Subsidence Measurements Around Geopressured-Geothermal Test Sites in Southwestern Louisiana

Drukell B. Trahan

First-order elevation surveys of benchmark networks established around geopressured-geothermal test sites in southwestern Louisiana have been conducted before, during, and after testing to determine the potential for growth fault activation and compactional subsidence due to depressurization of geopressured-geothermal reservoirs.

Subsidence increased to the south along the line from Lafayette, Louisiana, to the Parcperdue test site as expected as this line descends the flank of the Iberian structural axis. Subsidence varied for benchmarks around the site and by yearly rate although the yearly pattern was consistent. During the period from 1980 to 1982, the benchmarks on the site may have subsided more than other benchmarks due to compaction of soils by drilling and testing equipment.

Motion rates for benchmarks around the Sweet Lake geopressured-geothermal test site south of Lake Charles, Louisiana, were highly variable during the period from 1980 to 1984 and were less variable from 1984 to 1986. These rates and patterns of motion may reflect the instability of the Sweet Lake salt dome located south of the prospect but do not correlate with the subsidence expected from geopressured-geothermal development.

Relevelings of the benchmark network around the Gladys McCall geopressured-geothermal test site on the coast illustrate generally decreasing subsidence from west to east and slight variations among benchmark motions. The greatest subsidence occurred along the well access road and at the well site from 1981 to 1984 coincident with well-site preparation and drilling.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.