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SUMMARY:Passive seismic monitoring\, seismic anisotropy\, and the state-of
 -stress\nin a reservoir - Mike Kendall (Bristol University)
DTSTART:20061122T163000Z
DTEND:20061122T173000Z
UID:TALK5492@talks.cam.ac.uk
CONTACT:4047
DESCRIPTION:The stress regime of the shallow crust controls the migration 
 of  fluids and fault propagation\, subjects of interest to both industry 
  and academia. Earthquakes are a manifestation of stress release and can 
 be \nused to study the stress field. A relatively new technology in  the 
 oil industry is the passive seismic monitoring of microseismic  events\; 
 a typical downhole array of sensors will record many events  of magnitude
 s \nless than Mb-1 per day. Here we present results from oil reservoirs in
  the North Sea and Middle East. In the former\,  events were recorded usi
 ng sensors in a single borhole\, and located  using P- and S-wave travel 
 time delays and the P-wave particle  motion. A noise-weighted array-based
  analytic principle-component- analysis method was developed \nto estimate
  particle motion. The resulting event locations cluster on two neighboring
  faults. The  events were also used to measure shear-wave \nsplitting\, a
 nd hence  infer anisotropy\, in the overburden. Detailed modelling was us
 ed to  constrain the cause of anisotropy to vertically aligned fractures 
  superimposed on a more intrinsic anisotropy due to phylosilicate  align
 ment. An exciting observation is a temporal variation in the  shear-wave 
 splitting\, which is attributed to stress transfer between  the two adjac
 ent faults. A dataset from an Omani field was acquired  with 8 sensors in
  5 wells thus providing accurate source locations  and an image of detail
 ed spatial variations in anisotropy. We have also been able to put constra
 ints on fracture/crack size using analysis of frequency dependent anisotro
 py. Cumulatively\, these  results demonstrate how seismic anisotropy can 
 be used to infer detailed changes in stress and lithology\, valuable piece
 s of  information in the management of an oil field.
LOCATION:Marine/Wolfson Building lecture hall
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