Predicting Brine Mixing Deep Within the Reservoir, and the Impact on Scale Control in Marginal and Deepwater Developments

Sulphate scale deposition is a common problem in hydrocarbon reservoirs where injection seawater, rich in sulphate, mixes with formation brines, rich in barium, strontium and calcium. The deposition of these scales can cause significant production impairment if it occurs within zones near the production wellbores. To control scale deposition within the near wellbore region of a reservoir, scale squeeze treatments are commonly deployed. In cases where the scale severity is very high, removal of sulphate ions from the injection water is an alternative scale control strategy. Both these methods of mitigation have associated CAPEX (e.g. desulphation plant) and OPEX (e.g. scale squeeze treatments) costs. Typically, to assess the severity of the problem in new fields, thermodynamic calculations are performed to calculate the mass of scale that will form. Until present there has been little work carried out to identify the location of scale formation within the reservoir. In this paper, field data and flow simulations from three North Sea fields are presented to show that the formation of scale can in fact occur deep within the reservoir, and have negligible negative impact on oil production within the near wellbore region. Evidence is presented from these North Sea fields that shows the evolution of the brine chemistry as seawater breakthrough occurs and squeeze treatments were applied. The evidence from the produced brine chemistry is linked to flow calculations for these fields to show that in some systems scale is depositing deep within the reservoir, reducing the potential for damage in the near production wellbore region. The extent and impact of the deposition varies throughout the reservoir and can be quantified. The ability to model brine mixing and stripping of the scaling ions before the fluids reach the production wellbore has a very significant impact on the economic assessment of marginal fields and deepwater developments. In such fields, the technical challenge and cost (CAPEX/OPEX) of scale control might make development un-economic. This paper outlines the data requirements and methodology used to allow such an assessment to be made.