Subsidence in Geopressured Geothermal Resource Test Sites: Monitoring Assessment Combining Geodetic Leveling and Tidal Control Stations in Southwestern Louisiana

Karen E. Ramsey, Chacko J. John, Drukell B. Trahan

The Louisiana Geological Survey has an ongoing environmental monitoring program, sponsored by the U.S. Department of Energy, at geopressured geothermal prospect well sites in southwestern Louisiana. This paper presents the results from monitoring subsidence at some of these reservoir sites. Over 1,000 km of first-order surveys and data from several NOAA and U.S. Army Corps of Engineers tidal control stations were examined to determine regional trends. Tidal records were used to examine the history of sea level with respect to the land surface. Relative rates of land subsidence can be determined by comparing rates of water level rise over time with rates of rise from a stable craton. Regional subsidence ranges from 3 to 5 mm/year.

First-order bench-mark networks established at Parcperdue, Sweet Lake, and Gladys McCall prospects were used to determine local trends of subsidence. Repeated leveling surveys before, during, and after fluid withdrawal from Parcperdue and Gladys McCall indicate that an increase in subsidence was observed during the drilling of the wells. Data suggest subsidence was possibly due to surface loading by heavy drilling equipment. Historical leveling in the Sweet Lake region indicates differential compaction between sediments as a possible cause for subsidence. However, in all cases, virtually no increase in subsidence was observed during and after times of fluid withdrawal.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.