Determining Saturation Using Electrical Impedance Tomography (EIT)

Abstract

Three-dimensional Electrical Impedance Tomography (EIT) is a measurement technique that has the potential to provide estimates of reservoir saturation at multiple scales from the calculated resistivity distribution. EIT in theory has the capability to image geothermal reservoir systems due to the contrast in resistivity between the liquid and vapor phases. Here in our initial laboratory investigation we have applied EIT to measure the saturation distribution within a core. The initial EIT experiment presented here was on a Berea sandstone core with 48 electrodes attached in three rings of 16. The core was open to the atmosphere with saturation occurring by natural imbibition and desaturation occurs by evaporation. The voltage potential field is measured by applying a direct current pulse across the core and measuring the voltage potential at all electrodes, essentially applying the 4-wire resistance technique over all electrodes in turn. The result is a data set that embodies the resistivity distribution within the core, and by inversion the resistivity distribution is reconstructed, which allows for the inference of the cores saturation. The data processing was accomplished by utilizing the EIDORS toolkit developed for MATLAB. The toolkit was required due to the nature of EIT being a nonlinear and an ill-posed problem. The procedure utilizes a finite element model for forward calculations and a regularized nonlinear solver for obtaining a unique and stable inverse solution. (Polydorides et al. 2002) Initial tests on the core indicate that EIT is a viable alternative for measuring the saturation distribution within a geothermal rock core, and is capable of detecting saturation fronts in near real-time. Current efforts are focused upon calibrating the saturation inferred by EIT, and on increasing accuracy by examining the multiple aspects of the systems electrochemistry that may effect resistivity measurements.