Relaxation of Low-Sulfate Seawater Injection Based on Geochemical Reservoir Modeling

Injection of Low Sulfate Seawater (LSSW) instead of untreated Full Sulfate Seawater (FSSW) is widely used to reduce the barite scaling risk at production wells. However, LSSW injection may no longer be required when the barium concentrations in the produced water drop below a certain threshold. Relaxation of requirements for the sulfate Removal Plant (SRP) can significantly reduce operational costs. The case study concerns an African field which presents a shortage of production because of operational difficulties with membrane nanofiltration showing that it can be difficult to inject the required volume of LSSW for maintaining the pressure of the reservoir by respecting the maximum specification of 40mg/L of sulfate in the injection desulfated seawater. This study investigates the timing and degree of relaxation of the output sulfate concentration from the SRPs required to ensure barite risks are minimized in the nine production wells supported by LSSW injection since 2017. With the help of a reactive transport geochemical model, the BaSO₄ scaling risk at producers has been evaluated. The output of the simulations is water composition and rate profiles at the production wells, which is then used to identify the evolution of the BaSO₄ scale risk in each well. For a temporary switch from LSSW (SO₄=20mg/L) to FSSW in July 2021 before returning to LSSW again 6 months later, scaling risk may increase temporarily for three wells within a year’s time of the switch. For temporary switch (6 months) from 20mg/L to 100mg/L of sulfate then back to 20 mg/L of sulfate, the simulations showed that scaling risk may increase temporarily for these three wells. If this switch is considered, it is required to monitor these wells for 2-3 years after the switch in case some chemical management is required. For a permanent switch from 20mg/L to 50mg/L and 60mg/L of sulfate until the end-of-life production, the simulation results showed different behavior varying from no scaling risk to high scaling risk. For a permanent switch from 20mg/L to 100mg/L of sulfate until the end-of-life production, the simulation results showed that scaling risk may increase temporarily for two wells and scaling risk will increase significantly for six wells. This study showed that permanent switch to 100mg/L sulfate should be avoided due to the significant increase in scaling risk that would arise and a temporary (6 months) switch to 100mg/L sulfate may be considered, with monitoring of some specific wells for 2-3 years after switch. A close collaboration between well performance and flow assurance teams is crucial to validate the conclusions from the modeling work; this will contribute in the decisions to be taken for future management of scale mitigation in this field.