On the Impact of Reservoir Overburden Heterogeneity on Subsidence Modelling

There is an increased need for modelling the Dutch overburden to improve the prediction of earthquakes and subsidence due to the extraction of hydrocarbons. Depending on the surface location, the subsidence model of the NAM can under or over predict the amount of subsidence by 4 to 5 cm. This is around 15 % of the current maximum subsidence of 30 cm. The underlying assumption of the NAM subsidence model is that the reservoir, and the overburden above it, behave the same and thus have the same elastic parameters. The purpose of this research is to obtain a better understanding of the role overburden heterogeneities play in man induced subsidence. The main research question is: To what extent do overburden heterogeneities affect subsidence caused by reservoir depletion? Followed by the hypothesis: The heterogeneities of the overburden affect subsidence and should therefore be incorporated. The results show that modelling the overburden as multi-layered, rather than as one homogeneous layer, leads to a difference of 0.75 cm. This is greater than the measurement error of the InSAR data, which is used to determine the subsidence of the Groningen field, and is significant when compared to the current discrepancy of 4 to 5 cm. Elastic parameters have been calculated from acoustic well data for geological units around the Groningen area. The calculated values differ from the values used by the NAM. When the calculated values are used for the Upper North Sea and Lower North Sea groups, the difference with the reference model is up to 2 cm for a reservoir radius of 3 km and 0.87 cm for a Groningen scale reservoir. The results suggest that the heterogeneities of the overburden affect subsidence significantly enough to warrant further investigation. Due to the fact that the obtained results are for horizontal layers, there is much potential for more complex overburden geology e.g. salt structures and non-horizontal layers.