Cross-hole electromagnetic and seismic modeling for CO2 detection and monitoring in a saline aquifer

The injection of CO2 in saline aquifers and depleted hydrocarbon wells is one solution to avoid the emission of that greenhouse gas to the atmosphere. Carbon taxes can be avoided if geological sequestration can efficiently be performed from technical and economic perspectives. For this purpose, we present a combined rock-physics methodology of electromagnetic (EM) and seismic wave propagation for the detection and monitoring of CO2 in crosswell experiments. First, we obtain the electrical conductivity and seismic velocities as a function of saturation, porosity, permeability and clay content, based on the CRIM and White models, respectively. Then, we obtain a conductivity–velocity relation. This type of relations is useful when some rock properties can be more easily measured than other properties. Finally, we compute crosswell EM and seismic profiles using direct modeling techniques. P- and S-wave attenuation is included in the seismic simulation by means of White's mesoscopic theory. The modeling methodology is useful to perform sensitivity analyses and it is the basis for performing traveltime EM and seismic tomography and obtain reliable estimations of the saturation of carbon dioxide. In both cases, it is essential to correctly pick the first arrivals, particularly in the EM case where diffusion wavelength is large compared to the source–receiver distance. The methodology is applied to CO2 injection in a sandstone aquifer with shale intrusions, embedded in a shale formation. The EM traveltimes are smaller after the injection due to the higher resistivity caused by the presence of carbon dioxide, while the effect is opposite in the seismic case, where water replaced by gas decreases the seismic velocity.