This paper presents an analysis of the water injection conditions on a vast oil field complex operated by StatoilHydro in the Norwegian sector of the North Sea over a fifteen years period. The injection data were analysed from a geomechanical perspective such as to identify the main parameters governing the variations of fracturing pressure over the life of the wells, which in turn governs efficient and safe water injection.
The data highlight the importance of thermal effects in fracturing pressure variations even in relatively weak formations, where theory would deem them small based upon laboratory measurements. The magnitude of thermal effects quantified from field measurements is about one order of magnitude larger than what was expected a-priori from laboratory measurements.
It was also possible to track the evolution of the fracturing pressure of single wells for various reservoir pressures during phases of reservoir re-pressurisation. Here again the field data indicate behaviours that were not expected from a theoretical point of view – i.e. a constant fracturing pressure during reservoir re-pressurisation, while theory would predict a significant increase.
The practical implications of the study when designing water injection schemes on new fields are highlighted and examples are given on how the current theoretical short-comings identified during the study can be successfully by-passed in real life engineering. The need for R&D efforts aimed at matching the gap between field observations and existing theories is also emphasised.