4D - Monitoring the Reservoir
By periodically repeating 3D seismic acquisition over a reservoir it is possible to detect production related changes and update the reservoir model. Time is the 4th dimension. 4D surveys may be useful to identify bypassed reserves, place injector wells, to locate unidentified permeability conduits, and monitor depletion or plan CO2 sequestration.
High-density 3D seismic has become a common standard for reservoir monitoring and 4D projects can be carried out in almost any geological and environmental setting. They require a range of solutions. Through project risk analysis and management, our acquisition experts address environmental challenges such as marine fauna, or variable currents and weather. Busy oilfields, very deep water, and remote locations each present their own project challenges.
One of the requirements for 4D marine seismic surveys is to repeat the acquisition configuration as closely as possible. PGS applies acquisition controls that optimize lateral vessel, streamer and source position. These systems increase repeatability by minimizing the effects of variable currents on the acquisition geometry, while also improving survey efficiency.
Active control and monitoring of the wavefield emitted from the source is part of a standard 4D solution. When currents may result in cable feathering, PGS recommends adding redundancy in the streamer spread, by recording data with more streamers than the nominal configuration. Typically this means one or two additional streamers on each side on the spread in an overlap configuration.
Traditionally, the streamer tow depth of the baseline and monitor surveys has to be the same. However, with dual sensor recording this restriction can be relaxed. The collocated sensors make it possible to separate the seismic wavefield into up- and down-going parts. These can be independently re-datumed and summed (re-ghosted) to emulate the total pressure wavefield at any recording depth. This reconstruction process treats the amplitude and phase of the seismic signal correctly. Thus, the dual-sensor streamers may be towed at any depth in a 4D context. Deeper tow depth takes advantage of the quieter recording environment and increases the bandwidth of the data.
As the reconstruction of hydrophone-only data re-introduces the receiver ghost, the broadband uplift is sacrificed in order to maximize backward compatibility and repeatability. For that reason, 4D dual sensor data is commonly treated with an iterative approach:
- the reconstructed and re-ghosted data is used for 4D monitoring
- the broadband upgoing wavefield (P-UP) is the basis for high-resolution imaging and quantitative interpretation (seismic inversion)
The full benefit of the increased bandwidth for 4D projects is realized when both baseline and monitor surveys are acquired with advanced dual sensor streamer technology that enables full integrity and 4D-friendly wavefield separation.
Seventeen proprietary dual-sensor baseline and monitor surveys have been acquired since 2009. With time-lapse, reservoir characterization, and high resolution imaging objectives in mind, all the 4D acquisition projects made use of deep-tow and high-density spreads. Most of these surveys were acquired with 50 meter streamer separation and with up to 16 streamers.
3D acquisition and imaging will be repeated many times over the producing life of a field