1. John Downs Cabot Specialty Fluids
SPE Observations on Gas Permeability Measurements under HPHT Conditions in Reservoir Core Materials Exposed to Cesium Formate Brine
John Downs
Cabot Specialty Fluids
2009 SPE European Formation Damage Conference

2. Water vapour in natural gas
Natural gas is saturated with water vapour at reservoir conditions
Equilbrium water vapour content of gas:
- Increases with temperature (and acid gas content)
- Decreases with pressure (and salt content of reservoir fluids)
Gas pressure
(psi)
Equilibrium water content of methane*
(ppm)
20oC
75oC
125oC
175oC
200oC
100
3,498
57,198
342,907 -
1,000
465
6,849
39,176
147,234
254,610
5,000
238
2,545
12,293
41,762
61,183
15,000
34,902
* Source: AQUAlibrium 3.1
2009 SPE European Formation Damage Conference

3. Water vapour in HPHT natural gas – Field example (ref
Water vapour in HPHT natural gas – Field example (ref. SPE , Wat et al., 2008)
Kristin field, offshore Norway
- Reservoir temperature 170°C
- Reservoir pressure 13,400 psi
Prior to formation water production: 1 million Sm3 of produced
gas yields 25 m3 of condensed water (i.e. 25 grams/m3 gas)
AQUAlibrium prediction for water content of pure methane under
similar conditions: 18 grams water /m3 gas
Difference: Acid gas content of Kristin gas, and drawdown effect?
2009 SPE European Formation Damage Conference

4. Drawdown pressure gradients in gas wells drive vaporisation of water from the formation
Reduction in pore pressure makes gas into a flowing dessicant in vicinity of the wellbore
Removes water-blocks from tight gas reservoirs
- May take weeks, but should stimulate gas production
Potential for salt precipitation from any fluid residues in gas path?
- Most likely in HPHT gas reservoirs?
- High-salinity connate fluids
- High-salinity brine filtrates if clear completion fluids used
- Likely in tight sandstones/limestones requiring high drawdowns?
See SPE 10779, 13246, 30719, 63161, 84829
2009 SPE European Formation Damage Conference

5. Equilibrium water content of nitrogen gas
Water vapour content of nitrogen gas used in laboratory core flooding test
Water content of nitrogen gas:
- Increases with temperature
- Decreases with pressure
Risk of dehydrating the fluid in the core* if gas not fully humidified
at test conditions, or if large drawdown pressure applied?
Gas pressure
(psi)
Equilibrium water content of nitrogen gas
(ppm)
20oC
75oC
125oC
175oC
200oC
100
3,479
56,997
342,429 -
1,000
432
6,543
37,914
143,983
250,102
4,000
185
2,324
12,206
54,984
74,362
8,000
25,799
43,125
* See Zuluaga and Monsalve, 2003 – SPE 84829
2009 SPE European Formation Damage Conference

6. Consequences of fluid dessication by gas in laboratory core flooding tests
Possible reduction in permeability to gas as a result of fluid immobilisation (viscosity increase) or crystallisation?
Viscosity of a completion brine at 25°C
Brine viscosity rises sharply
as it is dehydrated to < 50 %
v/v water content
Possibility of immobilisation
of dehydrated viscous brine
in smaller pores?
2009 SPE European Formation Damage Conference

7. Factors that might increase risk of perm impairment of core by fluid dessication
Gas not fully saturated with water at test T/P conditions
High-salinity connate water and high-density brine filtrates*
Large pressure drops across core plugs during clean-up phase
Throughput of > 1,000 pore volumes of gas during clean-up
phase prior to measuring return perm
* High-density brines may contain < 50% v/v water, meaning that water volume in a brine-saturated core plug
is already < 1 ml before any dessication processes get to work...
2009 SPE European Formation Damage Conference

8. Does fluid dessication ever happen in laboratory core flooding tests with gas?
High-drawdown HPHT tests with cesium formate brine on North Sea field core samples
Flood: 25 PV of brine followed by 96-hour soak period
Drawdown: > 1,000 PV of nitrogen humidified at room temperature
Permeabilities measured under HPHT test conditions
Cryogenic SEM showed some evidence of filtrate retention
Lower perm core with higher drawdown showed greatest reduction in return permeability
Brine
density
(s.g.)
Test
temperature
(oC)
Initial
permeability
(mD)
Drawdown
pressure
(psi)
Final permeability
Reduction in
(%)
2.30
190
25.8
2,000
23.3 10
1.98
178
0.35
2,500
0.22 37
2009 SPE European Formation Damage Conference

9. Does fluid dessication ever occur and cause impairment in HPHT gas wells?
No reported formation damage from > 200 applications of cesium
formate brine in HPHT wells over past ten years
PIs generally exceed expectations after cesium formate brine use:
- “Well performance was above expectation with initial rates of 33 MMscf/d and
12,000 31% choke” (SPE )
- “Use of cesium/potassium formate brine… has resulted in highly productive
gas wells with low skin” (SPE )
- “The well is flowing significantly above expectation… The expected production rate
was 40–50 MM scf/day but the well is actually flowing at 79 MM scf/day” (SPE 97694)
2009 SPE European Formation Damage Conference

10. Could gas humidification levels be influencing results of HPHT core flooding tests with cesium formate brine?
2009 SPE European Formation Damage Conference

11. HPHT laboratory core flooding test to determine effect of gas humidification on return permeability
Key features of methodology:
HPHT reservoir core sample – gentle clean out with solvents
Saturated with reservoir water and centrifuged to irreducible
Measure permeability to gas under HPHT conditions
Forward flow of test brine, followed by soak period
Realistic drawdown build-up, simulating production start-up
Flow large volume of gas under drawdown to achieve clean-up
Measure permeability under HPHT conditions with humidified gas
- HPHT humidified gas
- LTHP humidified gas
Complete SEM on core samples to identify source of any damage
2009 SPE European Formation Damage Conference

12. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Test conditions
- 200°C
- 5,800 psi pore pressure
- North Sea reservoir core flooded with reservoir water and then
centrifuged to irreducible saturation
Programme
- Measure initial permeability to gas at Swi under HPHT conditions
- 10 PV flush with 2.2 s.g. cesium formate brine at 1 ml/minute
- Soak for 48 hours at balance
- Drawdown ramped up in stages to 100 psi (5,700 psi in wellbore)
using > 1,000 PV of humidified gas
- Measure return permeability to gas under HPHT conditions
- Examine core (dry/cryo SEM) for any signs of damage
2009 SPE European Formation Damage Conference

13. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Core source*, dimensions and properties
Core sample
Coring
depth
(m)
Length
(cm)
Volume
(cc)
Pore volume
Porosity
(%)
Grain density
(g/cc)
Gas permeability
(mD) 2B
6,242.39
4.77
22.176
2.902
13.1
2.61
3.73 3B
6,242.42
4.732
23.866
3.518
14.7
2.64
4.95
* Core from major HPHT field in UK North Sea where cesium formate brine has been used as the
completion/workover fluid since 1999
2009 SPE European Formation Damage Conference

14. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Core face under SEM – before exposure to Cs formate brine
Coarse silt and fine-grained sand, with moderately abundant
grain-coating and pore-filling illite clay. Also grain-coating
Quartz and pore-filling dolomite
2009 SPE European Formation Damage Conference

15. Ionic composition of the reservoir water
Ion concentration (mg/l) Na K Ca Mg Ba Fe Cl
HCO3
31,190
300
2,300
350
1,000 10
53,500
610
2009 SPE European Formation Damage Conference

16. Effect of gas humidification on HPHT core flood test results with cesium formate brine
24-carat gold film wrapped around circumference of core to create
a barrier to gas diffusion/leakage under hydrothermal conditions
Encased with layers of PTFE tape, heat-shrink tubing and
an outer Kalrez sleeve before mounting in core holder
2009 SPE European Formation Damage Conference

17. HPHT humidifier for gas used in core flooding
Dry nitrogen gas enters base of
humidifier, passes through column
filled with high surface area spheres
saturated with water, and exits from top
Pressure vessel mounted vertically in
oven at test temperature/pressure
Materials all in Hastelloy C-276
2009 SPE European Formation Damage Conference

18. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Pressure development across core during injection of 10 PV of
cesium formate 1ml/min (frontal advance rate of 80 cm/hour)
Pressure stabilised after approx. 1.7 PV ( 5 minutes=5 ml)
2009 SPE European Formation Damage Conference

19. Cumulative gas throughput
Effect of gas humidification on HPHT core flood test results with cesium formate brine
Drawdown pressure ramping, gas volume throughput and stabilised flow rate – gas humidified at HPHT
Drawdown pressure
(psi)
Cumulative gas throughput
(ml)
Cumulative gas
throughput
(PV)
Stabilised flow rate
(ml/min) 1 25
8.61
0.3 5 50
17.2
0.44 10
250
86.1
0.95
1,000
345
5.0
2,000
689
15.0 75
2,750
948
20.0
100
4,000
1,378
26.0
2009 SPE European Formation Damage Conference

20. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Gas flow rates and cumulative throughput during the drawdown sequence – gas humidified at HPHT
1,378 PV of gas pulled through core in 650 minutes – 400 PV
at high drawdown
2009 SPE European Formation Damage Conference

21. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Gas flow rates and cumulative throughput during the drawdown sequence – gas humidified at HPLT
Shorter, more aggressive drawdown: 1,137 PV of gas pulled
through core in 146 minutes – 1,000 PV at high drawdown
2009 SPE European Formation Damage Conference

22. Change in permeability
Effect of gas humidification on HPHT core flood test results with cesium formate brine
Humidification at room temperature resulted in permeability
impairment
Gas humidification
system
Test
temperature
(oC)
Initial
permeability
(mD)
Final permeability
Change in permeability
(%)
LT/HP
humidifier
200
2.36
2.01
-14.8
HT/HP
1.62
1.66
+2.47
2009 SPE European Formation Damage Conference

23. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Appearance of wellbore core face under SEM – after exposure to Cs formate brine and gas drawdown
Both cores showed some evidence of cleaner pore throats –
reduction in size and amount of illite clay and dolomite particles.
No unusual fluid retention
2009 SPE European Formation Damage Conference

24. Effect of gas humidification on HPHT core flood test results with cesium formate brine
Appearance of formation core face under SEM – after exposure to Cs formate brine and gas drawdown
Both cores showed evidence of some reduction in the number
of clear pore throats – indications of illite clay and dolomite
fines being re-injected into core
2009 SPE European Formation Damage Conference

25. Effect of gas humidification levels on HPHT core flood test results with cesium formate
Conclusions
Full HPHT humidification resulted in no reduction in return perm
Room temperature humidification gave 15% reduction in return perm
- No obvious fluid/crystal retention to explain reduction
- Some clay fines movement in both experiments
- Shorter, more intense drawdown period in LTHP experiment
Cannot categorically say that lower humidification level was source of
perm reduction, but prudent to use HPHT humidifier in future
Perm reduction effect by dessication cores
2009 SPE European Formation Damage Conference

26. Effect of gas humidification levels on HPHT core flood test results with cesium formate
Acknowledgements
I would like to acknowledge and thank Ian Patey, Murdo Munro
and the laboratory staff of Corex who planned, managed and
executed the experimental programme described in this paper
2009 SPE European Formation Damage Conference