Summary
Low-salinity waterflooding is an emerging enhanced-oil-recovery (EOR)
technique in which the salinity of the injected water is controlled to improve
oil recovery vs. conventional, higher-salinity waterflooding. Corefloods and
single-well chemical-tracer tests have shown that low-salinity waterflooding
can improve basic waterflood performance by 5 to 38%. This paper describes a
model of low-salinity flooding that can be used to evaluate projects; shows the
implications of that model and demonstrates its use to represent corefloods,
single-well tests, and field-scale simulations; and gives insight into the
reservoir engineering of low-salinity floods.
The model represents low-salinity flooding using salinity-dependent
oil/water relative permeability functions resulting from wettability change.
This is similar to other EOR modeling, and conventional fractional-flow theory
can be adapted to describe the process in 1D for secondary and tertiary
low-salinity waterflooding. This simple analysis shows that while some degree
of connate-water banking occurs, it need not hinder the process.
Mixing of injected water with in-situ water delays the attainment of low
salinity, potentially preventing attainment of low salinity all together if
very small slugs of low-salinity water are used. This paper demonstrates the
importance of mixing to modeling of low-salinity flooding and suggests
addressing it in engineering calculations. Care must be taken in representing
mixing appropriately in interpreting data and in constructing models. The use
of numerical dispersion to represent physical dispersion in 1D, radial, and
pattern simulations of this process is demonstrated (i.e., coarse-grid
simulations are shown to give the same result as fine-grid simulations with an
appropriately large physical dispersion). In many applications, the fine-grid
simulation necessary to represent appropriate levels of dispersion is not
practical, and pseudoization is necessary. We demonstrate that this can be
achieved by changing the salinity dependence and shapes of relative
permeability curves.
Introduction
Waterflooding is widely used to improve recovery from oil reservoirs but,
except to avoid formation damage, is largely designed without regard to the
composition of the brine injected. Yildiz and Morrow (1996) showed that changes
in injection-brine composition can improve recovery, thereby introducing the
idea that the composition of the brine could be varied to optimize waterflood
recovery. Tang and Morrow (1997) (Tang and Morrow 1999; Morrow et al. 1998;
McGuire et al. 2005) built on this idea by demonstrating the benefit lowering
brine salinity has on oil recovery. There has been a substantial amount of
research on low-salinity injection, which has included more than 20
reservoir-conditions corefloods on a range of sandstone reservoirs both in
secondary and tertiary mode, more than 10 single-well chemical-tracer tests
(SWCTTs), and a log/inject/log test (McGuire et al. 2005;Webb et al. 2004; Webb
et al. 2005; Lager et al. 2006). These tests have shown improvements of
waterflood-process efficiency by 5 to 38% by using low-salinity water or by
corresponding reductions in residual-oil saturation of 3 to 17% pore volumes
(PV). The purpose of this work is to present a simple extension to waterflood
simulators that can be used to translate corefloods or SWCTTs into field-scale
estimates of low-salinity waterflood (LSWF) oil recovery and demonstate this
with examples from a sandstone reservoir.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
10 July 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
11 July 2008
- Manuscript approved:
24 July 2008
- Version of record:
29 December 2008
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