Summary
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.
Introduction
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.
© 2009. Society of Petroleum Engineers
View full textPDF
(
1,376 KB
)
History
- Original manuscript received:
25 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
6 June 2008
- Manuscript approved:
25 July 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
View Cart
