Determination of SHmax using drilling induced fractures and focal mechanism

Abstract

Knowledge of in-situ stress is essential to well development technologies such as wellbore stability, hydraulic fracturing, well spacing design, and the monitoring of induced seismicity. As energy exploration has become more prevalent in unconventional reservoirs and as it has shifted focus to sustainable energy such as geothermal energy, the knowledge of in situ stresses plays a crucial role in the success of energy production. In this project, the in-situ stress S_Hmax in the geothermal reservoir of Utah FORGE is computed and analyzed using multiple stress analysis methodologies in both vertical and horizontal wells. In a stress analysis, the understanding of three principal stresses is needed and they are vertical stress (S_v), minimum horizontal principal stress (S_hmin), and maximum horizontal principal stress (S_Hmax). In general, S_v is calculated by integrating the density log over the depth, while S_hmin is determined by hydraulic fracturing-based tests such as DFIT, leak-off test, and micro frac test. The direction of S_hmin and S_Hmax can be interpreted from the azimuth of drilling induced fractures and/or borehole breakouts. However, the determination of the magnitude of maximum horizontal principal stress (S_Hmax) is challenging especially in an inclined wellbore. Traditionally, S_Hmax can be estimated using hydraulic fracturing data and the Kirsch solution. However, this method of using hydraulic fracturing data imposes uncertainties as it is sensitive to the models and the assumption to rock formation is too sample (Schmitt and Zoback, 1989; Zi et al., 2022). With the gaining use of image log tools such as FMI and UBI, the method of using borehole failures such as breakouts and drilling induced fractures gives rise to more access to assess in situ stresses (Zoback, et al., 2003). Thus, the need of integrating multiple stress analysis methods and the incorporation of drilling induced fractures and breakouts around the wellbore is required to constrain a more comprehensive S_Hmax magnitude estimation. In this paper, we present the wellbore in-situ stress models of the vertical and deviated wells in Utah FORGE based on the drilling-induced fractures and breakouts observed from borehole image logs. We also applied an in-situ stress inversion program based on focal mechanism and validated the program using the data from the Geyser geothermal reservoirs. This program can be used for the Utah FORGE reservoir once its focal mechanism data is available. In Utah FOREG, we integrated multiple methods such as fracture mechanics, and wellbore failure analysis with the incorporation of drilling-induced fractures, borehole breakouts, and thermal stresses to constrain the wellbore stress model and input parameters. The validated wellbore in-situ stress models and parameters were then applied to establish the in-situ stress profiles of wells. The stress results in all wells yield a range of S_Hmax of 0.80 – 1.05 psi/ft, suggesting a normal faulting regime in Utah FORGE. The orientation of S_Hmax has a range of N20E to N40E with a mean of N33E based on drilling induced fractures in vertical wells. In Geyser, an in-situ stress inversion program based on the focal mechanism has been applied and validated. The stress variation analysis confirmed that the Geyser reservoir has a mixture of normal and strike-slip fault regimes sandwiched by strike-slip fault regimes on the top and bottom of the reservoir. The Geyser injection test also was shown to alter the stress regimes of the reservoir and cause the stress regime to transit from strike slip to normal and to transtensional fault during a cycle of injection activities.