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