minimum horizontal stress magnitudes early in field development.
This paper begins by introducing pressure integrity test terminology, test procedures, and test interpretation techniques. The importance of interpretation within a geological and geomechanical context is introduced next and illustrated with an example. A method of interpretation based upon linear fracture mechanics is proposed. Using this method bounds upon the maximum horizontal stress can be obtained. Finally, application to drilling geomechanics is shown with a field example.
2. DEFINITIONS OF TERMS
One area in the analysis of pressure tests that often causes confusion is the nomenclature of leak-off testing events.