Controlled-source electromagnetic for 4D-monitoring of CO₂ storage in a geological reservoir

Abstract

Maximizing oil and gas production, or CO₂ sequestration capacity of a reservoir, requires detailed knowledge of reservoir properties and monitoring changes in fluid or gas content. Large contrasts of electrical resistivity between different fluids (gas/oil versus brine/connate water) and reservoir rocks make 4D controlled-source electromagnetic (CSEM) monitoring an effective tool for reservoir analysis. To examine the feasibility of CSEM monitoring at the Ghawar oil field, Saudia Arabia, we use porosity and permeability data from the Ghawar field to construct a geologic model of the reservoir. We simulate changes in water and CO₂ gas content within the reservoir over ten years in a CO₂ injection scenario. Archie’s law for carbonate reservoirs is then used to convert the simulation results to changes of reservoir conductivity. Last, the reservoir is embedded within a 3D homogenous seabed background conductivity model. A Finite Element Modeling technique is used to simulate time-lapse frequency domain CSEM data for reservoirs buried at a variety of depths. The results of the feasibility study show that the CSEM response exhibits small but measurable changes that are characteristic of reservoir-depletion geometry (conductivity). Our analysis demonstrates that CSEM can detect changes in reservoir properties at 1.5 years, 3 years, 6 years, and 10 years at 250m, 500m, and 1000m depths respectively. Moreover, high frequency sources result in better resolution, though resolution degrades significantly with reservoir depth.