Publication
Hydraulic fracturing is a key technique used to enhance subsurface reservoir permeability, with applications in oil and gas extraction, geothermal energy, and carbon sequestration. The closure phase of hydraulic fractures plays a critical role in determining reservoir properties such as the leak-off coefficient and in-situ stresses, which are essential for predicting fracture behavior and optimizing engineering design. Despite the widespread use of pressure-based methods to estimate fracture closure time and pressure, conventional techniques are limited by empirical assumptions and fail to account for critical factors such as poroelastic effects. Recently, analytical models such as the sunset solution have been proposed to provide a physics-based framework for characterizing closure, yet their experimental validation remains unexplored. This thesis presents a combined experimental and numerical investigation of fracture closure behavior. Laboratory-scale hydraulic fracturing experiments were conducted on cubic Molasse sandstone samples using a true triaxial apparatus. A key advancement was the integration of an eddy current (EC) probe to directly measure fracture opening at the injection point. This method was calibrated using high-resolution CT scans and thin-section analysis. Four hydraulic fracturing experiments were performed using different fluids with different viscosities to capture closure behavior across multiple injection and shut-in cycles. Two experiments were conducted in the viscosity-storage-dominated regime and two in the viscosity-leak-off-dominated regime. The results showed that fractures never fully close, with a residual opening persisting even after unloading, challenging the assumption of full closure in traditional models. The fracture stiffness, derived from simultaneous pressure and opening measurements, provided a more precise closure indicator than conventional pressure-based methods. In low-viscosity tests, a pronounced poroelastic back-stress effect was observed, increasing closure pressure in successive cycles. Moreover, the time evolution of fracture opening and pressure near closure exhibited asymptotic behavior consistent with the sunset solution. Micro-scale analysis of the core samples was also conducted to investigate fracture morphology and contact mechanics. High-resolution CT scans were used to reconstruct the upper and lower fracture surfaces, preserving their integrity without physical separation. The analysis revealed an increasing concentration of contact and bridge points near the fracture tip, indicative of a fracture process zone. Surface roughness characterization confirmed intergranular propagation, with a low Hurst exponent (~0.4). Residual opening profiles were anisotropic, showing a long-range gradient along the propagation direction. To complement the experimental results, numerical simulations were performed to assess two modeling approaches for fracture closure. A fixed-grid model, incorporat