Reexamining In-Situ Stress Interpretation Using Laboratory Hydraulic Fracturing Experiments

Knowledge of in-situ stress is important for many subsurface science and engineering issues. The minimum principal stress (S₃, or S_hmin in most cases) is typically measured through mini-frac tests. Several methods have been suggested to interpret S₃ using pressure data during the injection and/or shut-in phases of a mini-frac test. However, S₃ interpreted from different methods is often inconsistent with each other and could result in large uncertainty in determining net pressure. This paper presents a series of small-scale laboratory hydraulic fracturing experiments conducted under true-triaxial compression. The injection scheme consists of a hydraulic fracturing cycle followed by a few fracture propagation cycles and several injection/falloff (DFIT) cycles. The wellbore pressure and acoustic emission (AE) activities of each cycle were concurrently measured to monitor fracture initiation, propagation, and closure during fluid injection and shut-in. The pressure data were used to interpret S₃ using different hydraulic fracturing-based methods. The results illustrate that the spatial-temporal evolution of AE activities is well associated with fracture propagation. The stress interpretation results from the DFIT cycles demonstrate that fracture reopening pressure generally provides a reliable estimate of the minimum principal stress (S₃). ISIP consistently provides a relatively higher estimate of S₃ and can be used as an upper limit for constraining S₃. Fracture closure was observed using the so-called "tangent" method in all DFIT tests. However, the "tangent" method using a signature close to the peak GdP/dG tends to significantly underestimate S₃. The "compliance" method offers a relatively objective (yet still low) estimate of closure pressure. However, the signature associated with the change in system stiffness or compliance is observed but not consistently present in every DFIT cycle. It has been observed that the non-uniform fracture topography significantly impacts fracture closure behavior and the associated stress interpretation. Considering the complex nature of hydraulic fracturing in the subsurface, multiple techniques may need to be integrated for the determination of S₃.