Wave Propagation in Porous Media Saturated with Two Fluids: Is it Feasible to Detect Leakage of a CO₂ Storage Site Using Seismic Waves?

When it comes to geological storage of CO₂, monitoring is crucial to detect leakage in the caprock. In our study, we investigated the wave speeds of porous media filled with CO₂ and water in order to determine reservoir changes. We focused on deep storage sites where CO₂ is in a supercritical state. In case of a leak, CO₂ rises and eventually starts to boil as soon as it reaches temperatures or pressures below the critical point. At this point, there are two distinct phases in the pore space. We derived the necessary equations to calculate the wave speeds for unsaturated porous media and tested the equations for a representative storage scenario. We found that there are three modes of pressure waves instead of two for the saturated case. The new mode has a very small wave speed and is highly attenuated. This mode will most likely be very hard to detect in practice and therefore it may be necessary to use time-lapse seismic migration to detect leakage.