The evidence from the produced brine chemistry suggests that the Gyda field has experienced a variety of geochemical reactions due to the high temperature and initial calcium concentration, and so it is worth reviewing the produced water dataset and studying what in situ geochemical reactions may be taking place.

Produced brine chemistry data from 16 wells in the Gyda field are plotted and analysed in combination with general geological information and the reservoir description. A one dimensional reactive transport model is developed to identify the possible geochemical reactions occurring within the reservoir triggered by seawater injection, then extended with the inclusion of thermal modelling and also to be a two dimensional vertical cross section model.

Three possible classes of formation water compositions in different regions of the Gyda field have been identified by analysis of the produced water dataset. Anhydrite and barite precipitation are the two dominant mineral reactions taking place deep within the reservoir. Magnesium stripping may be a result of multi-component ion exchange, dolomite precipitation or a combination of both. Reservoir temperature is lowered during cold water injection. The solubility of anhydrite increases at lower temperature, and anhydrite will gradually dissolve in response to the movement of the temperature front, which is much slower than the formation/injection water mixing front. The extent of mineral precipitation within the reservoir can be reduced by the heterogeneity; the modelling shows that the extent of ion stripping caused by mineral reactions in the reservoir is greatest when simulating a single uniform layer. Brine mixing and the occurrence of geochemical reactions due to vertical mixing are not observable, even when assigning a high vertical permeability in a heterogeneous model.

Thermal modelling is included to evaluate the effect of non-isothermal processes and heat transport on the geochemical reactions, especially the anhydrite mineral reaction. We have investigated how the difference in horizontal permeability in the two layers affects brine mixing of formation and injection water and geochemical reactions.

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