Geophysical Methods and Strategies to investigate Rock-physical Properties at the Field-scale: Methane, Hydrogen and Heat Injection under Experimental Conditions

Abstract

Seismic borehole techniques offer the opportunity to characterize the nearsurface aquifer properties sensitively. Knowledge of rock-physical relations at the field scale is essential for interpreting geophysical measurements. However, transferring the results of existing lab-scale studies directly to the field scale remains challenging due to the use of different frequency ranges. To address this issue, we developed an experimental monitoring setup for gas and heat injection experiments to study rock-physics relations at the field scale. We successfully studied the dependence of temperature and gas saturation on seismic properties, respectively. The integration of geophysical measurements into a hydrogeological problem allows us to demonstrate the applicability of theoretical rock-physical concepts at the field scale, providing an essential link to the discipline of hydrogeophysics. After a thorough preliminary survey, which revealed a detailed picture of the subsurface conditions, we were able to define suitable test site areas for our injection experiments. With controlled injections of heat, CH4, and H2 at depth ranges between 7 - 18 m, we obtained controlled changes in sediment parameters such as temperature and water saturation. We monitored the temperature and saturation changes in a time-lapse experiment for at least twelve months at observation depths between 8 - 18 m. In each case, we analyzed P-wave velocity and amplitude change, including energy level and quality factor. A subsequent comparison of our seismic data with in situ water content measurements obtained from the gas injection experiments using different computational approaches resulted in a fit. It confirmed the method of seismic cross-hole measurement for gas leakage detection. With a comprehensive monitoring layout we furthermore detected parameter changes inferred from temperature variations in the subsurface and compared the results to in situ temperature measurements. We demonstrate in our experiments that we verify rock physics relationships at the field scale with our experimental setup and quantify relative water content changes in the subsurface.