Abstract

There has been renewed interest in the feasibility of extraction of geothermal energy from deep sands in the Louisiana Gulf Coast. Unlike concepts proposed in the early 1980s, in which heat would have been extracted at the land surface from produced overpressured waters, methods currently being explored at the Louisiana State University would involve engineered downhole heat exchangers with zero-mass-withdrawal. Our research focuses on what water-rock reactions could occur as reservoir temperatures are lowered during heat extraction and as native reservoir waters are made to circulate through a heat exchanger. The first phase of this study has involved characterizing formation water and sediment properties in a deep 23,000 ft (7000 m) borehole in the Lake Sand Field in southern Iberia Parish. Data available includes formation water salinities calculated from spontaneous potential and dual-conductivity techniques, temperatures and pressures from log header information, x-ray diffraction (XRD) and whole-rock chemical analyses, and chemical analyses of nearby produced waters. The wellbore was divided into three main zones based on large-scale changes seen in the spontaneous potential and gamma ray signature: shallow, transitional, and deep. Focus will be on the transitional and deep zone due to adequate temperatures for geothermal energy production. Ages within the lower zones ranged from mid to lower Miocene. Sediments deposits within these zones are estimated to be marine to deep-sea with sand packages reaching 150–175 ft (46–53 m) in thickness. Pressures, based on mud weight, range from 2235–21,612 psi at depth with temperatures reaching a peak of 337°F (169.4°C) at total depth (TD). Waters were found to become less saline with increasing concentrations of illite. It is reasonable to conclude this relationship corresponds to dewatering from the smectite to illite transition. The second phase of this study takes a more regional approach to characterizing waters by incorporating an additional dataset from a previously-published study. Water samples were collected from six oil and gas fields located in Vermillion, Iberia, and St. Mary parishes. Data from this study includes concentrations of major dissolved solutes, temperature and pressure, and pH. Thermodynamic modeling using the data from a previously published study was performed in order to evaluate the influence of rock buffering as a control on formation water chemistry. Waters were found to be partially buffered by calcite/dolomite and chalcedony. Precipitation of calcite and silica is a probable consequence of geothermal energy production based on our results. Mixing trends between waters of varying chemistries were also analyzed for possible precipitates as a consequence of geothermal production. It was found that there is potential for barite precipitation due to mixing of barium and sulfate rich waters.