Laboratory Insights on Hydraulic Fracture Closure and Stress Measurement

Abstract

Reliable knowledge of the magnitudes and orientations of in situ stress is essential for scientific and engineering activities in the subsurface, particularly in energy and storage applications. A common technique for determining the minimum principal stress (S₃) is to interpret the fracture closure pressure from the pressure decline transient during the shut-in phase of hydraulic fracturing. However, existing methods for interpreting fracture closure pressure often yield inconsistent results, leading to significant uncertainties in determining the minimum principal stress. To address this issue, we conducted a series of controlled laboratory injection/fall-off experiments on various rock types using different injection fluids to demonstrate the physical processes of fracture closure. Our results indicate that fracture closure follows a three-stage process, with the onset of mechanical closure during Stage 2 providing the most reliable estimate of S₃. Building on these findings, we propose using the early deviation on dP/dG versus G plot, corresponding to the beginning of Stage 2, for fracture closure analysis, as it consistently yields reliable stress estimates in both laboratory and field-scale hydraulic fracturing stress measurements. Additionally, our laboratory results suggest that the instantaneous shut-in pressure (ISIP) method could provide reasonable but higher-bound estimates of the minimum principal stress, indicating the potential value of integrating fracture closure pressure and ISIP for more accurate stress measurements. This study not only clarifies fracture closure analysis for stress determination but also offers a comprehensive understanding of the physics governing fracture closure during the shut-in phase of hydraulic fracturing operations.