Part of the excitement about 4-D seismic today is that we are in the midst of a technology revolution in which the outcome is quite promising. Within the exploration and production (E&P) industry, numerous 4-D technology options are being explored with different degrees of support. Concept Systems has provided a variety of technical support to 4-D seismic acquisition surveys, including towed streamers, ocean bottom cable, nodes and permanent seabed systems.

Oil and gas companies were initially cautious about embracing 4-D seismic for reservoir management purposes, even when the benefits had been well-demonstrated on a number of early surveys in the late 1990s. Faced with oil demand threatening to outstrip supply and with declining exploration success rates, companies have increasingly turned to maximizing production from existing oil and gas fields as the best way to address supply issues. It is in this context that 4-D seismic has a key role to play.

The technology has gone way beyond expectations of major oil and gas companies. There is no question that 4-D is an accessible tool for reservoir geoscientists and engineers dealing with producing offshore oil and gas assets worldwide. The focus of 4-D seismic surveys has been mainly in the North Sea and the Gulf of Mexico mature provinces, but interest is growing elsewhere.

Use of 4-D seismic technology is determined by a number of factors such as feasibility, projected complexity and cycle time, but above all by cost. A 4-D seismic survey must make economic sense. So far cost issues have translated into an overwhelming preference for towed streamer 4-D acquisition, despite some imaging shortcomings compared to seabed seismic recording techniques. Ocean-bottom cable (OBC) systems, nodes and permanent ocean bottom cable (POBC) systems are significantly more expensive operations and are relatively new survey methods. Towed streamers, by contrast, are very familiar territory for oil and gas companies after more than a decade of highly successful 3-D seismic exploration; the economics are well known and the results, as we shall discuss later on, can be surprisingly good.

According to Bob Peebler, chief executive officer of Input/Output (I/O), streamer 4-D seismic results in some compromises. For some time, he has been publicizing arrival of the "digital, full-wave" revolution, in which OBC systems like I/O's VectorSeis Ocean will play a part. OBC has several advantages over towed streamer 4-D: the ability to record shear wave information, acquisition in a lower noise environment and deployment of the hydrophone on the ocean bottom for better multiple suppression. Recording the "full wavefield" offers significant improvements in the subsurface information available, with greater insight into reservoir characteristics such as fracture orientation, permeability and fluid content. Full-wave acquisition and processing also enhances imaging of complex structures such as salt domes, not possible from towed streamer data based only on P-wave recording. Wide-azimuth seismic acquisition, now seen as useful for resolving reservoir fracture issues, is more easily acquired with ocean-bottom acquisition techniques than with towed streamers, although wide-azimuth streamer-based acquisition is not impossible if extremely long cables are used.

From an industry point of view, the perceived drawbacks of OBC systems revolve mainly around operational complexity, concerns about unresolved data processing issues and, of course, cost. Although the multi-company Teal South project piloted the potential for 4-D OBC starting in 1996, the industry is still waiting on the first commercial redeployable OBC survey dedicated to 4-D.

New entrants, including the Norwegian company Reservoir Exploration Technology (RXT), are testing new technologies and operational approaches to make step-change efficiency improvements in redeployable OBC seismic acquisition. The first RXT survey in the Gulf of Mexico deployed I/O's VectorSeis Ocean with a retrievable buoy system for recording data and Concept's Gator command and control system for communications and data management. The buoy recording system eliminated one boat, reducing the survey to a two-vessel project compared with three vessels for conventional OBC surveys, thereby significantly reducing the survey cost.

By placing cables directly on the seabed, RXT was able to acquire full-wave seismic data for its customer, a supermajor oil and gas company. GX Technology, which designed this integrated imaging program on behalf of this company, is now processing the data. The early indications are extremely promising. The shear wave information is helping to identify reservoir features previously hidden by gas clouds while advanced prestack depth migration techniques are being applied to better resolve steeply dipping beds that abut the many salt flanks in the area.

Similarly, Concept provided technical services and support to Seabed Geophysical, another Norwegian company, for its first survey using retrievable seabed nodes as seismic receivers. This customized application for the Mexican state oil company Pemex, again with 4-D seismic potential, was developed for acquiring seismic in an area heavily obstructed by existing offshore installations. Seabed Geophysical believes that nodes provide the optimal solution for this type of shooting, being more flexible than OBC and 100% better than towed streamers, which can be ruled out for this type of operation.

