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
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
Example Numerical Results
Transient data
Steady-state data
10
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 10-2 MPa, ?pbot 10 MPa

12
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
Modified Test Results
16
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!