ABSTRACT

The recently developed SUMIC technique enables the acquisition of high-quality shear wave data at the seabed. The additional information provided by the shear waves makes it easier to infer accurate geological information about the subsurface.

The availability of shear wave data enhances the reliability of Iithology and fluid predictions from seismic data. Furthermore, the shear waves have been successfully used to image reservoirs where gassaturation in the overburden completely distotis any imaging based on conventional seismic data. Thetechnique has also demonstrated potential for improving the overall data and image quality in traditional "bad data" areas.

Sub-sea shear wave technology has demonstrated potential for risk reduction in exploration through moreaccurate prospect definition. Reduced reservoir uncertainty may reduce the need for appraisal wells and thus make it possible to increase the net-present value of new fields by decreasing the time lapse from discovery to production. Finally, the improved reservoir description may facilitate more optimum reservoir management.

INTRODUCTION

While oil and gas from large and geologically simple fields have constituted a substantial part of the world production up to now, new resources are to a greater extent found in small traps in geologically complex areas or in subtle traps that are difficult to identify. Petroleum reserves thus become progressively more difficult to discover and develop and the financial risks increase while the economic margins tend to decrease. The demand is therefore high for technology that can reduce risk through more accurate prospect definition and reservoir description.

The ultimate objective of seismic surveying is to unravel accurate and detailed geological information about the subsurface in order to locate successful exploration and production wells. Conventional seismic techniques are unable to fully meet this objective. A high proportion of dry exploration wells can, for example, be attributed tolimitations in our ability to predict Iithology and pore fluid. Also, the reliability of modern reservoir simulation and production forecasting is limited by inadequate reservoir description.

Conventional seismic surveys use compressional (P) waves. Shear (S) waves provide additional information about the subsurface. P-waves propagate through the subsurface as local expansions and contractions as the medium is pushed or pulled upon. Thus the particle motion of the transmitting medium is in line with the direction of propagation. S-waves, on the other hand, are the result of shear forces acting on the rock leading to particle motion which is perpendicular to the direction of wave propagation.

P-waves react to changes in lithology and are equally sensitive to changes in fluid type filling the pore space in the rock. Shear waves also react to changes in Iithology, but are relatively unaffected by the pore fluid. Therefore, the comparison of P- and S-data helps discriminate between reflections that are Iithology related and those that are fluid related.

It is well known that the Vp/Vs ratio, i.e. the ratio between the P-wave velocity and the S-wave velocity, is a substantially better Iithology indicator than acousticimpedance alone. Figure 1 displays histograms for pure sand.

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