EXPLORATION FOR DEEP DIRECT-USE GEOTHERMAL HEATING IN THE APPALACHIAN BASIN: THE CORNELL UNIVERSITY BOREHOLE OBSERVATORY (CUBO) (Invited Presentation)

Deep direct-use (DDU) geothermal uses geothermal heat to warm buildings or drive industrial processes without conversion to electricity. Where commercially practiced, well pairs are drilled to depths where the temperature is high enough for HVAC system requirements. Water pumped down one well heats as it flows through bedrock; this hot water is produced from a second well and circulated through a heat exchange facility on the surface. Since 2014, technical feasibility and economic viability of DDU have been investigated across the Appalachian Basin of NY, PA and WV by combining archival subsurface data in a play fairway analysis and then conducting technoeconomic analyses of select sites. Those studies relied on assumptions about heat extraction efficiency from the expected low porosity and low permeability reservoirs. U.S. Department of Energy is supporting two deep drilling projects in the eastern US (Cornell University and WVU) to acquire and analyze data required for design of production wells and reservoirs. For Cornell, a >80 C temperature requirement places the targets in Cambrian strata and Precambrian basement, where vital rock, hydraulic and stress field characteristics were unknown. In 2022, a vertical borehole to 2983 m depth with 8.5” bottom diameter was drilled in Ithaca NY. From 2379 to 2865 m, Cambrian dolomite, quartz sandstone and minor limestone exhibit low porosity; fluid transmission between rock and borehole is indicated at three depths, each thinner than 10s of meters. Below 2865 m occur low grade Proterozoic metasediments, in whose upper part occurs the most pronounced fluid exchange between rocks and borehole. At 2650 m, stress conditions mark a strike-slip regime. For the Cambrian strata, CUBO data provide significant constraints for geothermal well completion design and to refine estimates of heat extraction potential. Further characterization of the basement is needed to advance an assessment of the highest temperature opportunities.