Borehole Seismology – The Tool for Reservoir Characterization and Monitoring

Borehole seismology is rapidly advancing in a number of key areas. The first example of new instrumentation for borehole seismic surveys is the 80 level, three-component, clamped borehole seismic receiver array developed by P/GSI. The most recent survey, shot in December 1999 in PanCanadian’s Weyburn Field in Saskatchewan, is a 634 shot 3D VSP recording 152,000 seismic data traces in 60 hours with a top frequency of about 200 Hz. The 3C geophone pod spacing in the array is either 25 or 50 ft, both common geophone spacing for VSP applications. We have demonstrated that data from the array can be used to image the entire drainage volume around the well at much higher resolution than what is possible from a surface seismic survey. The array also makes it possible to monitor the production from a field in real time with high resolution. In order to maximize the use of this new geophysical technique for development and production application the data is processed in the field and an image delivered to the clients within one to two days.

The second example of the new borehole seismic technology is a 3D Reverse VSP (RVSP) survey recorded at the MIT test site in Michigan using P/GSI’s downhole seismic vibrator. The 3D RVSP survey data was recorded using the downhole axial vibrator in one well and 325 geophones placed on the surface distributed around the source well. During the MIT RVSP survey the source was operated at depths between 3,000 ft and 4,500 ft. High quality data with a top frequency of 360 Hz was recorded with direct source-receiver paths of more than 6,000 ft. Good quality reflected data was also recorded with travel paths of 15,000 ft imaging geological features 5,000 ft below the source. 3D images with about four times better resolution compared with surface seismic techniques were generated using the Reverse VSP data. A cement bond log taken before and after the MIT Michigan survey confirmed that the borehole source is non-destructive to the casing - cement interface.

A number of large interwell distance cross well seismic surveys have also been recorded with the new downhole seismic vibrator. We have demonstrated that we can record data with transmission paths up to 15,000 ft and record frequencies from the source up to 1,000 Hz. Data examples as well as images generated by the data from the new downhole source will be shown.

The third new instrumentation technology we will discuss is an all fiberoptic hydrophone array. P/GSI has collaborated with Litton to design and manufacture a 96 channel all fiberoptic hydrophone array for boreholes. This array was tested for the first time in an oil field in October 1999 in Texaco’s Kern River oil field. The maximum OD of the array is 1.2” so it will fit inside tubing as small as 1.5”. The first survey with the all fiberoptic hydrophone array was a cross well survey using the P/GSI downhole vibrator as the borehole source. 500 Hz data was recorded in the unconsolidated formation and the images generated have a resolution better than 5 ft.