seawater as a 'smart' ior-fluid in chalk
TRANSCRIPT
Seawater as a “smart” IOR-fluid in chalk: A surface
chemical phenomenaTor Austad
University of Stavanger, 4036 Stavanger, Norway([email protected])
Question:
• Why is injection of seawater such a tremendous success in Ekofisk??– Highly fractured– Low matrix permeability, 1-2 mD– High porosity, 35-45%– Wettability
• Tor-formation: Preferential water-wet• Lower Ekofisk: Low water-wetness• Upper Ekofisk: Neutral to oil-wet
Coccolith chalk
Normal compaction prevented by:• Early invasion of hydrocarbons• High pore pressure• Low initial water saturation, Swi = 5-7%
Oil recovery prognoses
0
400
1972
1976
1980
1984
1988
1992
1996
2000
2004
2008
2012
2016
2020
2024
2028
OIL
RA
TE, M
STB
D (
GR
OSS
) NPD;2002: 50%2007: Goal 55 %
Brine compositionComp. Ekofisk Seawater
(mole/l) (mole/l)Na+ 0.685 0.450K+ 0 0.010Mg2+ 0.025 0.045Ca2+ 0.231 0.013Cl- 1.197 0.528HCO3
- 0 0.002SO4
2- 0 0.024
Seawater: [SO42-]~2 [Ca2+] and [Mg2+]~ 2 [SO4
2-]
[Mg2+]~4 [Ca2+]
Affinity of Ca2+ and Mg2+ towardschalk
0,00
0,25
0,50
0,75
1,00
0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6PV
C/C
o
C/Co SCN (Brine with Mg and Ca2+) at 23C[Magnesium] A=0,084
C/Co Mg2+ (Brine with Mg2+ and Ca2+) at 23°C
C/Co Ca2+ (Brine with Mg2+ and Ca2+) at 23°C
C/Co SCN (Brine with Mg and Ca2+) at 23C[Calsium] A=0,31
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0PV
C/C
o
C/Co SCN (Brine with Mg and Ca2+)at 130°C
C/Co Mg2+ (Brine with Mg2+ andCa2+) at 130°C
C/Co Ca2+ (Brine with Mg2+ andCa2+) at 130°C
NaCl-brine, [Ca2+]= [Mg2+]= 0.013 mole/l, SCN- as tracer
20 oC 130 oC
Wettability alteration by potential determining ions Ca2+, Mg2+, SO4
2-
Imbibition at 70 & 100oC (with/without Ca & Mg)
0
20
40
60
0 20 40 60 80 100 120Time, days
Rec
over
y, %
OIIP
25:SWx0CaMg(+Mg@43days)26:SWx0Sx0CaMg(+Mg@ 53 days)27:SWx2Sx0CaMg(+Ca@43 days)28:SWx4Sx0CaMg(+Mg@53 days)
70°C
100°C 130°C
Results hydrostatic tests with Stevns Klintchalk at 130°C with constant flooding
0
2
4
6
8
10
12
0.0 0.5 1.0 1.5 2.0 2.5
Axial strain [%]
Axi
al s
tress
[MPa
]
SSW
SSW
SSW2
SSW2
SSW-U
SSW-U
SSW-U2
SSW-U2
DW
DW
25% reduction in mechanical strength
Average yield point with sulfate ≈ 6.5 MPaAverage yield point without sulfate ≈ 8.5 MPa
Difference in strain by
a factor of 2.5
Creep phase with constant flooding at 130°C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.0 5.0 10.0 15.0 20.0 25.0
Pore volume
Axi
al c
reep
stra
in [%
]
SSW
SSW
SSW2
SSW2 without flooding
SSW2
SSW2 without flooding
DW
DW
SSW-U2
SSW-U2
SSW-U
SSW-U
Flooding of distilled water
At 9 PV: Difference in strain by a factor of 2.7Total: A factor of about 5
Suggested mechanism for enhanced water weakening of chalk by seawater
Enhanced water weakening of chalk when Mg2+ substitutes Ca2+ at inter granular contacts in the presence of SO4
2-
Conclusion
• Seawater contains the important ions, SO4
2-, Ca2+, Mg2+, to promote wettabilitymodification in chalk which increases oil recovery.
• Compaction as drive mechanism is increased by injecting seawater into high temperature chalk reservoirs.