Multicomponent, time-lapse acquisition developments expanding capabilities

• ROV on deck prior to deployment of Subseaco's four-component node system.
• Subseaco's four-component node system.

Recent developments in multicomponent and time lapse (4D) seismic acquisition are facilitating greater understanding of deepwater reservoirs and improving data from subsalt pay zones, according to Compagnie Générale de Geophysique researchers.

Expanded capabilities in seismic technology, says CGG's vice president of borehole and seabed seismic, Christophe Pettenati, will facilitate timely answers for asset managers facing high-risk decisions for production optimization, additional drilling, fluid injection strategy, and monitoring.

"Given the tough economics of deepwater production, developments in seismic technology are expected to deliver a significant reduction of costs and risks in offshore and subsea applications," said Pettenati, but the rewards to operators in terms of drilling costs or additional barrels recovered will dwarf the associated expenses for applying these new technologies, he added.

These technological developments also may facilitate continued integration of methodologies and work practices among multiple disciplines, particularly between the exploration and production groups. These multidisciplined efforts are critical to effective and long-term implementation of the new technologies.

The offshore environment, particularly deepwater, presents numerous challenges and opportunities, but deepwater intervention techniques necessary to facilitate these new technologies, however, are readily available. Deepwater developments in Brazil, the North Sea, and now the Gulf of Mexico, and industry associations such as Deepstar, are showing the way, says Pettenati.

Multicomponent acquisition

Improved understanding of the limitations and benefits of pressure and shear waves is helping to create technology advancements in seismic acquisition, said Claude Vuillermoz chief geophysicist of CGG. Shear waves (S) carry less energy as they propagate through rock, and they are slower than pressure waves (P), therefore are more difficult to emit and to record. This problem can be circumvented as P waves can convert partly into S waves at geological interfaces.

Until recently, these fundamental differences caused the industry to focus seismic acquisition methods solely on pressure wave recording. "Today, however, industry is finding that multicomponent acquisition is feasible by integrating the two modes," said Vuillermoz.

Shear waves present interesting features when combined with pressure recording: direct access to lithology parameters, eg. Poisson's ratio, indication of azimuthal anisotropy, eg. fracture density and orientation, and proper illumination of particular areas where P waves are blurred, eg. gas chimney and subsalt imaging. Onshore, increased noise and interference prevent adequate applications, but offshore applications are significantly different, since shear waves do not propagate in fluids, including water, oil, or gas.

To best record shear waves offshore, deployment of equipment on the seabed is necessary. Once seabed deployment is achieved, the relative economic impact of multicomponent acquisition is marginal in view of the heavy marine spread necessary for shooting, deployment, and recording. By making repetitive ocean bottom surveys during the life of a reservoir, changes in the signals can be representative of actual evolution taking place while hydrocarbons are being produced: fluid fronts movements, development of a gas cap, saturation changes, etc. But, fundamental questions are still only partly answered, such as what can actually be detected, or how important is it to reproduce exactly the same acquisition conditions for successive campaigns?

"Despite these uncertainties, it is already clearly accepted that seismic waves are indeed detecting changes in a reservoir," said Pettenati.

Seabed seismic

CGG notes several advantages of seabed acquisition in the context of reservoir characterization: quality of the measurement, less affected by noise and other disruptions; control of actual positioning, making repeated surveys more reliable; and the ability to work in congested areas, such as producing fields where marine streamers can interfere with the infrastructure. On that basis, a segment of the seismic industry is developing a system called Ocean Bottom Cable or OBC. Initially limited to shallow water, the system is now capable of operating in water depths up to 150 meters with fairly good productivity, making OBC an economic alternative to marine streamer acquisition.

Initial acquisitions were limited to the P hydrophone component, with some difficulties of processing due to surface reverberation of the P waves, but this problem has essentially been eliminated with the addition of a vertical geophone to filter out this disturbance.

"Even still, at 150-meter water depth, two-component capability does not provide an optimum approach to meet the challenges of reservoir characterization and deepwater developments," said Vuillermoz. For this reason, the industry is moving to a full P and S, four-component capability at virtually any water depth, and certainly beyond 1,000 meters, he added.

Two different kinds of systems are under development at CGG and aimed at different targets: a node-based system aimed at small and highly focused high resolution surveys of a few square kilometers and a cable-based system aimed at larger scale acquisitions.

