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Ekofisk Revisited
G. A. Jones 1, D. G. Raymer 2, K. Chambers1 and J-M. Kendall 1
1. University of Bristol; 2. Schlumberger Cambridge Research
• Reservoir monitoring seismic experiment• Hypocentre determination using grid search methods• Monte Carlo hypocentre error analysis• Multiplet relocation• Fault reactivation and production induced deformation
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Challenges in microseismicityAnisotropy and shear
wave splitting
Focal mechanism
Fault/fracture identification
Repeating earthquakes
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The Ekofisk reservoir
• Located in the central Graben of the Norwegian North Sea
• Field discovered in 1969 and was the first economically viable chalk reservoir
• Sea-floor subsidence ~30cm/year
• The challenge: to monitor subsidence, compaction and its effects on reservoir permeability
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The Ekofisk microseismic experiment
• One of the 1st microseismic monitoring experiments experiments in oil industry
• Vertical downhole geophone array of 6, 3 component receivers spaced 20 meters
• Geophones located in producing part of reservoir
• 4490 events triggered over the 18 day experiment in April 1997
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Signal characteristics
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Event evolution with time
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Event evolution with time
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Velocity model construction
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Arrival time picking and polarisationsV
eloc
ity (
µm
)
P S
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Polarisation analysis – refining position and azimuth
Jones et al. in press
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• Use the array-velocity model symmetry to reduce problem from 3D to 2D
• Simplification of the problem allows for a dense grid search procedure to be implemented
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Which hypocentre method?
• Which minimisation function to use?– P- and S-times
individually? – Differential S-P?– All possible combinations
of differential arrival times?
• Or use EDT surfaces?
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EDT tolerance selection
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Arrival Time Monte Carlo TestS-P
All pairs
EDT
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Velocity Model Monte Carlo TestS-P
All pairs
EDT
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Summary of Monte Carlo Analysis
∆rtt(m) ∆ztt(m) ∆rvel(m) ∆zvel(m)
S-P 0.13 ± 9.6 -1.5 ± 17.5 1.5 ± 13.5 0.7 ± 29.6
All pairs 0.04 ± 1.3 -0.04 ± 2.7 2.0 ± 5.2 1.8 ± 6.7
EDT -0.06 ± 14.6 -0.4 ± 7.8 0.4 ± 22.5 1.3 ± 12.0
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Hypocentre Locations
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Multiplet Identification
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Arrival time re-pickingBefore After
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Multiplet polarisation analysis
• Modified polarisation analysis of de Meersman et al. 2006
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Location of 5 largest multiplet clusters identified with cluster analysis
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Cluster 1
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Cluster 2
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Cluster 3
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Cluster 4
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Cluster 5
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Results• Different mechanism of
failure seen based on waveform characteristics and location.
• Mechanisms include stress triggering - cluster 2, pore pressure diffusion cluster 4, and fault re-activation - clusters 1,3 and 5.
• Clusters dip away from monitoring well
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Conclusions
• Use of all available arrival time pairs result in most robust hypocentres at Ekofisk
• Errors in velocity model x2 those of arrival times
• Numerous possible mechanisms of microseismic activity present at Ekofisk:– Fault re-activation– Pore pressure diffusion– Stress triggering – Production induced activity around wells