A Reservoir Simulation Model for Ground Freezing Process

1. A Reservoir Simulation Model for Ground Freezing Process SPE Annual Technical Conference and Exhibition Florence, Italy 19–22 September 2010 Chonghui Shen, Bill Mckinzie , and Sepehr Arbabi

2. Outline • Green River Oil Shale Resources • Shell’s In-situ Conversion Process (ICP) • Ground freezing for containment • Simulation of ground freezing • Summary

3. Green River Oil Shale (From “World Oil”, August 2005) • The Green River Formation in Colorado, Utah and Wyoming, USA • 500 to 1,100 billion barrels of recoverable oil using a cut-off of 15 gallons per ton (gpt) (Bartis, 2005). • The Piceance Basin in Colorado • Largest deposit of three • Up to 1000 ft thick • Up to 2.5 mln bbl/acre

4. In-situ Conversion Process (ICP) • Direct formation heating via conduction • Tight spaced heating holes and slow heating • Uniform heating to ~650 °F • High quality product generated from kerogen pyrolysis • 25 to 40 °API oil • High liquid recovery efficiency (>60% FA) • 1/3 of energy in produced gas

5. Piceance Basin Regional Groundwater Flow • Fractured subsurface formations • Water flows from edge of basin to the creek drainage Piceance Basin – Cross Section Outcrop

6. Ground Freezing Process - Freeze Wall (FW) • Multiple water-bearing or permeable zones in target resources • Freeze ground to form a flow barrier (freeze wall) • Isolation of developing area from undeveloped • Production contained within a freeze wall barrier • Post-heating steam remediation removes residual products Outcrop

7. Shell’s ICP and FW Pilots • Seven in the past 29 years • Summary in SPE 121164 • By Fowler and Vinegar, 2009 • Test depth progressed from 20 ft to 1700 ft

8. Test period: 2002 - 2004 Reservoir consists of shale with fractured shale aquifers 18 Vertical freeze holes 50 feet diameter circle 1400 feet depth Monitor holes, heaters and producer Testing after freeze wall completion Seal was verified Mahogany Isolation Test (MIT) for Water Control

9. Vf oil gas water Vv V (fixed) “solids” rock Simulation of Ground Freezing Process • STARS Ice Model • To simulate water freezing (pore filling) and ice thawing processes • Based on pure water (only) • Water freeze reduces fluid porosity • Desired, but numerically troublesome • Numerical singularity with vanishing fluid porosity • Permeability decreases with fluid porosity • Used one of the permeability-porosity models • Void vs. Fluid Porosity

10. Freeze-hole Model Freeze Hole Cross Section • Coolant circulation through tubing and annulus to chill the formation • Used discretized wellbore (DW) for circulation modeling • Hybrid refined ring around DW allowing water/ice or air filled annulus

11. Calibration of Freeze-hole Model • Used MIT coolant injection rate and inlet temperature as control

12. Face the FW Simulation Challenges (1) • Numerical singularity • A “rubbery” fluid approach to circumvent near-zero porosity and to improve run performance • Introduce a “solid” like pseudo fluid (oleic) component • Repartition some of the “rock” volume to the pseudo oleic component • Same property values as rock • High enough viscosity to minimize movement • Improved simulation performance with the "rubbery" fluid model. • Runs at least one order of magnitude faster

13. Face the FW Simulation Challenges (2) • Freezing point depression (FPD) • The freezing point of a liquid decreases with increasing concentration of solute. • The freezing point of seawater is about 28.4°F (-2°C). • A work-around approach • Shift the initial temperature of bulk volume upwards by the amount of FPD • To ensure right amount of sensible heat to be removed before freezing (from Wikipedia)

14. Face the FW Simulation Challenges (3) • Variation of inter-hole distance • FW closure time strongly depends on hole spacing • Important to capture hole deviation • STARS hybrid refinement allows only in one column/row • “WellTraGrid” (Syihab, 2006) • A in-house developed grid generation tool • Deform grid to follow hole trajectories • Example of hole trajectories and a deformed grid ( Dimensions in feet )

15. Differential Cooling with Depth • Non-uniform growth of frozen zones • Function of formation temperature (gradient) - 1st order • Function of formation thermal properties – 2nd order

16. Simulated Freeze Zone and Water Flow Shut-off • Effects of water flow • Causes the deformation of frozen zone around a freeze-pipe • Can significantly delay or, in extreme cases, prevent the formation of a freeze wall • Example of simulated water flow shut-off in three zones

17. Assessing Sensitivity of Design Parameters

18. Summary • Developed a reservoir simulation model for ground freezing process • The details of freeze-hole configuration realized with a discretized well inside hybrid-refined cells • Some unique challenges encountered and dealt with • A “rubbery fluid model” for diminishing porosity • A temperature shift for freezing-point depression • A deformed grid for freeze-hole trajectory

19. Acknowledgement • All the pioneers in Shell who dedicated their works to the development of In-situ Conversion Process and the freeze wall technology

20. Main References • Fowler, T.D. and Vinegar, H.J. “Oil Shale ICP - Colorado Field Pilots”, SPE 121164, SPE Western Regional Meeting, San Jose, California, 24–26 March 2009 • Shen, C. “Reservoir Simulation Study of An In-situ Conversion Pilot of Green River Oil Shale”, SPE 123142, SPE Rocky Mountain Petroleum Technology Conference held in Denver, Colorado, USA, 14–16 April 2009 • Shen, C., Mckinzie, B.L. and Arbabi, S. “A Reservoir Simulation Model for Ground Freezing Process”, SPE 132418, SPE Annual Technical Conference and Exhibition to be held in Florence, Italy, 19–22 September 2010