Magnetotelluric and Controlled-Source Electromagnetic Pre-Injection Study of Aquistore CO2 Sequestration Site, Near Estevan, Saskatchewan, Canada

The Aquistore project is a large-scale CO2 sequestration operation at Estevan, southeast Saskatchewan, Canada. CO2 is being captured from the Boundary Dam power station, and injected to the base of the Phanerozoic Williston Basin, to be stored in a saline aquifer at 3.4 km depth. In this study, magnetotelluric (MT) and surface controlled-source electromagnetic (CSEM) methods are tested in a pre-injection setting at Aquistore for their applicability to sequestration monitoring goals. The MT and CSEM methods are complimentary in their ability to resolve structures at different scales using different current systems. Pre-injection MT soundings were conducted in 2013, 2014 and 2015 over a 2.5 km × 8.5 km area surrounding the Aquistore injection well. The Phanerozoic MT response is spatially uniform across the survey area. The resistivity structure of the Phanerozoic is one-dimensional from 0.001 to 10 s: the apparent resistivity decreases from 8 Ωm to 2 Ωm in this period range. Spectral and polarization analyses indicate that broadband noise in period bands of 0.05 to 1 s and 0.0077 to 0.0125 s recorded in 2014 is associated with a CO2 pipeline. At frequencies outside these bands, the MT responses define small differences between surveys (< 3%). Using an optimal MT response from these datasets and constraints from a resistivity well-log, a representative 18-layer 1D resistivity model for the Williston Basin sedimentary sequence has been recovered. CSEM surveys in 2013 and 2015 used a 1 km, 30 A electric horizontal dipole source. Recordings of the radial electric field component were made along an inline receiver profile from 3.5 to 9.5 km offsets. Preliminary characterization of these recordings indicates that the transmitted signals are observable at each of the profile locations. Fréchet derivatives of the CSEM response indicate that response sensitivity to changes in the Williston Basin electrical properties is attainable at the level of the reservoir. Low induction number soundings achieve sensitivity at the depth of the reservoir on the order of 0.012 Ωm/m and 0.22o/m changes in the apparent resistivity and phase responses at a transmission frequency of 0.5 Hz and offsets of 7.5-9.5 km.