Time-lapse (4-D) seismic is recognized as a key tool for optimizing hydrocarbon production. Extraction of hydrocarbons induces changes in the reservoir that cause changes in seismic timing, velocity and amplitude, which in turn affect inversion results. Monitoring these changes in the seismic signal by accurately repeating 4-D monitor surveys enables changes in reservoir properties to be derived. This enhances recovery by enabling undrained volumes to be mapped and providing input to dynamic geological and reservoir simulation models. Hydrocarbon producers have recognized 4-D seismic as an economic success, contributing directly to successful infill wells and enhanced recovery techniques, such as gas injection and horizontal drilling, with the added value being several times the cost of the 4-D seismic data.
For 4-D surveys to deliver these advantages, 4-D noise needs to be minimized to avoid obscuring differences caused by changes in the reservoir. More importantly, the positions of the sources and receivers need to be replicated as closely as possible to ensure that the same subsurface raypaths are recorded. High-end acquisition solutions for 4-D have focused on source steering and, more recently, CGG has developed integrated vessel and source steering to maximize repeatability. Advanced 4-D acquisition also needs to be complemented by the very latest 4-D imaging technology to deliver a complete solution.
Large-scale repeatability is achieved through 4-D operational planning, working with experts onboard to prepare and constantly update acquisition plans by taking environmental conditions (current, tide, etc.) into account to position the spread and optimize streamer (feather) matching. The geometry of the spread then is maintained through streamer steering using equipment such as Sercel Nautilus units, which also deliver acoustic positioning and depth to ensure accurate positioning.
CGG’s integrated source and vessel steering system consists of an automatic steering assistant to position the vessel so that the source is as close as possible to the pre-plot position while maintaining the integrity of acquisition geometry. Residual crossline positioning and short-period perturbations of the source position caused by swell are controlled by the automatic source steering system. The system also controls the shooting strategy and triggers the source firing to respect in-line positions rather than time intervals, correcting for smaller scale errors such as the skew of the sources. This automatic integration of the different parts of the system has delivered considerable improvements in repeatability over source steering only, let alone acquisition without any source steering at all, in the very different environments of the North Sea and offshore West Africa. Advanced monitoring systems and onboard quality control (QC) ensure acquisition of the best possible 4-D data.
In addition to repeatability of source positions, it also is beneficial to monitor the source signature to ensure it is consistent throughout the survey. The stability of the source signature in the changing sea environment is monitored via shot-by-shot near realtime QC of the recorded near-field signature. These recordings enable far-field signature reconstruction using a proprietary inversion method for use in onboard and onshore processing, delivering more accurate designature and therefore better images.
Ideally, a 4-D monitor survey would be acquired with exactly the same parameters and under the same conditions as the base survey. In practice, this often is not possible and, in cases where there are already several monitors dating back to the last century, would not really be desirable as it would prevent the use of any advances in acquisition technology, which might deliver benefits in terms of bandwidth, illumination or multiple attenuation. Nevertheless, it is generally agreed that repeatability of the source and streamer positions is of prime importance and that these should be replicated as closely as possible. Acquisition of additional longer offsets and broader bandwidths can be beneficial in providing a good basis for comparison with the next monitor, even if the benefit for the current comparison may be limited.
Multisensor streamers recently have joined the toolbox of broadband solutions, providing an additional option for broadband 4-D monitoring. These streamers use the same hydrophone components as single-sensor solid streamers, whose low noise characteristics and precise low-frequency response make them ideal for 4-D acquisition. Recording with multisensor streamers means that forward and backward compatibility can be straightforward because the hydrophone data can be matched to the base survey while the y and z accelerometer data can be used for multisensor 3-D deghosting using advanced algorithms to match future surveys for true broadband 4-D seismic. However, in general, conventional base surveys are usually deghosted using ghost wavefield elimination and matched to as broad a bandwidth as the signal-to-noise ratio of the conventional data allow. The maturity of these algorithms allows the joint 3-D deghosting of base and monitor surveys (known as 4-D deghosting) and enables variations in the sea surface state to be handled fully.
This technique results in acquisition solutions, both conventional and broadband, that are forward and backward compatible with all other towed-streamer 4-D acquisition techniques. Nevertheless, broadband acquisition remains the best system to enable successful deghosting and to preserve the broadest possible bandwidth for matching to future surveys. Broadband 4-D seismic can deliver enhanced reservoir modeling as all the benefits of broadband in 3-D also hold true for 4-D. For example, sharper wavelets without sidelobes provide a high-resolution 4-D signal with no interference and masking of seismic detail. Ghost-free wavelets remove the imprint of the sea surface on the data and hence deliver more reliable amplitude versus offset for accurate simultaneous 4-D inversion combined with better low-frequency content for quantitative inversion results.
The enhanced low frequencies improve the characterization of reservoir heterogeneities and enable the detection of 4-D signal due to pressure and/or saturation changes in a more quantitative manner. When there are clearer images of the reservoir from low frequencies at and between wells, 4-D signals will be detected more readily, especially where the 4-D signal is explained by low-frequency changes.
Advances in 4-D processing and imaging have been geared toward optimizing the sequence for minimal 4-D difference. Corrections based on knowledge of the acquisitions, such as the sources and the water-layer velocities, are used to deterministically remove 4-D noise. Standard QCs are regularly augmented by reservoir-domain attributes during the processing. The cost functions for 4-D can be designed to further attenuate nonrepeatable noise while preserving coherent 4-D signals. Although careful 4-D processing and imaging can help to ameliorate challenges caused by lack of repeatability in acquisition, the closer the acquisition is matched, the better the 4-D signal and the lower the 4-D noise tends to be, reducing the chance of false signals. This is demonstrated in the extreme case where permanent installations of buried subsea cables are used for monitoring at frequent intervals and deliver exceptionally high levels of repeatability for monitoring very small changes in the reservoir. However, advanced steering techniques can deliver very good repeatability and, combined with state-of-the-art 4-D imaging technology, provide reliable 4-D signals for reservoirs where less frequent monitoring is required.
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