Investigating Electrical-Impedance Tomography as a Technique for Real-Time Saturation Monitoring
- Robert Stacey (Stanford University) | Kewen Li (Peking University) | Roland N. Horne (Stanford University)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2009
- Document Type
- Journal Paper
- 135 - 143
- 2009. Society of Petroleum Engineers
- 5.9.2 Geothermal Resources, 1.6.9 Coring, Fishing, 5.6.5 Tracers, 6.7 Fundamental Research in HSSE, 4.3.4 Scale, 5.5.2 Core Analysis, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 5.3.1 Flow in Porous Media, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing
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3D electrical-impedance tomography (EIT) is a technique that has the potential to provide estimates of reservoir saturation at multiple scales by determining the resistivity distribution within the subsurface. In theory, EIT is well suited for researching oil and brine systems because of the large contrast in resistivity between the two phases. Here, in our initial laboratory investigation, we have applied the EIT technique to measure the saturation distribution of water within a core.
The initial EIT experiment presented here used 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 occurring by evaporation. The voltage-potential field was measured by applying a direct-current (DC) pulse across the core and measuring the voltage potential at all electrodes, essentially applying the four-wire resistance technique over all electrodes in turn. The result was a data set that embodies the resistivity distribution within the core, and, by inversion, the resistivity distribution was reconstructed, which allowed for the inference of the saturation.
The data processing was accomplished by using the Electrical Impedance Tomography and Diffuse Optical Tomography Reconstruction Software (EIDORS) toolkit, which was developed for application to this nonlinear and ill-posed inverse problem. The procedure uses a finite-element model for forward calculation and a regularized nonlinear inverse solver to obtain a unique and stable inverse solution.
Experiments have indicated that EIT is a viable technique for studying the displacement characteristics of fluids with contrasting resistivity and is capable of detecting displacement fronts in near to real time. The current system is also a quantitative technique able to measure saturation distributions accurately between 15% < Sw < 65% in a Berea sandstone core. These limitations were imposed because of connate-water connections to the electrodes and ion-mobility effects caused by the DC voltage source. It is anticipated that the applicability of EIT will increase with the implementation of an alternating-current (AC) voltage source.
In an oil reservoir, it is crucial to know the extent, the saturation distribution, and the connectivity of the resource. The extent is typically well understood compared to the connectivity and saturation distribution within the reservoir.
At the field scale, where the question of connectivity between wells is of critical importance, injection- and production-history data may be used to infer connectivity. However, the productivity of a field may be placed in jeopardy by improper placement of an injection well. Therefore, knowing the connectivity of the reservoir early in development would help minimize risk and maximize productivity throughout the life of a reservoir. For this reason, EIT at the field scale is of particular interest in identifying connective faults and fractures throughout the reservoir.
However, before any large-scale investigations may be pursued with EIT, a laboratory-scale EIT system has been developed to investigate core-scale fluid interactions that are of equal importance to the life of a reservoir. Core experiments may infer the microscale properties that influence the life of the reservoir significantly—primary and secondary porosity and permeability, relative permeability in fractures, and saturation distribution.
In laboratory experiments, ferrous core holders are often used to replicate high reservoir pressures and temperatures. However, the use of a ferrous core holder eliminates the application of the X-ray computed-tomography (CT) -scan technique to estimate in-place saturations because the X-rays cannot penetrate the steel vessels.
Consequently, because of the importance of understanding core-scale phenomenon and the limitations of the X-ray CT scan, EIT has been investigated as a new technique to image fluid distribution.
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