Near-surface CO2 leak detection monitoring from downhole electrical resistivity at the CO2 Field Laboratory, Svelvik Ridge (Norway)

Abstract : The objective of the CO 2 FieldLab project was to demonstrate that adequate monitoring methods can be deployed to document potential leaks of CO 2 from subsurface reservoirs. For this, a shallow injection experiment through permeable sediment was designed and conducted at Svelvik (Norway) in September 2011. The goal was to produce a CO 2 leak in order to assess the relative sensitivity of several geochemical and geophysical CO 2 monitoring tools. A total mass of 1.67tons of CO 2 was injected at a depth of 20m through a 45° inclined well over a 6 day period. Time-lapse induction logging and downhole resistivity monitoring from a permanent dipole–dipole array observatory prove to be very sensitive to the presence of CO 2 in the shallow subsurface after the start of injection. Electrical resistivity response correlates with progressive CO 2 dissolution in groundwater, with resistivity and pH values decreasing due to the increase of bicarbonate and dissolved species. The CO 2 plume is channelled by enhanced permeability layers, with updip migration of the gas plume to the North. Other processes responsible for resistivity changes were fresh/salt water mixing, tidal effects and rainfall.