Seawater as a “smart” IOR-fluid in chalk: A surface

chemical phenomenaTor Austad

University of Stavanger, 4036 Stavanger, Norway(tor.austad@uis.no)

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

Suggested wettability mechanism

Seabed subsidence

1973 1984

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.

Acknowledgements

• ConocoPhillips (Ekfisk group)• BP (Valhall group)• Norwegian Research Council (NFR)