In all major centers of world oil and gas production (the North sea, the Gulf of Mexico, West Africa shelf), seismic monitoring of fields under development has been performed for years and is a standard work.
These surveys are mainly aimed at increasing the oil recovery factor up to 25%, as well as ensuring uninterrupted technological and environmental monitoring of the offshore well operation, which, taking into account resonance accidents on offshore platforms, including in the Gulf of Mexico, is the most important mission of oil and gas companies. Up to now, Russia has not conducted suchlike surveys in offshore fields. However, a number of companies are very interested in them.
Today, our company offers the so-called "cyclic" seismic monitoring of hydrocarbon fields being developed in the water areas with depths up to 500 m. During the cyclic monitoring, seismic acquisitions are performed periodically, e.g., once a year, or in accordance with the technological scheme of field development, for example, after a certain stage. Cyclic monitoring implements active seismic acquisitions, involving the excitation of seismic signals with pneumatic or electric spark sources. The cyclic seismic monitoring consists of at least two acquisition series: Initial acquisition of seismic data (base line) and subsequent acquisition (monitoring line).
The monitoring is required to identify changes in key monitoring indicators, for example, oil-water contact positions, between geological and geophysical models, built at the base line and monitoring line stages, respectively. Subsequent acquisitions can be repeated many times: monitoring line 1, monitoring line 2, etc.
The seismic monitoring has the following advantages:
Option to obtain a highly reliable geological and geophysical field model at each development stage;
Cost saving, as the equipment involved in the monitoring process, as well as specialized vessels, can be used for executing other projects in the period between monitoring stages of this particular field.
When carrying out the cyclic monitoring, the key issue is to achieve full coincidence of the position of exciting and recording points for each monitoring stage. This requirement is determined by the fact that the change of monitoring indicators from stage to stage, on the one hand, is slightly reflected in the observed wave field, but on the other hand, these minor changes may have policy implications for further field development scheme.
Performing the repeated acquisitions in the same exciting and recording points is a difficult technical task, which is solved using high-precision underwater positioning systems and ROV.
One of the examples of active seismic monitoring is the British Petroleum activity on the Atlantis field in the Gulf of Mexico.