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

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

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.

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

Leakage Pathways in Wells

Modeling CO 2 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 k well in well segments

Experimental Design Disturbed zone, k w

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 (k w ), caprock (k’), and lower formation (k) Disturbed zone, k w r z rBrB Shale layers Permeable formations Fixed pressure B.C. Lower formation, k Intermediary caprock, k’ 0.5 m

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

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

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 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 ∆p top = ±10 -2 MPa, ∆p bot ≤ 10 MPa

Estimation of Sensitivity Limits Error in field data –∆p top /∆p bot = ±10 -3 Viable range of values –minimum pressure that can be measured reliably Insensitive response regions –Slope of curve is flat –Small error in ∆p top translates to large uncertainty in k w Viable range of values

Range of Detection range of detection

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

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

Improved Range of Detection range of detection

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 CO 2 leakage.

Thank you!