An empirical relationship for the seismic activity rate of the Groningen gas field

The Groningen field is the largest onshore gas field in Europe. Continuous production since 1963 has led to induced seismicity starting in the early 1990s. Production measures aimed at lowering the level of seismicity have been implemented since 2014. In this paper we start from an empirical relationship between the cumulative number of seismic events and cumulative gas production. We show that a better way to analyse the data is to relate the ratio of activity rate over production rate versus the cumulative production, such that the model parameters and their uncertainty can be determined. This also allows us to make predictions including the confidence intervals. Using this model, we first performed regression analysis based on the larger seismic catalogue which includes all recorded events with a magnitude of 1.3 and larger, because we consider this value to be the magnitude of completeness since 1995. We have also performed regression analysis based on a smaller seismic catalogue consisting of all events with a magnitude of 1.5 and larger. This was done in order to be able to compare our forecast with forecasts performed by others. Our prediction for 2016, based on the seismic catalogue of all events with a magnitude of M≥1.5 (using only the events recorded before 2016), was 16±8 events. By the end of 2016, 13 such events had been recorded. We discuss a number of factors which may influence the predictive power of the derived relationship and which require further study. For instance, we consider the delay between production and earthquakes which increases with decreasing reservoir pressure. In addition, the effect of seasonal fluctuation in Groningen production should be considered. Future work can be done to include these effects in the empirical model. We also investigated the challenges related to the applicability of the analysis to sub-regions of the Groningen field.