A model combining patchy saturation and moisture-induced elastic weakening with application to ultrasonic monitoring under varying water saturation

Water is known to induce weakening on the static mechanical properties of rocks. However, injection-based operations such as EGS or EOR are commonly monitored through seismic methods involving dynamic moduli. It is therefore important to understand and quantify the effect of water-weakening on dynamic properties. In this study, we performed water injection tests on microporous carbonate rocks (two chalks from the Mons Basin) with ultrasonic monitoring of P-wave velocity and attenuation in order to observe the evolution of the rock moduli with varying water saturation. Our experimental results were interpreted through (i) a classical patchy saturation or PS model and (ii) the same model coupled with water weakening effect through modulus reduction induced by surface energy decrease induced by water in the fluid-rock system, called the WW-PS model. We show that the WW-PS model can better fit the experimental data than the PS model for both selected chalks, but also the previously published data for Sherwood sandstone. Therefore fluid-rock interaction needs to be taken into account when dealing with a fluid not in equilibrium with the host reservoir in fluid injection operations. An extension to reservoir scale modelling is proposed to emphasize the potential impact of water weakening at larger scales.