Summary
Alkaline/surfactant/polymer (ASP) flooding using conventional alkali
requires soft water. However, soft water is not always available, and softening
hard brines may be very costly or infeasible in many cases depending on the
location, the brine composition, and other factors. For instance, conventional
ASP uses sodium carbonate to reduce the adsorption of the surfactant and
generate soap in-situ by reacting with acidic crude oils; however, calcium
carbonate precipitates unless the brine is soft. A form of borax known as
metaborate has been found to sequester divalent cations such as Ca++
and prevent precipitation. This approach has been combined with the screening
and selection of surfactant formulations that will perform well with brines
having high salinity and hardness. We demonstrate this approach by combining
high-performance, low-cost surfactants with cosurfactants that tolerate high
salinity and hardness and with metaborate that can tolerate hardness as well.
Chemical formulations containing surfactants and alkali in hard brine were
screened for performance and tolerance using microemulsion phase-behavior
experiments and crude at reservoir temperature. A formulation was found that,
with an optimum salinity of 120,000 ppm total dissolved solids (TDS), 6,600 ppm
divalent cations, performed well in corefloods with high oil recovery and
almost zero final chemical flood residual oil saturation. Additionally,
chemical formulations containing sodium metaborate and hard brine gave nearly
100% oil recovery with no indication of precipitate formation. Metaborate
chemistry was incorporated into a mechanistic, compositional chemical flooding
simulator, and the simulator was then used to model the corefloods. Overall,
novel ASP with metaborate performed comparably to conventional ASP using sodium
carbonate in soft water, demonstrating advancements in ASP adaptation to hard,
saline reservoirs without the need for soft brine, which increases the number
of oil reservoirs that are candidates for enhanced oil recovery using ASP
flooding.
Introduction
This paper describes a laboratory and modeling approach to ASP flooding in
reservoirs containing very hard, saline brines without the need for soft brine.
Our target reservoir is a low temperature (52°C), light oil (45°API) sandstone
reservoir containing hard, saline formation brine with 157,000 mg/L TDS of
salinity, of which 8600 mg/L are Ca++ and Mg++. Our
objective was to design an ASP slug to use as much of the formation brine as
possible and eliminate the need for soft water. We show that a novel alkali,
sodium metaborate, can provide tolerance to high divalent cation concentrations
that the conventional alkali sodium carbonate cannot. Our laboratory approach
uses quick, inexpensive microemulsion phase-behavior experiments to screen
chemical formulations for both performance and tolerance to salinity and
hardness. Well performing formulations are validated in coreflood experiments
for good oil recovery, low pressure gradient, and low surfactant retention
using prepared Berea sandstone cores saturated with very hard, saline brine at
residual oil saturation.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2008
- Meeting paper published:
21 September 2008
- Revised manuscript received:
5 March 2008
- Manuscript approved:
20 March 2009
- Published online:
15 October 2009
- Version of record:
12 March 2010
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