Impact of Reservoir on Relaxation of Requirement for SRP Whilst Protecting Production - Reactive Transport Modelling for a Mature Offshore Field

Injection of Low Sulphate Seawater (LSSW) is widely used to reduce the barite scaling risk. LSSW may be used in addition to Produced Water Re-Injection (PWRI). LSSW injection may no longer be required when the barium concentrations in the produced water decline. This study investigates the timing and degree of relaxation of the output sulphate concentration from a Sulphate Reduction Plant (SRP) required to minimize the barite risk, accounting for the impact of PWRI. Reactive transport modelling is used to calculate brine mixing and in situ precipitation. The output of these calculations is water composition and rate profiles at the production wells, which is then used to identify the evolution of the barium sulphate scale risk in each well. Five scenarios have been considered: a one-off increase of sulphate, Full Sulphate Seawater (FSSW) injection, gradual increase of sulphate concentration over a given period, and a mixture with PWRI, where the injected brine is assumed to be 95% formation brine and 5% FSSW. Both aqueous speciation reactions and carbonate and sulphate mineral reactions are modelled. Different injection strategies are addressed, based on a geochemical transport reservoir model of a mature field. The tolerance of sulphate increase for injection brine will help in determining relaxation requirements for the SRP to reduce operational costs as well as seeking other possible late field development strategies (e.g. PWRI).