OTC: Ocean-bottom seismic improves resolution for offshore subsurface

Streamer technology is not new, however, it has gone through many advances within the last 30 years. In 1980, 120 channels were considered advanced for a typical streamer run when shooting offshore seismic. “Today, we see 10,000 channels towed behind a single boat,” said David Monk, director, geophysics and distinguished advisor, Apache Corp.

Speaking May 5 at a panel discussion at the Offshore Technology Conference in Houston, Monk said, “In some cases, a single boat can tow up to 50,000 channels.” Within the next 10 years, the industry will use up to 1 million channels in a single run.

Given this complexity, the goal of producing high-resolution images of the subsurface is to broaden the bandwidth using ever-increasing numbers of streamers. The problem arises from increased activity in most offshore areas. New infrastructure—both on the surface and on the sea floor—can impede access for newer surveys. While streamer technology has matured since the 1980s, it is now confronted with working around platforms, subsea structures, and newer technology is providing enhanced imagery as opposed to surface seismic shot offshore.

Improvements such as steerable technology are providing further enhancements by allowing streamers to be positioned for optimal reception from the source. Predictive tools using tide and current information are also fine tuning seismic shoots. Yet, ocean bottom seismic is vastly gaining in momentum as a potential exploration tool.

“The value of 4D seismic is that it drives down uncertainty and improves dollars per barrel of oil over time,” said Rocky Detomo, manager, Shell Areal Monitoring Research, Shell International Exploration & Production Inc., also part of the OTC panel. Life-of-field systems are still in their infancy, but are becoming more commonplace. Monitoring a reservoir over time requires accuracy and permanent installations mitigate problems associated with repeatability when placing and removing seismic sensors. “These are the most expensive parts of this process,” Detomo said.

Recent advances in autonomous nodes are making ocean-bottom seismic more accessible to monitor offshore reservoirs, but the process is highly dependent on accuracy. Because the technology costs more, it is not yet widely used. “For any prospect that will undergo 4-6 seismic surveys in its lifetime, a permanent monitoring system becomes comparable in price with conducting single seismic shoots,” Detomo said.

Although the benefits of continuous seismic monitoring are lending themselves to smaller placements to better understand novel geometries. With the benefit of ocean-bottom nodes, companies like Shell are designing programs to monitor very specific areas to observe certain objectives.

Detomo added that more advances will be needed for this technology to become more commonplace. More fiberoptics, long-life autonomous recorders, advanced acoustic sensors, and an increased use of passive seismic data will each contribute to this emerging trend.

Another panel member, Shuki Ronen, chief geophysicist, Seabed Geosolutions, said, “Ocean-bottom seismic is now comparable to where 3D seismic was 30 years ago.” To date, the technology has been successful in reservoir monitoring and development programs but the high cost is prohibitive for exploration programs.

The use of ocean-bottom nodes eliminates seismic blind spots caused by either surface obstructions caused by offshore platforms or subsurface equipment and infrastructure. The important point for many of the panelists was that streamer technology and the use of autonomous nodes were not mutually exclusive. In many cases, these technologies are combined to form a much more robust image of the subsurface than previously known.

Streamer technology will remain as a highly advanced seismic tool, but ocean-bottom nodes may be used in the near future to fill in where streamers fall short in collecting subsurface data. When compared, the introduction of wide azimuth seismic came with an increased cost. According to Ronen, as late as 1984, streamer technology came at a high cost, but did not display a high value. Now 30 years later, wide azimuth seismic has drastically increased its value, but improvements on imaging the subsurface come with an increased cost, i.e. increasing the number of channels to broaden a survey’s bandwidth.

A major benefit associated with ocean-bottom seismic is the ability to capitalize on S-wave reflections as P-waves are predominant for most offshore seismic. “Producing higher quality data improves the quality of drilling targets,” Ronen said.

As wide-azimuth offshore seismic continues to mature, ocean-bottom seismic will continue to advance. Production and development surveys have benefitted greatly with ocean-bottom technology. As this technology is continually combined with streamer technology, the subsurface offshore should come into better focus.

Contact Tayvis Dunnahoe at tayvisd@ogjonline.com.

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