Electrical imaging using a well casing as an antenna, a case study from a CO2 sequestration site in Montana

Abstract:

Steel well casings offer a convenient and readily available method to draw electrical current deeper into the earth than surface sources allow. When a source is connected to the well casing, the well casing acts to distribute the current along the casing and into the formations adjacent to the well. The currents leaving the casing can be modelled as a distribution of smaller sources along the well path. In this study we show through modelling both synthetic and field data that using a well casing to draw current deeper into the earth offers the ability to image electrical resistivity structure at depths deeper than surface transmitters allow.

Electric field data have been collected at a future CO₂ sequestration site in Montana, where a steel-cased well drilled to about 1.4 km below the surface was utilized for the source. Electric field measurements were made in two scenarios, 1) while current was transmitted between surface electrodes, and 2) while current was transmitted from the wellhead to a surface return electrode. Modelling has indicated that the electric fields measured during the wellhead transmission result in sensitivity to deeper structure than the surface measurements. 3D Inversion of surface and casing electrode data provided results consistent with the data and knowledge of background structure.

This particular survey was completed prior to CO₂ injection into the reservoir layer, and has provided the baseline for a time lapse field experiment. The model produced by this baseline survey will be used as a starting model for the inversion of data collected after CO₂ has been injected.