Utah FORGE: Injection Test Interpretation

Abstract

This study analyzed the in-situ stress regime of the Utah FORGE Well 58-32 based on a variety of measurements, including drilling reports, wireline logs, mud logs, drilling-induced wellbore failures, injection stimulation, and core samples. The focus of the study was Zone 1 (the deepest test interval). We concentrated on the analysis of injection stimulation tests to quantify the minimum horizontal stress (Shmin). The overburden stress (Sv) gradient was first estimated as 1.13 psi/ft using density log measurements, rock cuttings, and shallow measurements of neighboring wells targeting the Roosevelt Hot Spring. Three stimulation injection techniques were investigated to estimate Shmin: microfrac hydraulic fracturing tests, Diagnostic Fracture Injection Tests (DFIT) and Step-Rate Test (SRT). While the majority of microfac test cycles across Zone 1 were terminated prematurely, two cycles were interpretable. Assuming negligible friction at very low flow rates, these microfrac tests indicate a Shmin gradient in the range of 0.71-0.76 psi/ft. Both the tangent and compliance interpretation methods were used to analyze the DFIT data. The tangent method resulted in very low Shmin magnitudes. Given no clear evidence of fracture stiffness change after shut-in, the compliance method interpretation seems to be more appropriate as it indicates that the fracture walls closed immediately after shut-in such that the initial shut-in pressure (ISIP) should yield the best estimate of Shmin. The ISIP readings were found to vary between 0.72 and 0.88 psi/ft depending on the fluid flow rate and cumulative volume associated with different cycles. Meanwhile, SRT data yielded an Shmin gradient estimate of 0.79 psi/ft. Overall, the Shmin gradient appears to be 0.75 +/- 0.4 psi/ft. Given that drilling-induced fractures indicate SHmax direction of N25°E, a stress polygon was constructed to constrain the SHmax gradient indicating normal/strike-slip regime at the greatest depth in Well 58-32. The large uncertainty in SHmax results from the marked effect of wellbore cooling on the occurrence of the drilling-induced hydraulic fractures.