Cornell University is exploring deep hot rock under its campus in Ithaca, New York to develop Earth Source Heat, Cornell’s approach to deep direct-use geothermal energy. In the process of evaluating geothermal heat potential, it is important to assess the natural thermal conditions of the subsurface to find depth targets with favorable temperature. To characterize the subsurface of Ithaca, New York, the university recently drilled, constructed, and tested a 3-km deep vertical exploratory well known as the Cornell University Borehole Observatory (CUBO). Key thermal datasets collected during the testing of CUBO include bottomhole temperatures from several logging runs immediately after drilling and cessation of borehole circulation, high-resolution downhole temperature logs conducting prior to, during, and after whole borehole production and injection tests, as well as lithostratigraphic logs, drill cuttings, and sidewall cores from which the distribution of thermal properties can be determined. The interpretation of borehole temperature is important because it records a combination of background conductive heat flow, the influence of thermal property variations, and transient effects caused by drilling the borehole and hydrologic processes. Here, we assess the natural thermal conditions of the subsurface under Ithaca, New York by interpreting borehole temperatures using analytical methods to inform reservoir modeling and strategies for Cornell University’s development of deep direct-use geothermal energy. Borehole temperature measurements are disturbed by borehole circulation during drilling and take considerable amounts of time to re-equilibrate to background formation values. Therefore, we estimate the equilibrium formation temperature near the bottom of the borehole at 2.95 km depth using a modified Horner plot analysis which takes into account the duration of borehole circulation at that depth and the time since circulation completed. We use 5 bottomhole temperature values recorded during wireline logs immediately after drilling and estimate a bottomhole formation temperature of at least 80 degrees Celsius. This value is within the anticipated range for potential low temperature reservoirs for deep direct-use applications. Subsequent higher resolution temperature logs recorded as part of hydrologic testing are affected by subsequent borehole circulation effects but provide insights into depth-variations and the influence of thermal property variations and advection due to fluid inflow or outflow between the borehole and formation. Large advection signals Indicative of formation fluid flow are not immediately apparent. However, analysis of 7 high-resolution downhole temperatures reveals 9 separate depth zones between 2.5 to 3.0 km depth with distinct thermal gradients that do not directly correlate lithostratigraphic boundaries. Assuming a constant vertical conductive heat flow we evaluate the thermal conductivity variations implied. Although these profiles are complicated by secondary circulation effects, by looking at the thermal recovery over time at various depths we assess intervals potentially affected by fluid flow within the formation. Further assessment of natural thermal conditions will be improved by the availability of time-series downhole temperature measurements from a fiber-optic distributed temperature sensing (DTS) cable, anticipated to be installed in late 2022.

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