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

The objective of the CO2FieldLab project was to demonstrate that adequate monitoring methods can be deployed to document potential leaks of CO2 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 CO2 leak in order to assess the relative sensitivity of several geochemical and geophysical CO2 monitoring tools. A total mass of 1.67 tons of CO2 was injected at a depth of 20 m 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 CO2 in the shallow subsurface after the start of injection. Electrical resistivity response correlates with progressive CO2 dissolution in groundwater, with resistivity and pH values decreasing due to the increase of bicarbonate and dissolved species. The CO2 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.