A permanent borehole seismic setup may typically consist of one (or more) of the following configurations:
1) Downhole sensor array and surface (marine) seismic source (vertical seismic profiling, VSP).
2) Downhole sensors continuously monitoring microseismic events.
3) Borehole source with receivers in neighboring wells for cross hole tomography.
4) Borehole source and receiver array in the same well for inhole reflection seismicity.
Illustration of a borehole seismic system showing inhole seismics and permanent sensors for VSP.
By bringing sensors and source into the borehole, close to the reservoir, the resolution of the seismic monitoring can be improved from tenths of meters as typical for surface seismics, to the one-meter range, using a borehole seismic source in the kHz frequency range5. Borehole seismic receivers will typically be multi-component, clamped geophones, to resolve both compressional (P-) and shear (S-) waves. Borehole seismic receivers for permanent installation are available5. The integration of such receivers together with a downhole source in an intelligent completion system can bring excellent repeatability and thus an optimal basis for trend analysis. Different borehole sources also exist5, but development of a reliable source for permanent installation, integrated in the well completion is still a challenge.
Another challenge is to define the most appropriate applications for permanent well seismic systems, to understand the seismic responses, and to predict system performances.