Coupled geomechanics-flow modelling at and below a critical stress state used to investigate common statistical properties of field production data

An areal model of a fractured/faulted reservoir with 49 wells is developed that incorporates fully-coupled geo-mechanics and fluid flow. It is a generic example of a pattern water-flood although it is inspired by a parallel study of the Gullfaks reservoir in the North Sea, in which stress-related, fault-related and long-range correlations in rate fluctuations are observed. Based on this model, three scenarios are examined in terms of different initial stress states prior to production, each of which involves 36 months of production and injection in the presence of fracture sets and faults. The results support the concept that the long-range, stress-related and fault-related characteristics of correlations in rate fluctuations, observed not only in the Gullfaks data, but also in several other fields worldwide, are symptomatic of a system near a geomechanical critical point. These characteristics are not observed in models that are sub-critical. Short-range rate correlations are likely to exist where there are highly permeable zones between producers and injectors. Long-range rate correlations occur only within critically-stressed regions where there is active shearin or fault reactivation. The modelling results are consistent with field evidence suggesting that incipient shearing is an important mechanism coupled with reservoir flow behaviour. © The Geological Society of London 2007.