Paper No. 5
Presentation Time: 9:25 AM
ENGINEERING GEOTHERMAL SYSTEMS IN SEDIMENTARY BASINS: EXAMPLES OF POTENTIAL IN COLORADO
MORGAN, Paul, Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, CO 86203 and SARES, Matthew, Colorado Geological Survey, 1313 Sherman Street, Room 715, Denver, CO 80203, Paul.Morgan@nau.edu
Sedimentary rocks have received less attention than crystalline rocks in efforts to economically extract heat from low-permeability systems. In part this bias is because temperatures increase with depth, and the ratio of crystalline rocks to sedimentary rocks increases with depth. As binary technologies develop to generate electricity with low-temperature geothermal fluids, sedimentary rocks become of greater interest as thermal reservoirs. Perhaps the greatest technical challenge in EGS systems is creating a large-area fracture system with sufficient hydraulic connectivity between two wells to make an efficient heat exchanger from which to extract sufficient heat for electricity generation. The fracture system is usually generated by hydraulic fracturing but experience with this process in crystalline rocks is limited. However, hydraulic fracturing is routinely used in sedimentary rocks to enhance production of oil and gas. Creating an EGS system in sedimentary rocks will benefit from decades of experience in hydraulic fracturing from the hydrocarbon industry.
An alternative to hydraulic fracturing to produce connectivity between two wells would be to use natural permeability. This natural connectivity has been successfully used at several locations in Germany, two of which produce >3 MW of electricity each from a pair of wells. Geothermal gradients in sedimentary basins in Colorado are above average for continental crust, ranging from 35 to 70°C/km. Temperatures of 150°C are indicated from bottom-hole temperature data in some basins as shallow as 2-3 km. There are probably permeable aquifers at these depths. Many hydrocarbon wells penetrate to these depths, so permeability testing either in situ or with core samples should be possible. Abandoned wells may even be available for a pilot project to test connectivity and heat extraction through a natural aquifer. Colorado is ready to engineer sedimentary geothermal systems with natural permeability.
