In recent years, seafloor multi-component seismic surveys have emerged as a valuable tool for oil companies in appraising disc-overies and defining field development strategies. Over the past year, Petroleum Geo-Services (PGS) has performed four such surveys for customers in the Asia-Pacific region:
- Lasmo: In the fourth quarter of 1999, in Indonesian waters
- Arco: In Chinese waters in the first quarter of this year
- Phillips: In the second quarter in Bohai Bay, also off China
- Shell: In late summer, in Malaysian waters.
The Phillips' project targeted the PL 19-3 Field, where before exploration drilling took place, PGS had acquired seismic data by means of a conventional streamer survey. Part of the PL 19-3 Field was obscured on the streamer data by a gas cloud. The primary objective of the ensuing seafloor multi-component survey was to provide an improved structural image of the reservoir for Phillips' appraisal and development planning needs. According to Mike Saur, PGS Reservoir Services' Vice President, Marketing and Sales, this was the largest exclusive multi-component survey performed in the industry to date.
Shooting took about three months, from April to early July, and used two vessels, the Bergen Surveyor and Orient. The data is being processed at the company's Houston headquarters, with the final deliverables due to go to Phillips in the fourth quarter. PGS was happy with the initial results, and understood the client was also, Saur said.
Multi-component seismic involves the recording of shear wave energy as well as the compressional wave (P-wave) energy collected by conven- tional seafloor and streamer techniques. One of the benefits of this approach is the ability to image beneath gas clouds. Conventional P-wave seismic data is highly affected by gas-charged sediments, resulting in the attenuation and distortion of P-wave energy. S-waves, on the other hand, travel relatively undisturbed through the gas-charged sediments, thus making imaging possible.
Additional applications of multi-component seafloor surveying includes improved imaging of complex structures using orthogonal survey designs, the imaging of transparent - P-wave reservoirs, lithology prediction, fluid discrimination, amplitude anomaly validation, and fracture mapping.
Recording of the seismic waves is made using sensor packages set at 25-meter intervals along a cable which is laid in a series of parallel lines on the seabed over the target location. Each sensor package contains a hydrophone, a vertical geophone, and two horizontal geophones orientated perpendicular to each other. A cable will typically be six km in length.
The PL 19-3 survey was conducted in shallow waters, which has been typical for such seafloor surveys until recently, Saur says. However, PGS has also invested in developing the technology for deep-water application down to depths of 2,000 meters plus.