Condition for Low Salinity EOR-Effect in
Carbonate Oil Reservoirs
T. Austad, S. F. Shariatpanahi, S. Strand, C. J. J. Black*, K. J. Webb*T. Austad, S. F. Shariatpanahi, S. Strand, C. J. J. Black*, K. J. Webb*
University of Stavanger, 4306 Stavanger, Norway*BP Exploration Sunbury, UK.
The IEA 32nd Annual Symposium and Workshop,
Vienna 17-19. October, 2011.
1
Low salinity EOR-effects in carbonates
SPE 137634 Ali A. Yousef et al. (Saudi Aramco)
2
No low salinity EOR-effects in chalk
Forced displacement at 110 oC Spontaneous imbibition at 120 oC
3
Question
• Why is Low Salinity EOR-effects possible in
limestone but not in Chalk ???
4
Reminder
• Low salinity EOR-effects are linked to
Sandstone reservoirs containing clay.
• Wettability alteration in carbonates by “Smart
Water”Water”
– SO42- acts as a catalyst in combination with Ca2+
and/or Mg2+
– The efficiency of the wettability alteration process
is increased if the injected fluid is depleted in non-
active salt like NaCl.
5
Wettability modification in Chalk:
Effect of Sulfateand Salinity in SW
50.0
Rec
ove
ry (
%O
OIP
)
CS100-5 - SSW*4S
•Effect of sulfate:
•Imbibing fluid: Modified SSW
•Spontaneous imbibition at
100 oC
•Effect of salinity/NaCl
•Spontaneous imbibitionn at
110 oC.
0.0
10.0
20.0
30.0
40.0
0 5 10 15 20 25 30 35 40 45
Time (days)
Rec
ove
ry (
%O
OIP
)
CS100-5 - SSW*4S
CS100-2 - SSW*3S
CS100-4 - SSW*2S
CS100-1 - SSW
CS100-3 - SSW/2S
CS100-6 - SSW/US
6
SO42- can compensate for low Tres
30
40
50
60
70
oil r
ecov
ery
(%O
OIP
)
100°C (Oil A, AN=2.07)
130°C (Oil A, AN=2.07)
0
10
20
30
SSW-US SSW-½S SSW SSW×2S SSW×4S
Imbibing fluids
Maximum oil recovery from chalk cores when different imbibing fluids were
used (SW with varying SO42- conc.). Oil: AN=2.07 mgKOH/g).
7
Example: “Smart Water” in Sandstone
40
50
60
Rec
ove
ry (%
)
Low Salinity
0
10
20
30
0 2 4 6 8 10PV Injection
Rec
ove
ry (%
)
B15-Cycle-2
High SalinityLow Salinity
HS: 100 000 ppm; LS: 750 ppm
8
Outline
• Experimental conditions
• Tertiary Low Salinity effects:
– Diluted FW
– Diluted GSW
• Parameters affecting low salinity effects:• Parameters affecting low salinity effects:
– Presence/dissolution of Anhydrite, CaSO4
– Reservoir temperature
– Composition of low the LS water
• Conclusions
9
Experimental data
• Reservoir core material very homogenous
– Porosity: 0.16 - 0.18
– Permeability: 0.4 – 1.2 mD
– Containing small amounts of Anhydrite– Containing small amounts of Anhydrite
• Crude oils used
– Oil A: Low AN = 0.08
– Oil B: High AN = 0.70
– Oil C: Intermediate AN= 0.34
• Reservoir temperature: 110 oC
10
Brine composition
Ions FW GSW
mole/l mole/l HCO3
- 0.003 0.003 Cl- 3.643 0.662 SO4
2- 0.000 0.032 SO42- 0.000 0.032
Mg2+ 0.076 0.059 Ca2+ 0.437 0.010 Na+ 2.620 0.594 TDS, ppm 208 940 42 220 IS, mole/l 4.158 0.829
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Acid number as wetting parameter
Spontaneous imbibition at of FW at 110 oC, Swi=0.10.
Core 16A: Oil A; Low AN = 0.08
Core 11B: Oil B; High AN=0.70
12
Low salinity EOR-effect with diluted FW
HS: ≈ 208 000 ppm
LS: ≈ 2080 ppm
T=110 oC
About 5% of OOIP low salinity effect.
Differential pressure over the core
13
Simulated dissolution of CaSO4
0,010
0,012
0,014
0,016
, m
ol
FW
100 TIMES DIL. FW
10 times Dil. FW
0,000
0,002
0,004
0,006
0,008
0 20 40 60 80 100 120 140
SO
42
- , m
ol
Temp., ̊ C
14
Diluted GSW as low salinity fluid
Flooding conditions for Core 18C:
•FW and 10× diluted GSW; 4222 ppm
•Temperature: 110°C.
•Oil:B: AN≈ 0.70 mg KOH/g.
•Low salinity effect of 3% of OOIP
The presence of anhydrite confirmed:
• Injecting DI water at 110°C into FW
saturated core. Rate: 0.01 ml/min.
15
Simulated dissolution of CaSO4 in diluted GSW
16
Codition for observing low salinity EOR-effects
in carbonates
• The carbonate rock must contain anhydrite, CaSO4(s)
• Chemical equilibrium:
CaSO4(s) ↔ Ca2+(aq) + SO42-(aq) ↔ Ca2+(ad) + SO4
2-(ad)
• The concentration of SO42-(aq) depends on:4
– Temperature (decreases as T increases)
– Brine salinity and composition (Ca2+ , Cl-)
• Wettability alteration process depends on:
– Temperature (increases as T increases)
– Salinity (increases as NaCl conc. decreases)
• Optimal temperature window
– 90-110 oC ???
17
Great possibilities for low salinity EOR-
effects in carbonates
High concentration of Anhydrite present in the formation.
18
Anhydrite in Oil Reservoirs
• Morrow et al. (SPE-113410) observed tertiary low
salinity effects (3 to 9.5 % of OOIP) in sandstones
with very low clay content, but significant amount of
dolomite and anhydrite.
• Hirasaki et al. (SPE-141420) observed that alkaline
(Na2CO3) surfactant flooding of carbonate reservoirs
containing Anhydrite failed because of precipitation
of CaCO3.
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Conclusions
• Low saline EOR-effects can be observed in mixed wet
carbonates provided the formation contains precipitated
Anhydrite, CaSO4.
• The chemical mechanism is the same as published previously,
i. e. improved water wetness by a symbiotic interaction i. e. improved water wetness by a symbiotic interaction
between, SO42-; Ca2+ and/or Mg2+ at the carbonate surface.
• The low salinity EOR-effect is depending on:
– The reservoir temperature
– The concentration of non- active salt, NaCl.
– The amount of dissolvable anhydrite present in the formation
20
Acknowledgement
• Thanks to BP Sunbury for economical support
and for providing material for the
experimental work. experimental work.
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