Ahmad Fuad Muhammad Izzuljad Bin

Hydrogen accumulation in the subsurface relies on an effective "hydrogen system," sharing elements of "hydrocarbon system," such as reservoir and seal (Prinzhofer et al., 2018). Therefore, seismic exploration is useful for detecting and evaluating hydrogen plays. Understanding the physical properties of hydrogen-bearing reservoirs is essential for seismic-based evaluation, but research in this area is limited. Hydrogen's ultra-light density and small molecular size create knowledge gaps in how these gases affect the elastic properties of gas-bearing rocks. Questions about distinguishing hydrogen from other reservoir fluids (e.g., gas and brine) using seismic methods remain unanswered. This study utilizes laboratory rock physics high-frequency measurements and modelling to investigate hydrogen gas's impact on sandstone's elastic properties under varying temperatures and pressures. The same procedure is applied to methane and brine for comparison. Helium and Nitrogen are used as proxies for hydrogen and methane due to their non-reactive and non-absorbing properties, allowing the study of gas effects without chemical reactions. These tests provide insights into rock behavior under pore-pressure changes caused by ultra-light gases. Specialized equipment is needed for prolonged experiments with fluids at high temperatures and pressures. The rock mechanics testing system was modified to contain nitrogen, helium, and brine without leaks. This system includes a servo-mechanical frame press that can reach 250,000 lbf within a temperature-controlled enclosure, a pump for applying up to 15,000 psi of confining pressure and 10,000 psi of pore pressure, and high-resolution pumps for handling supercritical fluids and monitoring volume for saturation estimates. Berea sand cores, simulating high-quality clean sand reservoirs, were used as test samples. The test matrix was designed to replicate hydrogen accumulation scenarios at shallow and intermediate depths with varying pore pressures. Laboratory testing began with fully brine-saturated samples. For shallow depth intervals, effective pressure was initially set at 5 MPa and increased to 14 MPa at 50°C. For intermediate depth intervals, effective pressure started at 10 MPa and increased to 30 MPa at 100°C. This process was repeated with helium- and nitrogen-saturated sands. Additionally, conceptual rock physics modelling based on measured data investigated the applicability of the Mavko-Jizba squirt flow model for rocks saturated with ultra-light gases at high frequencies. Laboratory measurements revealed that brine-saturated rocks exhibited the highest P-velocity and density, followed by nitrogen and helium-saturated cases, as brine's density and incompressibility facilitate effective P-wave transmission. Helium-saturated rocks showed the highest shear velocity due to the lower density effect of ultralight gases, which reduces pore pressure and increases effective stress. There is separation between elastic properties of Helium and Nitrogen saturated rock, suggesting the potential of using seismic for natural hydrogen exploration. Rock physics modelling results indicated that while the common approach could accurately model the density of fluid-saturated rocks, it showed significant variances for P-wave and S-wave velocity modelling in nitrogen and helium-saturated cases, highlighting its inadequacy for ultra-light gases. These findings are crucial for developing adaptive rock physics models for rocks saturated with ultra-light gases, enhancing the use of seismic data for natural hydrogen exploration and storage evaluation.Co-auteur : Hongwen Zhao, Ernest Austin Jr Jones