The importance of being able to characterise fractured reservoirs in oil field development is widely accepted. This paper discusses two recently developed borehole seismic techniques which have proved to be useful in achieving this. In particular, case histories gathered from recent experience illustrate how these well seismic techniques can help to determine main fracture orientations and densities. One technique involves monitoring microseismic activity taking place in active fractures at the reservoir, the other an analysis of the birefringence effects that apply to shear wave energy when passing through the fracture medium.

Microseismicity occurs in association with geological stress changes. In the case of hydrocarbon reservoirs it can arise naturally from straightforward production, as the result of injection, or hydraulic fracturing operations. In two major surveys in the North Sea the SST500 VSP tool has been used by CGG to successfully record microseismic activity.

In late 1997, CGG participated in a major Reservoir Characterisation Project (RCP VII) designed to monitor the effects of prolonged CO2 injection over the Vacuum field, New Mexico. Seismic data was acquired using conventional surface seismic techniques together with downhole-recorded data, simultaneously recorded using an SST500 tool deployed in a well within the RCP area. One Pwave plus two orthogonal Swave sources shooting into a downhole array comprising of 12 3C receivers gives rise to a 9C/3D dataset. This dataset was processed to provide information on the character of reservoir fracturing through the analysis of shear wave

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