Reexamining In-situ Stress Interpretation Using Laboratory Hydraulic Fracturing Experiments



Key Words:

In-situ stress, hydrualic fracturing, DFIT, closure pressure, EGS


Stanford Geothermal Workshop




Enhanced Geothermal Systems



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1486 KB

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Knowledge of in-situ stress is important to many subsurface science and engineering problems. The magnitude of minimum principal stress (S3, or Shmin in most cases) is generally measured through hydraulic fracturing tests. Several methods have been suggested to interpret Shmin using the pressure data during the injection and/or shut-in phases of a hydraulic fracturing test. However, Shmin interpreted from different methods are often not consistent with each other and could lead to large uncertainty in net pressure determination. In this paper, we present a laboratory hydraulic fracturing experiment conducted on a granite block with a side length of 13 inches under controlled true-triaxial stress conditions. In the experiment, the injection scheme includes a hydraulic fracturing cycle followed by a few fracture propagation cycles and several diagnostic fracture-injection/falloff tests (DFIT). The wellbore pressure and the acoustic emission (AE) activities during fluid injection and shut-in were concurrently measured to monitor fracture initiation, propagation, and closure within the span of fluid injection and shut-in. The pressure data were used to interpret Shmin using different HF-based methods. The results show the spatial-temporal evolution of AE activities is well associated with fracture propagation. In addition, the overall geometry of the hydraulic fracture created in our experiment is planar, however, a clear non-uniform topography is evident with heterogeneous distribution of asperities. The stress interpretation results from DFIT test demonstrate fracture reopening pressure generally provides a very good estimate of Shmin. Fracture closure was observed using the so-called tangent method in all DFIT tests and the 1st, earlier signature tends to offer a better stress estimate when compared to the traditional tangent method using a signature close to the highest point on the GdP/dG curve. The signature corresponding to the change in the system stiffness or compliance is observed although not consistently. It is found 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 Shmin, nevertheless, the results demonstrate that the traditional tangent method clearly underestimates the stress value thus overestimating the net pressure.

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