Title:

Preliminary Determination of in-situ Stress Orientation and Magnitude at the Cornell University Borehole Observatory (CUBO) Geothermal Well, Ithaca NY

Authors:

Daniela PINILLA, Patrick FULTON, Teresa JORDAN

Key Words:

stress state, orientation, magnitude, borehole breakouts, CUBO, hydraulic fracturing stress test, direct use

Conference:

Stanford Geothermal Workshop

Year:

2023

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Pinilla

File Size:

905 KB

View File:

Abstract:

Understanding the in-situ stress regime is vital for two central challenges of geothermal projects that rely on permeable fracture zones: to predict the preferential fracture orientation for the design of a future stimulation plan and to mitigate induced seismicity. Here, we present a preliminary wellbore in-situ stress analysis for the Cornell University Borehole Observatory (CUBO), a deep geothermal exploration well drilled on Cornell's campus in Ithaca, NY in 2022. The objective of this analysis is to guide the development of the next step, a doublet to produce geothermal heat for direct use in the university's district heating system. Four-arm caliper surveys (Power Positioning Caliper), hydraulic fracturing stress tests, and repeated borehole acoustic and micro-resistivity image logs (BHI) were carried out in CUBO and used to constrain the orientation and magnitude of in-situ stresses. The caliper logs and BHI reveal borehole breakouts (BBOs) and tensile fractures that mark the orientation of the main stresses. Only the four-arm caliper data are available for the three cased-well sections between the surface and 7800 ft. These data indicate that the minimum horizontal principal stress orientation is ~148° on average. In the open section of the well between 7800 ft and 9790 ft, caliper and image logs differentiate borehole breakouts with a preferential orientation of ~125° in the shallower sedimentary formations and somewhat rotated to 146° in the basal formations and crystalline basement. Notwithstanding the subtle change in orientation, the main direction of the maximum horizontal stress for CUBO is northeast, following the regional trend reported in historical data. An estimation of the magnitude of principal stresses is possible for at least two depth intervals where minifrac tests were conducted with a dual-packer system. The minimum horizontal principal stress (Shmin) and the Vertical stress (Sv) are obtained from the minifrac test and density logs, respectively. The approximation of the Shmin was determined from fracture Closure Pressure (CP). The maximum horizontal main stress magnitude was estimated with a theoretical elastic equation and yielded values from 74.3 MPa at 7885 ft to 96.2 MPa at 9360 ft. The magnitudes of the main horizontal stresses relative to the overburden stress indicate a strike-slip regime at depths below 6000 ft per the general geological regime in the region.


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