Seismic reservoir monitoring has demonstrated its effectiveness to help understand the dynamic behavior of the reservoir and its value for reservoir management. The method has a yet unrealized potential that will open for wider use, if higher repeatability can be obtained. Wider applications will be more quantitative interpretations, monitoring over shorter time intervals, and application of the technology to smaller, tighter, and more complex reservoirs.
Perturbations, (errors and differences), between different phases of a 4D seismic study create noise when datasets acquired at different times are analyzed to detect changes in the subsurface. This noise can mask the subtile variations in the seismic response of the reservoir that indicate changes in pore fluids. The effects of perturbations have been studied and the impact on seismic data quality quantified. The specification criteria for Q-Marine have been set to address the sources of perturbations that impact the quality of seismic data, both for high-resolution imaging and repeatability in 4D studies. This paper will describe the new technology and show examples of the ability to remove the perturbations experienced in conventional seismic data.
We conclude by explaining how 4D seismic surveys utilizing the new Q-Marine concept will be a very costeffective tool for management of the reservoir throughout its lifetime.
The requirements for repeatability and fidelity of the seismic data are significantly greater for reservoir monitoring than for exploration surveys because we are looking for small features and changes. These details are most often masked by noise in conventional seismic. Noise can come from external sources or may come from perturbations in the data introduced by variations within the seismic systems themselves, the so-called "noise behind the signal." We can divide the perturbations experienced in seismic into:
Those that affect the received signal, such as sensor sensitivity variations and ambient noise, such as swell.
Those that affect the emitted source signature, such as Shot-to-shot variations in the source output and the array directivity.
Those that affect positioning repeatability and accuracy, such as sea currents and positioning system.
The feasibility studies leading to the development specifications of Q-Marine included significant effort to study and quantify the effects of these perturbations. As a result, with the launch of the system, we are now able to control these perturbations, and the concept of "calibrated seismic" from acquisition through processing has become a reality. The system features a new sensor and streamer system, new source signature calibration technology, and new positioning and active streamer control. These features will all be described with respect to their impact on time-lapse repeatability.
It has been shown that for seismic to be an adequate tool for reservoir management, the perturbations in the data should not exceed a level of "30 dB below the seismic signal. The two most significant perturbations affecting the signal received by a towed streamer are sensor sensitivity variation and ambient noise.