Geothermal Energy from Mature Gas Reservoirs; an Analysis of the Frio and Vicksburg Formations, South Texas, Hidalgo County

Over the past decade there has been an increasing interest in developing renewable and unconventional energy resources. Of the many renewable energy technologies being developed geothermal energy is one of the select few that can provide baseload energy production. In the past geothermal energy has focused on geographically limited magmatic areas, but this may not be true in the future. This investigation examines the potential to develop geothermal energy from natural gas fields which are no longer economically viable to operate. Existing infrastructure, including production and brine injection wells, power supplies, pipelines and the reservoir characteristics available from the producing zones, provides an economic advantage over drilling new geothermal production wells. The authors have conducted a re-assessment of a previous study done by the Bureau of Economic Geology in 1975 (Loucks) for fields in southwest Texas in the Frio and Vicksburg Formations. Reservoir properties are re-assessed by analyzing electric logs of nearby wells which penetrate the producing formations. Zones with high gross sandstone thickness and high fluid temperatures are identified. Potential production rates of hot brines from these zones are assessed. Identified reservoir zones are classified based on their ability to provide high fluid flow rates and high temperature fluids. High-quality reservoirs are defined as being located between the 300°F and 350°F isotherms and having in situ permeabilities greater than 10 mD. Low-quality reservoirs are defined as being located between the 200°F and 250°F isotherms and having in situ permeabilities less than 1 mD. Reservoirs found between the 250°F and 300°F isotherms represent moderate-quality reservoirs. Using this assessment approach, an approximation of geothermal production can be made using a simple block model analysis. Estimated production values are used to derive the economic viability given current market conditions. The economic analysis includes consideration of geothermal binary heat exchange systems, and existing infrastructure and determines the lowest projected cost of development for particular reservoirs in the Hidalgo county region.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California