Kinetics of in Situ Calcium Magnesium Carbonate Precipitation and the Need for De-Sulphation in Seawater Flooded Carbonate Reservoirs

Abstract

Abstract Mixing of incompatible injection and formation brines leads to the deposition of inorganic sulphate scales such as barite, celestite and anhydrite in and around production wells. This process is well documented in seawater-flooded clastic reservoirs. One technique to avoid the resulting formation damage is to remove sulphate from seawater before injection, using nanofiltration; however, this process is costly. This paper identifies that it may not always be necessary in higher temperature carbonate reservoirs. This paper 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 the sulphate scaling risk in waterflooded carbonate reservoirs. Dissolution and precipitation of calcite, dolomite, gypsum, anhydrite, barite and celestite are all modelled and found to be coupled through (various) common ion effects. The produced brine compositions are used to calculate the saturation ratios and mass of precipitate that may form in the production system. Sensitivity to mineral reaction kinetics, particularly for the dolomite reactions, is accounted for. Results identify that there is a strong relationship between calcite dissolution and dolomite (or other calcium/magnesium carbonate mineral) precipitation reactions, which drive each other and are affected by availability of carbon dioxide in the residual oil phase. This evolves over time and as the thermal front propagates, impacting the concentration of calcium and magnesium in the brines traversing the reservoir. Temperature changes around the injection wellbore impact carbon dioxide and mineral solubilities. The concentration of calcium in the displaced brine mix is thus determined more by contact with rock and temperature than by mixing between injection and formation brines. Depending on location relative to the thermal front, this may lead to gypsum or anhydrite precipitation, thereby stripping sulphate out of the injection brine. Thus, the sulphate scaling risk at the production wells is significantly reduced by this sulphate depletion process: the sulphate is stripped out of the seawater as it warms up in the reservoir, before it mixes extensively with the formation water, and significantly before any mixture of the two brines reaches the production zone. Thus, any loss of permeability is restricted to deep within the reservoir, where the pore volume that can accommodate mineral precipitation is very large. This work identifies that for carbonate reservoirs above 90°C to 100°C, stripping of sulphate due to coupled mineral reactions may reduce or eliminate the need for use of a sulphate reduction plant. The process is modelled for the first time, accounting for the impact of carbon dioxide partitioning and thermal front propagation. Knowledge of the kinetics of calcium/magnesium carbonate precipitation is shown to be critical in predicting the extent of sulphate depletion.