Puntervold_subbed 30/3/09 09:27 Page 114
Enhanced Oil Recovery by Injection of Seawater and Mixtures of Seawater and Produced Water
Figure 1: Precipitation of CaSO
4
(A), SrSO
4
(B) and BaSO
4
(C) versus Temperature for Mixtures of Ekofisk Produced Water and Seawater
A 0.009 B 1.40E-03 C 9.00E-06
0.008
1.20E-03
8.00E-06
0.007 7.00E-06
1.00E-03
0.006 6.00E-06
(mole/l) (mole/l)
4 0.005 4
8.00E-04 (mole/l)
4 5.00E-06
0.004
6.00E-04
4.00E-06
0.003 3.00E-06
4.00E-04
0.002 2.00E-06
Precipitation CaSO 0.001
Precipitation SrSO 2.00E-04
Precipitation BaSO 1.00E-06
0 0.00E+00
0.00E+00
80 100 120 30 80 130 30 80 130
Temperature (ºC) Temperature (ºC) Temperature (ºC)
PW PW1 SW2 PW1 SW8 PW PW1 SW2 PW1 SW8 PW PW1 SW2 PW1 SW8
PW1 SW1 PW1 SW4 SW PW1 SW1 PW1 SW4 SW PW1 SW1 PW1 SW4 SW
PW = produced water; SW = seawater.
Figure 2: Precipitation of CaSO
4
as a Response to Varying [Ca
2+
] and
interactions between injected water and the chalk surface, such as
[Mg
2+
] in Seawater as a Function of Temperature
substitution of Ca
2+
by Mg
2+
and adsorption of SO
2-
4
. In the reservoir,
the injected SW or PW–SW mixtures are not in equilibrium with the
0.018
0.016
formation, thus component exchange takes place. Chalk dissolution
0.014
decreases at higher temperatures, while the substitution of Ca
2+
by Mg
2+
(mole/l)
0.012
at the chalk surface increases with higher temperatures.
1,4
At 100 and
4
0.01 130°C, core flooding experiments revealed that the amount of Ca
2+
in
0.008 the effluent increased by 20 and 50% relative to the initial concentration,
0.006
respectively, because of substitution of Ca
2+
by Mg
2+
. Analysis of the
0.004
Precipitation CaSO
concentration of Mg
2+
in the effluent confirmed a 1:1 substitution.
1,2
At
0.002
70°C, no changes in either concentration were detected.
0
50 70 90 110 130 150
The small Mg
2+
ion is strongly solvated/hydrated in water (∆H
hydr
=
Temperature (ºC)
-459kcal/mol),
11
which makes it less reactive at low temperatures.
SW
The Mg
2+
ion is partly dehydrated as the temperature increases, and
SW +0.003mol/l Ca
2+
, -0.003mol/l Mg
2+
the rate of substitution increases due to the decrease in the
SW +0.006mol/l Ca
2+
, -0.006mol/l Mg
2+
SW +0.009mol/l Ca
2+
, -0.009mol/l Mg
2+ activation energy of the reaction. Additionally, in SW, Mg
2+
forms an
SW +0.012mol/l Ca
2+
, -0.012mol/l Mg
2+
important ion-pair with SO
2-
4
:
12
SW -0.003mol/l Ca
2+
, +0.003mol/l Mg
2+
SW -0.006mol/l Ca
2+
, +0.006mol/l Mg
2+
Mg
2+
+ SO
2- 2-
SW -0.009mol/l Ca
2+
, +0.009mol/l Mg
2+ 4
= [Mg
2+ ...
SO
4
](aq) (1)
SW -0.012mol/l Ca
2+
, +0.012mol/l Mg
2+
Because [Mg
2+
] ≈ 2[SO
2-
4
] in SW, the formed ion-pair will lower
PW = produced water; SW = seawater.
the activity of SO
2-
4
significantly. As the temperature increases, the
equilibrium of reaction (1) is moved to the right, affecting
data, was used. Details of this study can be found in the published the solubility of CaSO
4
in SW. By removal of Mg
2+
from SW by the
article by Puntervold and Austad.
9
substitution process, the activity of SO
2-
4
is reduced to a lesser extent,
and SO
2-
4
is thus available for sulphate precipitation. The effect of
At the Ekofisk reservoir temperature, 130°C, the PW is saturated substitution of Ca
2+
by Mg
2+
from the chalk surface on CaSO
4
with CaSO
4
, SrSO
4
and BaSO
4
(see Figures 1A, 1B and 1C, solubility was modelled as a function of temperature by increasing and
respectively). Precipitation causes sulphate stripping in the reservoir. decreasing the molar concentrations of Ca
2+
and Mg
2+
(see Figure 2).
In the injection well, the temperature is lower due to injection of Using the Ekofisk field at 130°C as an example, a 50% increase in
cold PW/SW. As long as the injection well temperature is kept below Ca
2+
concentration was experienced as a result of substitution by
80°C and the PW is diluted with at least four volume parts of SW, Mg
2+
. The concentration of Ca
2+
in normal SW is 0.013mol/l, thus a
the injectivity can be maintained because sulphate scale precipitation 50% increase corresponds to an increase of 0.0065mol/l, which in
is negligible. However, it has been reported that injection wells in Figure 2 is represented by the pink triangles. Interpretation of the
fractured reservoirs can tolerate a substantial amount of scale modelling results shows that substitution of Ca
2+
by Mg
2+
at the chalk
deposition before loss of injectivity is experienced.
10
surface in the Ekofisk reservoir almost doubled the amount of
precipitated CaSO
4
, from 0.005mol/l from pure SW to 0.009mol/l.
Interactions Between Chalk and Water
Temperature plays a key role in predicting scale potential because of its Injection of SW into the Ekofisk field has been going on for more
influence on chalk dissolution, sulphate salt precipitation and ionic than 20 years, and the PW is therefore a mixture of FW and SW. The
114
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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