Consolidation of Geologic Studies of Geopressured-Geothermal Resources in Texas

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Hydrochemical data obtained from samples of brine produced from the Gladys McCall Zone 8 sandstone reservoir were used to estimate the importance of shale dewatering as a contribution to ultimate reservoir volume. Changes in chloride concentration of produced brines with time were generally small and close to analytical margins of error, but some correlation between production-related pressure drawdown and declining chlorinity was detected. More rigorous analysis of brine composition and source was hindered by nonstandardized sampling and analytical procedures. Geologic data suggest that sandstone interconnection is a more important source of extra reservoir volume than is shale dewatering at the Gladys McCall site. Methods for more definitively determining the effects of shale dewatering and reservoir interconnectedness include direct sampling and chemical analysis of shale water, pressure monitoring and fluid sampling in multiple reservoirs in a single well or a well field, and sidetrack drilling and coring. Petrographic analysis was used to document the effects of experimental compaction on core samples from geopressured-geothermal reservoirs. Experimental compaction simulates the increasing effective stress within these reservoirs as fluid pressures decline during production. Inelastic compaction and brittle failure (fracturing) are closely related to sandstone composition. Sandstones that contain abundant ductile rock fragments and clay minerals undergo large compaction-induced porosity reductions and fracture readily at effective stress levels comparable to those generated in the reservoir during high-volume production. Well-indurated, high-quartz sandstones, such as the Gladys McCall Zone 8, are extremely resistant to both inelastic compaction and brittle failure.


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