Optimization of Injection Brine Composition and Impact of Geochemical Reactivity to Reduce Mineral Scaling Risk

Abstract Scale problems pose significant challenges during oilfield production, especially when water flooding is employed. Mixing incompatible injection and formation brines can lead to the deposition of inorganic scales, such as barite, celestite, dolomite, and anhydrite, in production wells. This issue is well-documented in seawater-flooded clastic reservoirs. One technique to prevent the resulting formation damage is to remove sulphate from seawater before injection using nanofiltration; however, this process is costly. This study considers these processes in two carbonate reservoirs, and the impact on the scaling risk at the wells. This study describes the use of reactive transport reservoir simulation to investigate the impact of carbon dioxide partitioning and changes in pH, ionic concentrations, and temperature on carbonate reactivity and sulphate scaling risk in waterflooded carbonate reservoirs. The models calculate the dissolution and precipitation of calcite, dolomite, gypsum, anhydrite and barite, finding that these processes are coupled through various common ion effects. The compositions of the produced brine are used to calculate the scaling risk in the two fields, and to what extent CO2 partitioning from residual oil to injected water affects the scaling behaviour. In this research, seawater without any dissolved CO2 is considered as the injection brine. Also, various factors impact the system, for instance, compositional effects arising from the use of different types of seawater such as Full Sulphate Seawater (FSSW) and Low Sulphate Seawater (LSSW). The results indicate that SO42-, Mg2+, HCO3-, and Ca2+ are the principal ions exerting a significant influence on the system, thereby affecting the precipitation and dissolution of calcite, dolomite, anhydrite (or gypsum at lower temperatures), and barite. Furthermore, although there is no single ion that links all the reactions, all the reactions are directly or indirectly coupled, since no reaction involves ions that are not also involved in at least one other reaction. Particularly noteworthy is the dissolution of anhydrite, which is part of the initial mineral assemblage and has a significant impact on most scenarios examined. However, in cases where Full Sulphate Seawater (FSSW) is heated to reservoir temperature, in situ precipitation of anhydrite occurs, altering the outcome. On the other hand, the availability of carbonate minerals and anhydrite, and the temperature are identified as the primary factors influencing the precipitation (or even dissolution during LSSW injection) of barite, since the anhydrite reactions determine the SO42- concentrations. Of note is the finding that injected SO42- concentrations can impact the final fluid pH, since, first of all, CO2 solubility in the injected water is affected, CO2 is partitioning from the residual oil into the injected brine, and, secondly, the injected SO42- concentrations impact the extent of the anhydrite and/or gypsum reactions, affecting the Ca2+ concentrations, and hence the balance of the calcite and dolomite reactions, which influence pH.