Geomechanical Analysis for Target Sequences in the Cornell University Borehole Observatory (CUBO) Geothermal Exploration Well, Ithaca NY
Authors: Pinilla Daniela , Fulton Patrick , Fulcher Sean , Jordan Teresa 
Published: September 2023

Understanding the stress state in the Cornell University Borehole Observatory (CUBO) and its relation to the orientation of main fracture clusters is significant for building reservoir models and designing stimulation plans in the proposed geothermal project. Natural fracture orientations obtained using resistivity and acoustic image logs were reported previously. Among surveys and tests carried out in CUBO, the most relevant for stress analysis are four-arm caliper surveys, hydraulic fracturing stress tests, and repeated borehole acoustic and micro-resistivity image logs. The results reveal that the orientation of the main stresses vary between N 38 °E and N 50 °E. The likelihood of observed fractures slipping under current stress conditions was analyzed using 3D Mohr diagram for critically stressed fractures. The analysis shows that the regional fracture systems are primarily oriented in directions similar to the maximum horizontal stress and that the fractures are not critically stressed in the current state. Preliminary magnitude interpretations suggest a strike-slip geological regime at target depths.

INTRODUCTION

Cornell University aspires to utilize geothermal energy for heating its Ithaca NY campus. The primary objective for an exploration well drilled in 2022 was to provide critical data for decision-making regarding the potential Cornell geothermal system (Jordan et al., 2020; Tester et al., 2023). The exploration well, Cornell University Borehole Observatory (CUBO), was drilled to a depth of 3 km (T.D. = 9790.5 ft.) and was completed in five sections: a 36-inch conductor section with 30-inch casing to 110 feet; a 26-inch surface section with 20-inch casing to 789 feet; a 17.5-inch first intermediate section with 13.375-inch casing to 4,256 feet; a 12.5-inch second intermediate section with 9.625-inch hung liner to 7,809 feet; and an 8.5-inch open hole section to 9,790 feet.

Stress field interpretation is a critical parameter in reservoir engineering for Enhanced Geothermal Systems (EGS) like Cornell's anticipated "Earth Source Heat" (ESH) system. A clear comprehension of the magnitude and orientation of in-situ stress is essential to address several engineering challenges in the geothermal development stage: the stabilities and trajectories of deep development wells, the orientation in which hydraulic fractures propagate, the injection pressure required to activate stimulation, and to mitigate the risk of induced seismicity in the area (Min et al., 2020).The preliminary findings from CUBO, which include whole borehole hydraulic tests, microfrac tests, and geophysical logs, reveal that the potential geothermal reservoir has low natural permeability (Fulcher et al., 2023; Clairmont and Fulton, 2023). In order to increase permeability during subsequent stages of ESH development, reservoir stimulation may be necessary. Therefore, the interpretation of the in-situ stress is pertinent to direct the next phase of ESH, developing a doublet to produce geothermal heat for direct use in the university's district heating system.