Techniques for coping with missing data in pressure transient tests
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This paper presents techniques we developed to cope with data missing from most of 72 buildup tests run between 1956 and 1986 in a large North American oil field. Most of the tests were run before the industry fully appreciated the need for early-time data and for measurement precision sufficient to generate useful derivative plots. Frequently missing were pre-test rates, time and flowing bottom-hole pressure at shut-in. We needed the buildup test analysis to help with reservoir description in an ongoing integrated study of the reservoirs in this field. `Conventional' analysis suggested a large wellbore storage and skin factors of -4 to -5 in these unstimulated wells, assuming that the reported time and pressures at time zero were correct. Our work showed that missing only the first half-hour of the test can account for these anomalies. As a result, we developed guidelines for identifying invalid tests and for correcting erroneous interpretations in many of these tests to determine permeability and average pressure. This paper helps the engineer to cope with missing data from field pressure tests and still be able to salvage the test and determine permeability and average pressure which are essential for reservoir characterization. The technical contributions of this paper include criteria and techniques to cope with missing data in `old' field pressure tests; a procedure for obtaining useful permeability estimates and average drainage areas pressures from these tests; and guidelines for identifying tests that are simply invalid as reported because of incorrect shut-in times and missing initial shut-in pressures.