Four-component node subsea seismic acquisition system

CGG identifies three major challenges in trying to record geophone data on the seabed in the x, y, z directions: coupling to the ground, positioning of the system, orientation of the system. Unlike hydrophones, which record the pressure waves by way of their simple immersion, geophones require coupling to the ground to detect either P or S waves. Cables can be ballasted or even buried. Nodes, however, are physically planted into the seabed using remotely operated vehicles equipped with a manipulator arm.

The positioning difficulty increases with water depth and difficult sea states. In a shallow water environment, the touch down point on the seabed of the deployed system is not greatly different from the position of the surface spread at the time of deployment. A few acoustic beacons fitted on the cable ensure that any movement due to current or wave, after laying, can be detected.

Difficulties increase with water depth, requiring positioning techniques to be more precise. In medium water depth, monitoring of first break arrivals and the use of additional acoustic beacons facilitate improved installations. Deepwater applications, however, require more stringent positioning techniques during laying to ensure compliance with planned and real lay patterns.

Randomly laid x, y, z geophones referential present recording difficulties for effective orientation. Gimbal mounting on cable improve the readings, at least for the z and y geophones. However, gimballed systems are not the ideal approach.

The node system, which is physically and visually planted by a ROV, is from the start approximately vertical. It is also fitted with a digital compass and an inclinometer to further correct and calibrate the readings. The geophones are fixed inside the canister.

The node, rated for 1,500 meters of water, consists of a watertight caisson, housing the telemetry electronics, inclinometer, compass and geophones, and is easily manipulated by the ROV. The hydrophone is positioned on top of the node handle in contact with water to detect pressure changes.

The node is then connected to the main transmission cable by a short two-to three-meter take out cable. As many nodes as allowed by power supply can be fitted on the main transmission cable, without signal attenuation as each node acts as an amplifier. Nodes spacing depends on the survey parameters.

Stored on a winch, the system is deployed from the deck of a standard survey-type, dynamically positioned vessel and does not require a dedicated seismic vessel, representing a great potential for marine resource optimization. Positioning on the seabed is accurately monitored using an ROV and onboard acoustic positioning systems.

Once the nodes are planted, shooting can start on a given pattern. The lay vessel remains stationary to record the data. Alternatively, recording can take place from an existing field facility, contributing to cost reductions for extensive surveys, as the lay vessel can be released as shooting progresses.

A conventional seismic 2D vessel can be used for shooting, but ideally an offshore supply vessel fitted with a containerized, modular source is adequate, again contributing to cost reduction and resource optimization.

"The node system design is modular and completely versatile for a diverse set of lay patterns," said Eivind Berg, co-founder and director of Subseaco, a CGG subsidiary based in Trondheim, Norway. The system provides flexibility for spacing between nodes, number of nodes and position of nodes, he added. The system is equally adequate for 2D or 3D surveys. Typical spacing between nodes for a 3D survey would be several hundred meters in both X and Y directions, and 25 to 50 meters for 2D.

In the case of 3D, the wide inter-sensor gap must be balanced by a high density, large offset shooting grid to obtain an adequate size and coverage, as well as a reasonably large illuminated target. Effective coupling of the geophones to the ground provides high quality readings. The system allows the modular coupling to accommodate specific soil conditions. Project-specific designs facilitate adaption to shear strength and consolidation to the ground. The solid state design also contributes to quality readings.

CGG's first commercial 2D job, carried out through Subseaco, is currently under way off Norway for Esso Norge Balder in 130-meter water depth.

"Thus far, the data acquisition is illuminating horizons previously undetected, and with particularly excellent X and Y shear waves readings," said Berg.

Further projects are planned this year, including a large-scale 3D survey in the UK sector of the North Sea.

Node technology is becoming a reliable reservoir characterization seismic tool, with special interest for time-lapse surveys, as reproducibility of both coupling and positioning is intrinsic to this technology.

The nodes, however, do have limitations. Beyond a certain number of nodes, the installation work becomes overwhelming. But if the node density is kept to a minimum, the duration of the survey becomes excessive.

Developments underway will further improve the system's overall productivity, but at present, applications of node technology are limited to high quality surveys.

Pettenati notes that operators and contractors will continue to resolve these issues, with reservoir managers and geophysicists learning to understand each other's problems and constraints. "Seismic technology is at the eve of a new era of multidisciplined cross fertilization. Before long, seismic information will constitute one of the essential decision-making tools for reservoir managers."