HR: 1340h
AN: NG33B-0172    [Abstracts]
TI: Downhole seismic monitoring with Virtual Sources
AU: * Bakulin, A
EM: Andrey.Bakulin@shell.com
AF: Shell E & P, 3737 Bellaire Blvd, Houston, TX 77025 United States
AU: Calvert, R
EM: Rodney.Calvert@shell.com
AF: Shell E & P, 3737 Bellaire Blvd, Houston, TX 77025 United States
AB: Huge quantities of remaining oil and gas reserves are located in very challenging geological environments covered by salt, basalt or other complex overburdens. Conventional surface seismology struggles to deliver images necessary to economically explore them. Even if those reserves are found by drilling successful production critically depends on our ability to ``see" in real time where fluids are drawn from and how pressure changes throughout the reservoirs. For relatively simple overburdens surface time-lapse (4D) seismic monitoring became industry choice for aerial reservoir surveillance. For complex overburdens, 4D seismic does not have enough resolution and repeatability to answer the questions of reservoir engineers. For instance, often reservoir changes are too small to be detected from surface or these changes occur in such pace that all wells will be placed before we can detect them which greatly reduces the economical impact. Two additional challenges are present in real life that further complicate active monitoring: first, near-surface condition do change between the surveys (water level movement, freezing/thawing, tide variations etc) and second, repeating exact same acquisition geometry at the surface is difficult in practice. Both of these things may lead to false 4D response unrelated to reservoir changes. Virtual Source method (VSM) has been recently proposed as a way to eliminate overburden distortions for imaging and monitoring. VSM acknowledges upfront that our data inversion techniques are unable to unravel the details of the complex overburdens to the extent necessary to remove the distortions caused by them. Therefore VSM advocates placing permanent downhole geophones below that most complex overburden while still exciting signals with a surface sources. For instance, first applications include drilling instrumented wells below complicated near-surface, basalt or salt layer. Of course, in an ideal world we would prefer to have both downhole sources and receivers (e.g. in-situ 4D seismic), but for now VSM may be the most economical alternative. By performing data-driven redatuming with measured Green's functions, these data can be recast into complete downhole dataset with buried Virtual Sources located at each downhole geophone. This step can be effectively thought of as a time reversal and it's remarkable feature is that velocity model between sources and receivers is not required to perform it. We will show various applications of the VSM method to several synthetic and real time-lapse datasets to illustrate the following advantages:
1) ability of VSM to eliminate overburden distortions without knowing velocity model between surface sources and downhole receivers,
2) greater quality of Virtual Sources in strongly scattering environment,
3) beneficial downward only radiation pattern on the Virtual Sources,
4) ability to correct non-repeatability caused by slight changes in acquisition geometry and temporal changes in the near surface,
5) ability to create P-wave Virtual Sources without shear radiation and S-sources without P-waves. Versatility of VSM to handle 1D, 2D and 3D situations and its ability to handle overburdens of any complexity makes it an indispensable tool for the active geophysical monitoring in a challenging geological environments. Although examples presented all come from an oilfield, it is straightforward to envision analogous applications in many other fields ranging from global geophysics to monitoring man-made structures.
DE: 0629 Inverse scattering
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0910 Data processing
DE: 0915 Downhole methods
DE: 7200 SEISMOLOGY
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005