Redeployable OBC and node-based systems to acquire repeat "time-lapse" 4-D seismic surveys should become more cost-effective as more research and development investment is dedicated to them, especially for the processing part of the operation. Offering the promise of "seismic on demand," the ultimate solution would appear to be POBC systems. POBC represents a move closer to the instrumented oilfield idea, in which a new 4-D seismic image of the reservoir can be easily acquired, processed, interpreted and analyzed to provide timely information for reservoir management decisions.

With such a tantalizing prospect in view, the industry is paying close attention to the progress of BP's Life of Field Seismic (LoFS) project at the Valhall field offshore Norway. It is the first serious attempt to install a POBC system for acquiring repeat time-lapse seismic surveys over a producing reservoir as part of a strategy to increase ultimate field recovery. Official feedback from BP at the 2004 Society of Exploration Geophysicists Convention was positive. BP's Jan Kommendal reported to colleagues in one presentation that "current technology provides high-quality data and allows streamlined quality control (QC) and processing." He added, "The greatest challenge to rapid delivery of 'seismic on demand' seems to be coordination of field operational activities to reduce non-repeatable noise sources such as rig activity, vessels and other activities which lead to delays either in acquisition or processing."

LoFS remains virtually a "one-off," with no POBC as far advanced and only one other permanent installation underway at Shell's Mars field in the Gulf of Mexico. Expense is likely to restrict next-generation POBC operations to well-capitalized companies. One recent initiative addressing the cost of POBC has been the partnership of UK company Qinetiq and I/O to develop and deploy the world's first fiber-optic seismic acquisition system, co-sponsored by BP, ChevronTexaco, ConocoPhillips and Shell. Designed to operate with no electronic components in the water, the system is intended to increase reliability and reduce life-cycle ownership costs compared with cable-based seafloor reservoir monitoring systems.

Some of the most productive 4-D seismic research to date has been directed towards improving performance of towed streamers in delivering 4-D seismic surveys. Concept began working with Shell Expro in 2001 to develop tools and technology to measure and improve 4-D repeatability. At that time it was believed that a significant portion of data errors arose from factors including geometry changes between surveys, weather, tides and the like. In 2002, the solution was successfully applied in Shell's North Sea 4-D seismic program to QC and optimize acquisition parameters and make infill choices.
Early research led Concept to develop a systematic approach for oil and gas companies considering towed streamer 4-D seismic, although the principles apply equally to any type of technology adopted. The premise, based on experience with several surveys, is that errors stem from the towing of long cables and the differences which can creep into subsequent monitor surveys' source/receiver positions compared to baseline surveys. Therefore, positioning is the crucial issue that must be addressed at the planning stage. This may involve accepting a compromise between operational acquisition costs and design of optimal scenarios to ensure accurate and measurable repeatability.

Planning should begin with what amounts to a forensic analysis of the survey being adopted as a baseline. In many cases, this will be a survey not originally intended as a 4-D seismic baseline survey. It would not be unheard of to recommend a "no go" if the data of the proposed baseline survey are inadequate. Only when the integrity and completeness of the proposed baseline data are understood can monitor surveys be successfully planned. All the normal blemishes in a survey, such as reshoots/infill decisions, etc., have to be considered along with such potential problems as feather matching.

Monitor 4-D surveys must take into account the baseline review analysis as well as economic and operational constraints on a repeat survey. In one solution, a set of deliverables to ensure an optimal 4-D seismic survey can be generated once the geometry has been agreed upon. These include a referenced set of 4-D attribute difference and repeatability map data to show any deviations from the plan during operation; recommended feather for each monitor line to ensure best possible matching; pre-plot generation to drive the acquisition system, for example, a navigation solution; and baseline feather generation to drive acquisition monitoring. Once in the field, software tools such as 3-D coverage analysis, 4-D attribute and repeatability analysis, and real-time 4-D navigation software can contribute to meeting the 4-D seismic standard required.

There is a further upside from meticulous planning, design and acquisition of 4-D surveys. By removing many potential sources of non-repeatability, post-survey processors are provided with data "fit for processing" rather than data to be "fixed in processing." Some observers suggest that lack of more widespread adoption of 4-D seismic methods actually hinges on progress in improving turnaround of the data once it has been acquired. Oil and gas companies need rapid 4-D seismic results if they are to have maximum impact on reservoir optimization decisions.