Frans Smit

Seismic Data Acquisition Using Ocean Bottom Seismic Nodes At the Deimos Field, Gulf of Mexico

Seismic data acquisition using Ocean Bottom Seismic (OBS) nodes was conducted over the Deimos field in the Gulf of Mexico, in about 1000m of water. The field is located under a salt overhang. Wave equation modeling was used to determine the optimal location of the nodes. The ability to acquire data at very long offsets was particularly useful. Seismic operations proved to be flexible in an area with considerable surface and subsea infrastructure, operational interference and strong currents. As the market for OBS node technology develops and more surveys get acquired in the future, it is expected that operational best practices will lead to increased efficiency in the field. Pre-processing of the data included node positioning, timebreak alignment, and optimized wavefield separation and noise attenuation on vertical geophone data. Migration of the downgoing wavefield data proved to be successful down to the target depth. Processed results achieved to date show significant improvements in sub-salt imaging compared to existing narrow azimuth data.

Toward affordable permanent seismic reservoir monitoring using the sparse OBC concept

In recent years time-lapse seismic has become a widely established reservoir monitoring technique. This is especially so in the offshore environment where the vast majority of all 4D seismic has been acquired using streamer technology. While there have undoubtedly been significant successes, especially in the North Sea, there is a continuous drive to further improve 4D signal detectability, operational flexibility, and efficient diagnostic generation for decision taking in reservoir management.

Experiences With OBS Node Technology In the Greater Mars Basin

A small, but growing, number of Wide Azimuth (WAZ) surveys have been acquired using fully autonomous ocean bottom seismic (OBS) nodes. The early successes with this method have led to a recent uptake in the application of node technology in deep water. One such survey, in the Greater Mars basin, was mainly justified by expected improvements in sub-salt illumination and ease of access in an area with infrastructure and currents. Results after processing had a significant business impact, and further surveys in the area have been planned. OBS nodes have also been tested successfully for 4D. A comparison with permanently installed ocean bottom cable (OBC) technology suggests similar 4D quality. Which technology is most suitable, however, depends on further non-geophysical considerations. Node technology is under continuous development to improve turn-around and reduce cost. Future benefits are expected from improved node hardware and handling, as well as more efficient shooting and processing technology.

First dual-vessel high-repeat GoM 4D survey shows development options at Holstein Field

In the Gulf of Mexico (GoM), loop and eddy currents can cause large errors in 4D shot and receiver locations between baseline and repeat streamer surveys, which invariably lead to poor data quality. In a recent 4D acquisition, a dual-vessel 3D acquisition method addressed the repeatability problem and showed reliable time-lapse measurements over Holstein Field. The time-lapse seismic data show time shifts up to 6 ms over depleting sands and amplitude changes over swept and compacted sands. This 4D information has improved understanding of the field and can support optimal placement of injection and production wells.

Permanent seismic reservoir monitoring using the sparse OBC concept

Summary The sparse ocean bottom cable (OBC) technique is based on acquisition of low fold, but highly repeatable data, resulting in relatively small data volumes. Short acquisition and processing turn-around times enable cost-efficient and rapid 4D decision taking. Permanent seismic reservoir monitoring systems using ocean bottom cables are expensive to install. The sparse OBC concept minimizes the number of receiver cables deployed, and is therefore an attractive proposition as installation costs are much reduced. The design of a sparse survey is based on maximizing 4D repeatability, and aims at imaging 4D changes in the reservoir. A minimal “sparsity” of a survey is required for a basic level of noise suppression and imaging. A dedicated 4D demultiple tool complements the sparse technology. A real data example demonstrates the effectiveness of sparse 4D imaging. The results compare well with those of high multiplicity 4D datasets. A certain ability for noise suppression is lost, however, and a trade-off with low costs, efficiency and short turn-around times is required.

A strategy for optimal marine 4D acquisition

Repeating source-receiver azimuths can be an important aspect of 4D acquisition. Seismic repeatability will decrease with an increase of source-receiver azimuth differences between base and monitor surveys. This paper discusses a marine acquisition strategy with respect to the optimal preservation of source-receiver azimuths in the presence of feathering. We show that repeating shot positions is favorable for azimuth preservation in 4D acquisition in combination with overlap configurations (additional outer streamers). With a dense streamer separation source-receiver azimuths can be repeated very accurately by using this strategy. In a base survey, overlap configurations allow the vessel to follow the survey’s preplot sail lines with significantly reduced crossline deviations. A well-conditioned base survey simplifies and optimizes the repetition of vessel/source positions in a future monitor survey.