Reservoir Modeling of Geothermal Energy Production from Stratigraphic Reservoirs in the Great Basin

Abstract

Large, potentially commercial geothermal resources exist in sedimentary rocks beneath high heat-flow basins of the United States. Geothermal reservoir modeling was performed to explore the available power density (MWe/km2) attributable to two general classes of reservoir: a multi-layered “sandwich” and single high permeability layer. Variations in reservoir temperature (i.e. conductive heat flow), permeability, and layer thickness were evaluated. The high permeability layers were assumed to be horizontal and laterally extensive. Production wells were assumed to be pumped at a constant rate, and all produced water was injected at 75 oC after being cooled in a power plant. Modeling was undertaken using the STARS Advanced Process and Thermal Reservoir Simulator, Version 2010, by Computer Modeling Group. Five reservoir models were simulated: 1. Sandwich (base) reservoir model to test heat sweep for a reservoir-seal configuration with an average reservoir temperature of 200 oC at 3 km depth; the reservoir comprised four 25 m thick layers with a permeability of 100 millidarcy and an overall transmissivity of 10 Darcy-meters. 2. Single layer reservoir with same initial temperature and transmissivity of the sandwich reservoir. 3. Low temperature (150 oC) sandwich reservoir model. 4. Low permeability sandwich reservoir model, involving lower permeability layers than the sandwich base model which results in a reduced transmissivity (3 Darcy-meters) for the overall reservoir. 5. Short-circuit sandwich reservoir model where a high permeability layer results in a higher transmissivity (30 Darcy-meters) than the base sandwich model. All models assumed isotropic permeability, uniform porosity (10%), and initial thermally conductive vertical temperature gradient and hydrostatic pressure gradient. All models utilized a five spot pattern with a 500 m well spacing, with the flow rate in producer and injector wells being 1000 gallons per minute. The base sandwich model, which may be representative of stratigraphic bedrock reservoirs beneath some basins of the Great Basin, has a power density 3 to 10 MWe/km2 over a 30 year period.