Managing Injectors Impairment in a Deepwater Field

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Abstract In this deepwater soft rock field injected water is equivalent to oil as the reservoir management strategy is to maintain 100% voidage replacement and keep reservoir pressures above bubble point. The producers are high rate wells operating at 10-50kbpd range. This means the injectors are always expected to inject reliably at high rates. However, over the life history of the field, waterflood performance has been one of the limiting constraints on achieving full potential of the field. Water injectors' impairment and failure has contributed immensely to this challenge of waterflood performance. A lot of efforts have gone into understanding the cause of these impairments and failures and the means to eliminate them. At the beginning of the field life, the impairment observed was mostly attributed to poor injection water quality. Water hammers resulting from trips were also identified as another cause of impairment. The earlier wells were completed as Frac & Pack wells and there were attempts to understand how this completion types impact the impairment particularly as regards whether the proppants will be intact after significant amount of injection. In recent times, horizontal wells are being drilled and though the data from them is still limited, possible impairment models are being developed for them as well. The choice of the mode of injection – fractured or matrix – also seems to impact on the impairment mechanism of injectors. Increase of the injection pump discharge pressure have been attempted to manage wells' rate and pressure interaction with each other through the subsea flowlines. The extent to which this can be carried out is limited by the pressure rating of topsides, the subsea equipment and the shale (cap rock) strength. Consideration of all these factors has improved understanding of impairment mechanism which in combination with learning's from operational procedure that led to failure of injector wells has resulted in better management and improved the operating envelope for these wells. Reduced failure rates have been observed in longer well's life for newer wells than earlier ones. It has also led to development of mitigation measures like backflushes, refracs and acid stimulation when impairment is being observed. There is still a large area of uncertainties or gaps in current understanding, although plans are being made for data gathering e.g. HUD checks, downhole gauges, injection profile logs and temperature warm- back logs to further to close the gaps in the existing impairment models for the water injectors.