Effect of Limestone and Buffer Solution in the Aqueous Speciation and pH of Brines for CO₂ Sequestration

Oil-field brines, a by-product of oil and natural gas extraction whose fate is mainly disposal, is gaining attention for the storage of CO₂ into geologically stable mineral carbonates. The suitable pH range for the formation of carbonates is 7.8 or higher, where CO₃ ^2- dominates. Therefore, to boost the precipitation of mineral carbonates by reaction between brine and CO₂, the pH of the brine must be modified before any brine-CO₂ interaction takes place. In this work, pH stability studies were conducted to study how synthetic brines respond in the presence of a natural limestone host rock or a tris buffer solution in both closed and open atmospheres and to evaluate the efficiency of both buffers at increasing brine pH. An aqueous and solid speciation model was used to investigate the influence of the buffer used in the aqueous and solid speciation of two synthetic brines. pH stability studies identified that both buffers can enhance brine pH, being tris buffer the one with the strongest buffer efficiency. Both XRD and geochemical modelling results suggest the suitability of brines containing SO₄ ^2-and the use of both host rock and tris buffer, respectively, to enhance oil-field brines pH for mineral carbonates precipitation.