Potentiometric Mapping from Incomplete Drill‐Stem Test Data: Palo Duro Basin Area, Texas and New Mexico

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Abstract
Drill‐seem test (DST) data from wildcat wells and wells sponsored by the U.S. Department of Energy were used to construct pressure‐depth diagrams and regional potentiometric surfaces of the Wolf camp and Pennsylvanian aquifers. These deep‐basin aquifers represent the two regionally important downgradient aquifers that underlie the host rock at the proposed high‐level nuclear‐waste repository sites in the Palo Duro Basin area of Texas and New Mexico. More than 5,500 DSTs were screened and classified according to shut‐in pressure and shut‐in time criteria devised to evaluate the quality of the DST data and to delete dubious DST data. After screening, three sets of pressure‐depth diagrams and potentiometric‐surface maps were constructed, corresponding to three levels of data refinement.The initial Wolfcamp and Pennsylvanian regional potentiometric surfaces contained several prominent local mounds and depressions with unrealistic variations in the direction and magnitude of hydraulic gradients. Pressure‐depth data and records and maps of historic oil and gas production showed that many DSTs were performed in depressured oil and/or gas production zones. The low shut‐in pressures recorded during these tests caused abnormally low heads to be calculated and contoured. Formation pressures recorded in these depressured zones represent local temporal pressures in a regional flow system that is probably steady state. Deletion of depressured DSTs produced regional potentiometric surfaces that were more realistic but contained a few large local mounds and depressions caused by local aberrant DST data. Deletion of local aberrant grossly underpressured DSTs not attributable to oil and/or gas production and local aberrant grossly overpressured DSTs, based on comparison of initial shut‐in pressures and heads in the same well and/or in adjacent wells at a similar depth in the same geologic unit, produced potentiometric surfaces that show regional and some local variations in flow directions and hydraulic gradients. Linear regression of the pressure‐depth data showed that most of the refinement in the data was due to culling depressured DSTs. Some additional refinement was due to culling local aberrant grossly underpressured and grossly overpressured DSTs.