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, 2005End of 2004End of 2004

Leakage Pathways in Wells

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

Experimental Design

Disturbed zone, kw

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)

Disturbed zone, kw

r

z

rB

Shale layers

Permeable formations

Fixed pressure B.C.

Lower formation, k

Intermediary caprock, k’

0.5 m

Example Numerical Results

range ofdetection

Transient data

Steady-state data

Dimensionless Results

k=10-2 Dk= 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. 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 = ±10-2 MPa, ∆pbot ≤ 10 MPa

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

Viable range of values

Range of Detection

range ofdetection

10 3 d ptop pbot

d kw k

1

1

kw k

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

intermediary caprock

lower formation

upper formation

Modified Test Results

k=10-2 Dk= 1 D

Improved Range of Detection

range ofdetection

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.

Thank you!