Geomechanical Aspects of 15 Years Water Injection on a Field Complex: An Analysis of the Past to Plan the Future

Abstract

Abstract 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. Introduction Efficient water injection in oil reservoirs is often performed under fracturing conditions such as to avoid the plugging of the sand face by dirt contained in the produced water. The fact is that even if the injected water is cleaned up on surface it will gather particles in the surface lines and in the injection tubing - e.g. pipe dope, corrosion particles, etc. - and contain significant amount of solids down hole 8. Injecting above fracturing pressure is then the only solution to maintain injectivity on the wells. As a consequence predicting the fracturing pressure of the wells throughout the life of a field is therefore of paramount importance for efficient design - e.g. decision about the pumping capacity. From a theoretical perspective the fracturing pressure of a well is known to vary with both temperature 1,6,8 - i.e. the cooler the injection fluid the lower the fracturing pressure - and reservoir depletion 2–5,7 - i.e. the larger the depletion the lower the fracturing pressure. As a consequence it was decided to gather the fracturing data recorded over a fifteen years period on a field complex operated by StatoilHydro in the Norwegian North Sea such as to check the evolution of fracturing pressure with both parameters. The purpose of the study was twofold.The first goal was to check the applicability of existing theories predicting the evolution of the fracturing pressure with various parameters during the life of a field - i.e. essentially reservoir pressure and to a lesser extent temperature.The second goal was to establish a solid base of field data to be used for the design of water injection schemes in future fields - e.g. pump pressure capacity.