Closure Pressure Diagnostics in Diagnostic Fracture Injection Tests Using G-Function and Total System Stiffness Methods

Summary Accurate characterization of hydraulic-fracture closure is fundamental for reliable in-situ stress estimation. Despite the widespread use of G-function diagnostics, persistent debates over the physical validity of the tangent vs. compliance methods often lead to inconsistent stress interpretations. In this paper, we evaluate these traditional diagnostics and propose the integration of the total system stiffness (TSS) method into the standard diagnostic fracture injection test (DFIT) workflow. By transforming pressure-decline data into an evolution curve of TSS, the TSS method identifies objective inflection points to define the upper and lower bounds of closure pressure. We systematically compare these methods using five field cases spanning diverse lithologies, including granite, basalt, mudstone, and shale. Results demonstrate that the TSS approach effectively yields unambiguous closure intervals even when compliance-method indicators are absent (or obscured) by near-wellbore transients. The traditional tangent pick carries a systematic risk of underestimating closure pressure and can be nonunique. Furthermore, we analyze the mechanical reasons behind the limitations of these methods, particularly the mathematical necessity of peak formation on the GdP/dG curve. This work provides a direct and practical tool for closure analysis, enhancing the reliability of in-situ stress estimation and fracturing design.