Alkaline Earth Metals Enhance CO2 Mineralization in Saline Aquifers

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225534, “Enhanced CO2 Mineralization in Saline Aquifers Using Alkaline Earth Metals,” by Tatenda R. Marapira, SPE, Saad F. Alafnan, SPE, and Arshad Raza, King Fahd University of Petroleum and Minerals, et al. The paper has not been peer-reviewed. _ Subsurface saline aquifer formations have emerged as a promising natural sink for mineral CO2 storage. However, a disconnect exists between the speed and permanence of the different trapping mechanisms that come into play when CO2 is injected into subsurface formations. This study aims to investigate the efficacy of chemical additives strontium chloride (SrCl2) and barium hydroxide (Ba(OH)2) in accelerating the two slowest CO2 trapping mechanisms, dissolution and mineralization; the chemicals’ effect on the dissolution of the carbonate-bearing minerals; and the changes in poroelastic properties of the host rock. Methodology Reactants. CO2 gas with a purity of 99.99% was used as the nonaqueous fluid phase in the experiment. Three 12-in. limestone cores were used as a representative reference for a carbonate reservoir. These cores were cut and shaped into smaller samples with specific dimensions suitable for the various analyses. A synthetic brine solution mirroring the constituents of natural seawater was prepared to provide a realistic representation of the fluids present in subsurface reservoirs by diluting sodium chloride, hydrated magnesium and calcium chlorides, sodium bicarbonate, and sodium sulfate in deionized water. To identify the effect of additives on the precipitation process, Ba(OH)2 and SrCl2 were used as additives to the brine solution in two batch experiments (Batch A and Batch B) at concentrations of 0.01 M to investigate their combined effects on carbon dissolution and mineralization by adding them after treatment for Batch A and at the beginning of treatment for Batch B. Batch A Experimental Setup. One 12-in. core plug of Indiana limestone was prepared into seven samples of varying sizes highlighted in Table 2 of the complete paper. The samples in Batch A were then saturated with the synthetic seawater over 24 hours using a vacuum cell and added to the bottom of a high pressure, high-temperature (HP/HT) autoclave. Three liters of synthetic brine were prepared and added to the autoclave and a vacuum applied for 15 minutes. High-purity CO2 (5,860 ml) was then added to the autoclave until a pressure of 1,400 psi was achieved. The autoclave was heated to a temperature of 333.15 K and left for 48 hours before the first effluent sample was taken. Pressure readings were recorded daily. Effluent samples were collected from the bottom of the autoclave at 48-hour intervals three times weekly. The autoclave was depressurized after 45 days to prevent fracturing in the samples. After treatment, the largest samples (1A, 2A, and 3A) were cleaned over 48 hours in methanol, and then together with the smaller samples dried at 348 K for 24 hours before further analysis. SrCl2.6H2O (6.799 g) and Ba(OH)2.8H2O (8.044 g) were added to the carbonated brine and the precipitate filtered out. The core samples were returned to the autoclave and the process repeated using the additive-rich carbonated brine in lieu of the original seawater formulation (Batch A2) over 19 days, after which the final analyses were run on the samples.