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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 - PowerPoint PPT Presentation
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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!