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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