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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 * 1

2 Challenges in microseismicity
Anisotropy and shear wave splitting Focal mechanism Fault/fracture identification With accurate earthquake locations we can start to pinpoint the origins of deformation: is the reservoir or the cap rock – implications for carbon sequestration! Using the radiation pattern from these events can help characterize the deformation – compression or extension. Helps understand the type of deformation. Then understand seismicity as a function of time: is there a link with production? Do the events repeat? Finally we can use the earthquakes to “image the reservoir”: seismic tomography. Also deformation results in fabrics, which can be measured using shear-wave splitting. Importance of microseismic events to further analysis of stress in reservoir Identification of fractures and fluid flow direction Multiplets may be used to understand the rupture process along a fault Multiplets may be used to improve event locations through removal of picking uncertainities SWS used to image fractures and their orientations Repeating earthquakes 2

3 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 Located in Central Graben of the Norwegian North Sea Field discovered in 1969 and went into production in 1972 One of the earliest giant fields which made North Sea a major oil producing province 3

4 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 Ekofisk microseismic experiment conducted between 13th to 30th of April 1997 Instruments deployed within the Ekofisk reservoir 6, 3C sensors deployed within a vertical borehole spaced 20m apart 4490 events triggered during the 18 days - approx events triggered/day 4

5 Signal characteristics

6 Event evolution with time

7 Event evolution with time

8 Velocity model construction
8

9 Arrival time picking and polarisations
Velocity (µm) 9

10 Polarisation analysis – refining position and azimuth
Can use the dip information extracted during polarisation analysis Dip usually not used in event location due to dependency on velocity model used Triangulate dip back and get a binary answer Do this for all station pairs and calculate mean to decide if event should be rotated Need more than 1 pair of dip measurements for dip analysis to work Jones et al. in press 10

11 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 11

12 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?

13 EDT tolerance selection

14 Arrival Time Monte Carlo Test
S-P All pairs EDT

15 Velocity Model Monte Carlo Test
S-P All pairs EDT

16 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

17 Hypocentre Locations

18 Multiplet Identification

19 Arrival time re-picking
After Before

20 Multiplet polarisation analysis
Modified polarisation analysis of de Meersman et al. 2006

21

22 Location of 5 largest multiplet clusters identified with cluster analysis

23 Cluster 1

24 Cluster 2

25 Cluster 3

26 Cluster 4

27 Cluster 5

28 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

29 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


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