Electrical Impedance Tomography (eit) Method for Saturation Determination

3D Electrical Impedance Tomography (EIT) is an alternative to the traditional CT scan technique that has been used to measure saturation distribution within a rock core. The primary motivation for investigating EIT is the applicability to high pressure, high temperature geothermal reservoir conditions, which would require a metal core holder, therefore eliminating the use of the CT scan technique. Secondary advantages of the EIT technique are that it is an inexpensive, practical, compact, and safe alternative to the CT scan. The initial EIT device presented here instrumented a Berea sandstone core with 48 electrodes attached in three rings of 16. The core is open to the atmosphere and saturation occurs by natural imbibition. The resistivity measurements are conducted by applying a direct current pulse and using the 4-wire resistance technique over all electrodes. This resulted in a data set that embodies the resistivity distribution within the core. Processing of this data set results in the reconstruction of the resistivity distribution, which is the saturation distribution. The processing was accomplished by utilizing the EIDORS toolkit developed for MATLAB. The toolkit was required due to the nature of EIT being nonlinear and an ill-posed problem. The EIDORS application utilizes a finite element model for forward calculations and a regularized nonlinear solver to obtain a unique and stable inverse solution. (Polydorides et al. 2002) It was found that EIT can detect changes in the core’s resistivity distribution quickly and with relatively good accuracy. However, currently the diffusive nature of EIT makes the reliable detection of sharp saturation fronts difficult, but not impossible. Therefore future efforts are being applied to increase resolution and quantify accuracy. INTRODUCTION The concept of measuring the saturation distribution in a core using ERT was described in a paper by Van Weereld, et al. (2001). The paper showed that EIT techniques were able to image a two-phase system (oil and brine). However the techniques shown in the paper did not convert resistivity images to saturation, or validate their results against other methods. Therefore we have developed an apparatus to investigate the feasibility of using EIT to measure geothermal core saturation.