Transcript Measuring Effective Wellbore Permeability

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
End of 2004
From IPCC SRCCS, 2005
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
Fixed pressure B.C.
– axi-symmetric coordinates
– transient, single-phase flow
Permeable formations
• 7 permeable layers (10mD),
7 shale caprocks (0.1mD)
– Fixed pressure at top and
outer boundaries
– Impermeable bottom
boundary
Shale layers
• Explore parameter space
– Vary permeability in well
(kw), caprock (k’), and lower
formation (k)
Disturbed zone, kw
Intermediary caprock, k’
z
0.5 m
r
Lower formation, k
rB
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. 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
• 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
Range of Detection
1
dp p 
1
top
bot
3
 
10  

 dkw k  
 kw k

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
upper formation
intermediary caprock
lower formation
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 CO2 leakage.
Thank you!