High Resolution 3D Seismic Imaging Using 3C Data from Large Borehole Seismic Arrays

Abstract

The highest resolution 3D imaging and monitoring technology for managing oil and gas reservoirs today is provided by large borehole seismic arrays recording 3C data generated by standard surface seismic sources deployed over a wide area around one or several receiver wells.

3D images generated from large borehole seismic array surveys have been successfully used by the petroleum industry for making drilling and reservoir management decisions in complex reservoirs, previously regarded as being impossible to image by regular surface 3D seismic methods. In addition to providing infill or side-track drilling information, 3D images from large borehole arrays have been used to monitor and evaluate the injection of CO2 for enhanced oil recovery and to assess the sequestration of green house gases.

By recording multi-component seismic data using receivers positioned deep in the earth, and closer to the target zone, one can overcome many of the limitations experienced by surface 3D seismic methods.

Inserting seismic sensors deep into oil and gas wells allows the recording of much higher frequencies as compared to placing sensors at the earth’s surface. The reason for this is simple: By placing receivers deep into a borehole, seismic waves have to propagate through the weathered layer only once, confined to a zone near the source. In contrast, during surface seismic surveys, waves must travel through the weathered layer twice. Each traverse of the weathered layer attenuates high frequencies much more than the low frequencies, thus reducing the resolution in the images. The frequency content of borehole seismic data is typically 2 – 4 times higher and leads directly to an increase in image resolution.

In addition to recording higher frequency data, there are a number of other advantages provided by borehole seismic sensors over surface seismic data. Borehole seismic data can typically achieve a much higher signal-to-noise ratio than what is possible in surface seismic data. The combination of a quiet borehole environment and strong sensor coupling to the borehole wall enables a high signal-to-noise ratio to be recorded by borehole seismic data. Surface geophones are often poorly coupled in weathered rock and exposed to cultural and environmental noise at the surface all lowering the data quality recorded on surface geophones. Another advantage of borehole seismic surveys is the favorable geometry to illuminate complex structures such as sub-salt targets, salt flanks or steeply dipping faults.

Good sensor coupling in the borehole enables 3-component seismic data to be recorded with high vector fidelity. It allows shear and converted-wave imaging, as well as determination of anisotropy by shear wave splitting analysis . Combining P and S wave images allows for attribute inversions of rock properties, such as fluid content, pore pressure, stress direction and fracture patterns.