Measuring Effective Wellbore Permeability

1. Measuring Effective Wellbore Permeability Sarah Gasda, Princeton University Michael Celia, Princeton University Jan Nordbotten, Univ. of Bergen

2. Objective • Propose a simple field test to determine effective (bulk) wellbore permeability • Use numerical analysis to determine the feasibility of this test • Define the range of detection given constraints on instrument accuracy

3. Approach • We design a test to determine well permeability. • If we can estimate permeability values for the formation and caprock, we can find well permeability from pressure response. • We do this by using simulations to generate response curves that relate pressure response to well permeability.

4. Existing Oil and Gas Wells End of 2004 From IPCC SRCCS, 2005

5. Leakage Pathways in Wells

6. Modeling CO2 leakage • Large spatial and temporal scales • Multiple leaky wells • probabilistic framework • No data exist on wells • Need to pin down statistical distributions • Need a simple test to identify kwell in well segments

7. Disturbed zone, kw Experimental Design

8. Numerical Experiments • Standard finite-difference simulator • axi-symmetric coordinates • transient, single-phase flow • 7 permeable layers (10mD), 7 shale caprocks (0.1mD) • Fixed pressure at top and outer boundaries • Impermeable bottom boundary • Explore parameter space • Vary permeability in well (kw), caprock (k’), and lower formation (k) Fixed pressure B.C. Permeable formations Shale layers Disturbed zone, kw Intermediary caprock, k’ z Lower formation, k 0.5 m rB r

9. range of detection Example Numerical Results Transient data Steady-state data

10. k=10-2 D k= 1 D Dimensionless Results

11. Limits on Field Measurements • Instrument measurement accuracy • Pressure transducers rated for high P,T • ±0.1 bar (Schlumberger, UNIGAGE Quartz) • Fracture pressure • Minimum horizontal fracture stress ~17 kPa/m • Bachu et al. 2005. Underground Injection Sci. & Tech. • Maximum pressure change must be less than fracture pressure minus initial pressure • Average hydrostatic gradient ~11kPa/m • Order-of magnitude sensitivity limits • Error in ∆ptop = ±10-2MPa, ∆pbot ≤ 10 MPa

12. Viable range of values Estimation of Sensitivity Limits • Error in field data • ∆ptop/∆pbot = ±10-3 • Viable range of values • minimum pressure that can be measured reliably • Insensitive response regions • Slope of curve is flat • Small error in ∆ptop translates to large uncertainty in kw

13. Range of Detection range of detection

14. Alternative Test Design • Purpose • Reduce influence of lower formation permeability on pressure response • Expand range of detection • Move perforations to location within intermediary caprock • Repeat numerical experiments

15. k=10-2 D k= 1 D Modified Test Results

16. range of detection Improved Range of Detection

17. Conclusion • There is a lack of meaningful data available for well properties. • A simple downhole pressure test can identify effective well permeability values that are in the critical range of values. • Field experiments are needed to reduce the uncertainty associated with current estimates of CO2 leakage.

18. Thank you!