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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
Challenges in microseismicityAnisotropy and shear
wave splitting
Focal mechanism
Fault/fracture identification
Repeating earthquakes
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
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
Signal characteristics
Event evolution with time
Event evolution with time
Velocity model construction
Arrival time picking and polarisationsV
eloc
ity (
µm
)
P S
Polarisation analysis – refining position and azimuth
Jones et al. in press
• 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
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?
EDT tolerance selection
Arrival Time Monte Carlo TestS-P
All pairs
EDT
Velocity Model Monte Carlo TestS-P
All pairs
EDT
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
Hypocentre Locations
Multiplet Identification
Arrival time re-pickingBefore After
Multiplet polarisation analysis
• Modified polarisation analysis of de Meersman et al. 2006
Location of 5 largest multiplet clusters identified with cluster analysis
Cluster 1
Cluster 2
Cluster 3
Cluster 4
Cluster 5
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
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