A Preliminary Wellbore In-Situ Stress Model for Utah FORGE

Abstract

ABSTRACT: Knowledge of the in-situ stress is very important to the geothermal reservoir development as well as to the planned R&D activities at the Utah FORGE. Whereas the vertical stress (Sv) and the minimum horizontal principal stress (Shmm) can reliably be determined through density logs and hydraulic fracturing tests, respectively, the determination of the magnitude of maximum horizontal principal stress (SHmax) is challenging. In this paper, we established a preliminary wellbore in-situ stress model for the Utah FORGE based on the drilling-induced fractures and wellbore breakouts observed in borehole image logs. We utilize stress polygon (frictional faulting theory), wellbore failure analysis, drilling-induced fractures, and borehole breakouts to constrain the magnitude of the SHmax in the well 78B-32 where both tensile and compressive failures occur. The constrained SHmax is in the range 0.83-0.98 psi/ft. The results are compiled to obtain a wellbore in-situ stress profile for the well 78B-32 which is in good agreement with other data. 1. INTRODUCTION Understanding the magnitude and orientation of in-situ stress is important to many subsurface science and engineering problems. In the development of an Enhanced Geothermal System (EGS), the knowledge of in-situ stress impacts several aspects such as reservoir characterization, deep well drilling, hydraulic stimulation, and induced seismicity. Constraining the in-situ stress requires the determination of a stress tensor with six independent unknowns (the magnitudes of three principal stresses and their orientations). In most geological settings within the Earth’s upper crust, we can assume that the three principal stresses are vertical stress (Sv), and two horizontal principal stresses (Shmin and SHmax). In general, the magnitude of vertical principal stress (Sv) is obtained by integrating the density logs, and the magnitude of minimum principal stress (Shmin) can be measured through hydraulic fracturing tests (e.g., DFIT, leak-off test, microfrac test). In addition, the orientations of Shmin and SHmax can be identified using observed wellbore failures (breakouts and drilling-induced fractures), crossed-dipole sonic logs, and seismic focal mechanisms. However, the determination of the magnitude of maximum horizontal principal stress (SHmax) is challenging.