Monitoring subsurface changes in and above oil- and gas-producing reservoirs
An important mode of seismic in the hydrocarbon industry is the so-called time-lapse mode, i.e., surveys are carried out over the years of the production of a field. This is a step towards making seismic movies of the subsurface. This is of particular importance for reservoir management and for guiding the production process. These types of investigations are expensive and require very repeatable ways of recording, and are getting more and more standard; still, many challenges lie ahead. Especially on land, little has been tried and major issues remain, such as noise and repeatability issues over time, caused by, e.g., traffic, wind, rain fall and changing water tables.
The permanent monitoring network of LOFAR facilitates the repetition of large-scale seismic measurements (passive or active) over time with intervals of, say, 3 to 12 months. This is carried out in a selected area in the northern part of the Netherlands, where changes in the structure and stress build-up are to be expected, for example due to gas- or salt-production, which may lead to subsidence. Key questions are:
- Around and above the level at which production of gas by the industry takes place: does the subsurface change due to this production? And if so, where does the change take place? And what is the character of the change?
- What new monitoring strategies can be developed on the basis of the seismic monitoring network?
The LOFAR-project goal is to allow monitoring the shallower subsurface (< 4 km deep) such that processes
in the subsurface can be monitored. This entails:
- Design and construct a fixed dense spatially well-sampled array of multi-component seismic sensors that allows monitoring changes in the subsurface.
Once the network is built, the processes occurring in the subsurface will be monitored in the years afterwards.