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Enhanced Oil Recovery by Injection of Seawater and Mixtures of Seawater and Produced Water
Table 1: Molar Compositions of Seawater, Ekofisk and Valhall Figure 3: Calculated and Measured Component Concentration in
Formation and Produced Water Produced Water from the Ekofisk Field
SW FW
a
PW
a
FW
b
PW
b
0.06
Ekofisk Ekofisk Valhall Valhall
HCO
3
- 0.002 0.004 0.008 0.009 0.013
0.05
Cl
-
0.525 1.423 0.765 1.066 1.096
SO
2-
4
0.024 0.000 0.007 – 0.001 0.04
Mg
2+
0.045 0.022 0.021 0.008 0.008
Ca
2+
0.013 0.100 0.049 0.029 0.031
0.03
Na
+
0.450 1.156 0.635 0.996 1.027
K
+
0.010 0.007 0.007 0.005 0.005 Concentration (mole/I)
0.02
Ba
2+
0.0E+00 0.002 8.2E-06 – –
Sr
2+
0.0E+00 0.009 2.2E-03 – –
0.01
Li
+
0.0E+00 0.000 9.8E-04 – –
0
Ionic strength 0.657 1.559 1.150 1.112 1.150
Ca
2+
Mg
2+
SO
2-
4
TDS (g/l) 33.39 83.09 45.81 62.8 64.96
Component
a. Data received from ConocoPhillips, Norway. b. Data received from BP, Norway.
FW = formation water; PW = produced water; SW = seawater; TDS = total dissolved solids.
(PW)
calc
* (PW)
exp
Linked to substitution of Ca
2+
by Mg
2+
at the rock surface, adsorption of SO
2-
4 onto the rock and
volumetric mixing of FW and SW can be calculated by assuming that
precipitation of CaSO4. *Calculated based on mixing 0.264l fresh water (FW) and 0.736l seawater (SW).
Na
+
and Cl
-
are inert ions, i.e. they do not react with the formation
PW = produced water. (PW)calc = calculated produced water; (PW)exp = experimentally measured
composition of PW.
in any way. For a given composition of PW and FW, calculations
showed that 0.264l of FW were mixed with 0.736l of SW, resulting various ions in the PW, shown in Figure 3, confirmed that
in 1l of PW (see Table 1). substitution of Ca
2+
by Mg
2+
at the chalk surface takes place in
the reservoir, i.e. the concentration of Ca
2+
increased while that
Based on these values, the expected concentrations of the other ions of Mg
2+
decreased, which was also observed in the laboratory.
1,2
in the PW can be calculated, (PW)
calc
, assuming that no interaction A slightly lower increase in Ca
2+
compared with that seen with a
with the formation takes place. A comparison with the 1:1 substitution may be due to precipitation of CaSO
4
. SO
2-
4
is
corresponding experimentally measured composition of PW, (PW)
exp
, strongly retarded in the formation, as seen by the reduction in
will then indicate possible interactions with the formation. The measured concentration compared with the calculated
differences between measured and calculated concentrations of the concentration. The concentration of Ba
2+
in the PW was significantly
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