Reactive Transport Modelling for a More Rigorous Scaling Risk Evaluation and Mitigation in a Giant U.A.E Carbonate Field

Abstract

After years of peripheral water injection with no significant scaling issues, pattern water injection and water injection at the GOC (Inner Ring Water Injection, or IRWI) are planned to be implemented in various reservoirs of this giant field. In a few pilot pairs, seawater injection is already taking place at a smaller spacing than historically applied. This allows testing of the injection schemes and assessment of the effect of heterogeneities before deploying pattern water injection and IRWI in the longer term. In this context, the scaling risk at the producer has been evaluated. The scaling risk assessment carried out with a thermodynamic prediction model has shown both SrSO4 and CaSO4 risks due to the mixing of formation water with injected seawater. This modelling fails to take account of geochemical reactions occurring in the reservoir; consequently, the scaling risk is usually overestimated. In this work, a reactive transport reservoir modelling tool has been used to investigate the impact of injection water composition on the scaling risk at the producer. In this model, the following are incorporated: aqueous component transport, partitioning of CO2 between aqueous and hydrocarbon phases, aqueous speciation reactions, mineral precipitation/dissolution reactions and heat transport. The simulations have considered full and reduced sulfate seawater injection with and without the presence of a thief zone. When full seawater is injected, the producer faces a risk of CaSO4 and no risk of SrSO4. This is the consequence of various coupled in situ mineral reactions, including dissolution and precipitation of carbonates and sulfates, which depend on propagation of temperature and CO2 desaturation fronts, as well as the other aqueous components. With the presence of a thief zone, SrSO4 presents a small scaling risk soon after seawater breakthrough; CaSO4 deposition has an initial peak soon after SrSO4 scaling. When reduced sulfate seawater is injected with and without the presence of the thief zone, there is no scaling risk for either SrSO4 or CaSO4. The results obtained by the reactive transport modelling tool match the general trends of scale deposition observed in the pattern injection well pair pilot to date. In this pilot a thief zone was identified in the vicinity of the injector and has contributed to accelerated water breakthrough at the producer. A peak in SrSO4 scale was observed in the early phase of water production, in agreement with the modelling results. A geochemical transport reservoir model was able to provide a full picture of seawater breakthrough at the production well, considering the impact of the thief zone. The required level of sulfate in the injected seawater, to prevent sulfate scales at the producer, has been determined. These results will help determine the scale mitigation strategy for the future development of this field.