Background
A challenge in seismic modelling during the last years has been to develop new methods that can be applied in connection with 4D or time lapse seismic monitoring. As opposed to the applications in 3D survey planning and imaging, where relatively large scale macro models are constructed for simulating seismic wave propagation, these new modelling schemes must be able to simulate the seismic response of much more detailed complex layered reservoir models, in order to model the small scale changes in the seismic wavefield due to fluid flow in the reservoir during production. The 4D seismic difference signal obtained by repeated seismic surveys must be interpreted, and this is presently to a large extent done in a qualitative way. The new challenge is to tailor advanced seismic modelling for the 4D process and obtain a tool that can be used quantitatively as well. NORSAR has been working for some time with such modelling concepts, and we have recently implemented a new modelling framework called SeisRoX. This modelling scheme is applied at the transition between seismic interpretation and reservoir modelling (fluid simulation), trying to strengthen the bridge between the geophysicists/geologists and the reservoir engineers. This bridge did hardly exist some years ago, and has been heavily addressed by the oil companies in recent years.
Project summary
This project represents a considerable research effort with the main objective to develop a new modelling system for transforming 4D (time lapse) seismic data to reservoir model changes, which can improve the reliability of fluid simulation. The modelling system will be integrated in an already established modelling framework called SeisRoX. The SeisRoX model contains physical properties for a number of rock layers in a petroleum reservoir zone, and these properties are classified in 3 different domains: geologic domain (G= reservoir properties like porosity, permeability, lithology, oil/gas content), elastic domain (E= elastic properties like seismic velocities and density), and reflectivity domain (R= reflection properties of seismic waves). In the present forward version one can import a model (e. g. Eclipse) from a fluid simulation (F) at different time steps, and define G at each step. By forward modelling procedures one can do the transformation G->E->R, and from R calculate a seismic response (S). This forward procedure is e. g. used for predicting the 4D seismic differences for varying fluid scenarios (4D feasibility studies). The project focuses on processes in the inverse work-flow, that is, S->R->E->G->F, aiming at modelling changes in the fluid parameters from observed differences in 4D seismic data. To be able to perform the above transformations in a proper (unique) way, assumptions or constraints must be added at critical steps in the process. An important add-on functionality of the SeisRoX system is the possibility of perturbing the model parameters in any chosen domain, and calculating the implications in the other domains. This allows valuable sensitivity analysis at all stages of the modelling process within a unified and consistent framework.
This project is funded by the Research Council of Norway as a Strategic Institute Program.
The SeisRoX modelling framework.
The core is the multi-domain model, travelling between the geological domain, the elastic domain, and the reflectivity domain. A main objective of the project is to establish both forward and inverse transformations between the model domains, also including external constraints.
|
