The Palo Duro basin, a subbasin of the Permian basin, is being investigated by the Bureau of Economic Geology at The University of Texas at Austin as a potential site for the location of a nuclear waste repository. As the program changes from a regional to a site-specific investigation, it becomes necessary to optimize the expenditure of funds for drilling and testing. Hydrogeologic information is of critical importance in evaluating the long-term stability of a potential repository; consequently, deep multipurpose wells should be drilled in locations that maximize the opportunity to obtain both hydraulic and geochemical data.

Prior to our own borehole testing, the only hydrogeologic data available came from petroleum exploration activities, namely, drill-stem test (DST) pressure measurements and brine samples collected with the test. Computer data files of DST information were purchased from commercial sources or obtained directly from operators who had worked in the basin, and the data were merged into a master file. Approximately one thousand tests have been sorted according to geologic formation and lithology. The DST data were then screened and ranked according to their level of confidence based on shut-in pressure characteristics, fluid recoveries, and flowing times.

Automatic computer contouring of the selected data produced an unsatisfactory map because of the varied quality of the tests. An objective geostatistical method was subsequently employed to map the regional pressure or hydraulic head distribution in the basin. Geostatistical analysis of the data revealed that a spatial dependency existed which could be modeled by a two-dimensional spherical variogram. The method of kriging was then applied to the data to estimate the regional hydraulic head surface.

A chemical equilibrium computer program was used to determine the reaction state of the deep basin flow system, using as input data the chemical composition of the brines collected during drill-stem testing. The program then incrementally added the CO₂ lost during collection back into the initial brine composition until it reached the calcite phase boundary. This mass transfer approach results in the computation of the most likely mineral constraints on the brine at measured formation temperatures, pressures, and computed pH conditions.

The results of these studies provide interpretations of the regional hydrogeologic processes. Consequently, exploration decisions can be made concerning the location of future test wells to further define the geologic, hydrologic, and geochemical characteristics in this sedimentary basin.

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