4D modelling refers to monitoring the changes in a reservoir over time. This is done to ensure that the production of a reservoir is optimised.
Various activities fall under the umbrella of 4D modelling:
- The first is forward modelling. This involves modelling the effect that changes in the saturation and pressure, due to the production of a reservoir, will have on seismic. This can be done to determine when to shoot repeat surveys. I.e. it answers the question: How much does my geology have to change before it is measurable with seismic.
- The second is fluid injection/depletion mapping – or reservoir monitoring. Given that we don’t expect rocks to move around – at least not quickly - mapping changes in the seismic tells us where our hydrocarbons are being drained and where injected fluids are going. This is invaluable information to production engineers who wish to know if their injector and production wells are performing as intended. Reservoir monitoring provides a qualitative map of changes in the reservoir; however depending on the geology, it is sometimes possible to apply reservoir characterisation workflows to quantify changes in saturation, pressure or both.
- The third objective of 4D modelling is to validate reservoir models – sometimes referred to as “closing the loop”. Using rock physics as the link between geology and seismic, it is possible take a geological model and simulate its seismic response (create a synthetic seismic volume). As the geological model is progressed through time new synthetic seismic responses are modelled. The seismic response from the model is then compared to the actual 4D seismic surveys. A mismatch between the measured and modelled seismic response indicates that the model needs to be updated. The model can then be updated and the process repeated until a match is achieved. In this way we have closed the loop between the reservoir model and the measured seismic response.