Feasibility analysis of surface-to-reservoir electromagnetics for waterflood monitoring

Summary We analyze, by means of a synthetic model, the feasibility of detecting the electromagnetic (EM) field variations related to waterflood in a large Saudi Arabian Jurassic reservoir. We utilize a 3D structural reservoir model and derive the geoelectric parameters from careful analysis of well logs acquired at the Saudi Aramco Technology Test Site. The resistivity variations as a result of water flooding are derived using characteristic parameters and injection water salinity of the field. We model a geometry consisting of a radial surface galvanic source and four EM receivers located at the reservoir level. The full EM field is modeled in the time domain and the horizontal and vertical electric field (Ex and Ez) and horizontal crossline magnetic field (dBy/dt) components are interpreted and analyzed. Results indicate that all the modeled fields show substantial variations as a result of water saturation changes with field strength values above the noise level expected for EM sensors. The results will be validated next by modeling more complex and realistic 3D patterns of water saturation in the reservoir, as derived directly from reservoir simulation. Given that one of the modeled components is the vertical electric field, the electrical anisotropy of the overburden is expected to play a significant role in the response and will be taken into consideration in the next round of modeling. Modeling results also suggest that the type of borehole EM sensors currently available in the industry may not be adequate for surface-to-reservoir EM applications