This thesis considers anisotropy of seismic wave propagation at two oil fields in the North Caucasus region of Russia. In both oil fields, the reservoir zone displays a strong lateral variation in productivity which is thought to be caused by variations in fracture intensity. Such fractures may cause azimuthal anisotropy which can be detected in Vertical Seismic Profiles (VSPs) and Walkaway VSPs. The main aim of the thesis is to characterise this azimuthal anisotropy at three of the oil wells in these fields and to compare this anisotropy with productivity. At each of the three wells, I determine azimuthal anisotropy from VSPs by the application of techniques for estimating shear-wave splitting. I find that the polarisation direction of the fast shear-wave at all three wells is aligned approximately NNE-SSW. At two of the wells, forward modelling shows that the shear-wave splitting parameters in the top 1km can be closely matched by a model containing aligned, vertical fractures, striking NNE-SSW, in approximately the top 1 km. I am unable to resolve the anisotropy of the reservoir zone at these two wells. At the third well, strong azimuthal anisotropy of the reservoir zone is indicated by a large decrease of time delay between shear-waves propagating along vertical raypaths. This decrease is interpreted as an orthogonal rotation of the fast shear-wave polarisation direction at a depth just above the reservoir zone. Using forward modelling, I successfully match these observations with three different fractured reservoir models: the first model contains vertical fractures striking orthogonal to the presumed maximum horizontal stress direction; the second model has dipping fractures striking parallel to the maximum horizontal stress direction; and the third model has a distribution of fractures with a high internal pore-fluid pressure. Consideration of only vertical raypaths through the reservoir cannot discriminate between these models. However, modelling of non-vertical propagation from far-offset VSPs suggests that the dipping fracture model is the better model, although the lack of observations above the reservoir at this well means that other interpretations cannot be excluded.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:662023 |
| Date | January 1997 |
| Creators | Slater, Colin P. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11395 |
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