Puntervold_subbed 30/3/09 09:29 Page 116
Enhanced Oil Recovery by Injection of Seawater and Mixtures of Seawater and Produced Water
Figure 4: Spontaneous Imbibition of Mixtures of Valhall Produced
to promote wettability modification. When the core exposed to PW
Water and Seawater into Chalk Cores at 110ºC
had reached a production plateau, it was removed from the
imbibition cell and subjected to viscous flooding at a controlled rate
80
of ~0.1PV/D with PW still at 110°C. After two days of viscous
70
flooding, the differential pressure had reached 4psi, and the first
60
drop of oil was produced. Total oil recovery increased and stabilised
50
at about 40%.
40
PW SW
30
As a final step, PW was exchanged with SW as the injection fluid in
Recovery (%OOIP)
20 a tertiary water flood process. A sudden and drastic increase in oil
10 recovery was observed; the total oil recovery amounted to about
0 70%, which is somewhat higher than that obtained with a
0 10 20 30 40 50 60
spontaneous imbibition process with a completely water-wet
Time (days)
system. SW therefore increased oil recovery in a viscous flood due to
PW PW1 SW2 FI
its ability to improve the water-wetness of chalk. These results have
PW1 SW1 PW1 SW8
been verified by experimental observations using Valhall reservoir
FI = forced imbibition; OOIP = original oil in place; PW = produced water; SW = seawater.
cores at reservoir conditions.
14
The whole capillary pressure curve
changed when SW was substituted for FW as the imbibing fluid.
lower than expected, supporting the modelling results; this was
ascribed to trapping of Ba
2+
in the formation by precipitation of
BaSO
4
. The concentration of Sr
2+
in the PW was similar for both Due to its smart composition,
(PW)
exp
and (PW)
calc
, reflecting very little interaction with the
seawater can act as a tertiary
formation. Ba
2+
and Sr
2+
have been left out of Figure 3.
enhanced oil recovery fluid by
Enhanced Oil Recovery by Injection of Mixtures of
imposing a favourable
Produced Water and Seawater
Finally, some experimental imbibition studies on moderately water-
wettability modification
wet chalk core plugs were performed to investigate the enhanced
of chalk.
oil recovery potential of injecting mixtures of SW and Valhall PW.
13
The composition of PW was quite similar to that of the Valhall FW,
which indicates that the PW was sampled early in the SW injection Conclusions
phase, i.e. before SW breakthrough. Traces of Ba
2+
and Sr
2+
were If the PW is diluted at least four times with SW, the simulated results
present in the PW, but were left out in the experiments to avoid showed that scale problems related to precipitation of CaSO
4
, SrSO
4
possible precipitation. and BaSO
4
in the injection well and in the nearby formation
appeared to be very small.
The imbibition rate and ultimate oil recovery indicate the relative
ability of the fluids to displace the oil by wettability modification. The At 110°C, the tested mixtures of PW and SW were able to increase
spontaneous imbibition tests performed at 110°C using an oil with oil recovery from cores of low water-wetness in a spontaneous
acid number 0.70mgKOH/g were sensitive to the composition of the imbibition process by wettability modification. The oil recovery was
imbibing fluid (see Figure 4). After 20 days of imbibition, only 32% at least as high as for a completely water-wet system. Imbibition of
of the oil was recovered when imbibing pure PW. The different pure PW gave an oil recovery that was only half the recovery from
mixtures of PW and SW (1:1, 1:2 and 1:8 by volume) gave an oil imbibing PW–SW mixtures.
recovery of around 55% for the same time interval.
In chalk, and probably in other carbonates, SW acts as ‘smart water’
Obviously, the presence of SO
2-
4
in the imbibing fluid is crucial for both in a spontaneous imbibition and in a forced displacement
improving oil recovery by wettability modification. The different process. Due to its smart composition, SW can act as a tertiary
mixtures of PW and SW appeared to contain high enough enhanced oil recovery fluid by imposing a favourable wettability
concentrations of all of the important ions – Ca
2+
, Mg
2+
and SO
2-
4
– modification of chalk. ■
1. Zhang P, Tweheyo MT, Austad T, Colloids and Surfaces A, 6. Nielsen CM, Olsen D, Bech N, Paper SPE 63226, Presented 11. Burgess J, Metal ions in solution, John Wiley & Sons
Physicochem Eng Aspects, 2007;301:199–208. at the SPE Annual Tech. Conference and Exhibition, Dallas, Limited, 1978.
2. Korsnes RI, Madland MV, Austad T, EUROCK 2006 – Texas, 1–4 October. 2000. 12. Carlberg BL, Matthews RR, Solubility of calcium sulfate in
Multiphysics Coupling and Long Term Behaviour in Rock 7. Korsbech U, Aage HK, Andersen BL, et al., SPE Reservoir brine, Oilfield Chemistry Symposium of the Society of
Mechanics, London: Taylor & Francis Group, 2006. Eval. & Eng., June 2006. Petroleum Engineers of AIME, Denver, Colorado, US, 1973.
3. Strand S, Høgnesen EJ, Austad T, Colloids and Surfaces A, 8. Bader MSH, Desalination, 2007;208(1–3):159–68. 13. Puntervold T, Strand S, Austad T, Energ Fuel, 2009
Physicochem Eng Aspects, 2006;275:1–10. 9. Puntervold T, Austad T, J Petrol Sci Eng, 2008;63:23–33. (accepted for publication).
4. Korsnes RI, Madland MV, Austad T, et al., J Petrol Sci Eng, 10. Mackay EJ, Collins IR, Jordan MM, Feasey N, Paper 14. Webb KJ, Black CJJ, Tjetland G, A laboratory study
2007;60(3–4):183–93. SPE80385, presented at the SPE 5th International investigating methods for improving oil recovery in
5. Puntervold T, Strand S, Austad T, Energ Fuel, 2007;21(6): Symposium on Oilfield Scale, Aberdeen, UK, 29–30 carbonates, International Petroleum Technology
3425–30. January 2003. Conference, (IPTC), Doha, Qatar, 2005.
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EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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