John Downs Cabot Specialty Fluids

1. SPE 121649 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) * Source: AQUAlibrium 3.1 2009 SPE European Formation Damage Conference

3. Water vapour in HPHT natural gas – Field example (ref. SPE 114079, 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. 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? * 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 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 bopd @ 31% choke” (SPE 103244) • - “Use of cesium/potassium formate brine… has resulted in highly productive • gas wells with low skin” (SPE 105733) • - “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 from major HPHT field in UK North Sea where cesium formate brine has been used as the completion/workover fluid since 1999 Core source*, dimensions and properties 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 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 brine @ 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. 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 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. Effect of gas humidification on HPHT core flood test results with cesium formate brine Humidification at room temperature resulted in permeability impairment 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