Modeling Enhanced In Situ CO2 Mineralization in the Samail Ophiolite Aquifer

The Samail Ophiolite aquifer in the Sultanate of Oman is a site of exceptionally well-developed naturally occurring in situ CO2 mineralization, and serves as a natural analog for an engineered CO2 sequestration process. Natural processes within the aquifer can be described by the following reactions [e.g.1,2]: near the surface, infiltrating rainwater dissolves peridotite, increasing dissolved Mg, Ca, and Si; interaction with soil CO2 and carbonate rocks and dust further increases Ca and dissolved C. At deeper levels, groundwater is cut off from the atmosphere-and hence its CO2 source- but continues to dissolve peridotite, and precipitates serpentine, magnesite, and dolomite. The resulting water has a high Ca-OH concentration, essentially no Mg or dissolved C, and ultrabasic pH. When this alkaline water reaches the shallow subsurface or surface, it mixes with CO2-saturated shallow groundwater or absorbs CO2 directly from the atmosphere. Dissolved C reacts with Ca to precipitate calcite on the surface, lowering the pH to basic. This process forms abundant carbonate minerals, both in the subsurface and in surficial travertine terraces. Water chemistry data can be used to determine the amount of CO2 sequestered. The quantity of CO2 mineralized at the surface as CaCO3 can be calculated from the removal of Ca from alkaline water once it discharges at springs, assuming CaCO3 precipitation is the only surficial Ca sink. Water samples from 22 alkaline spring outlets and 16 surface water bodies were used to calculate the average decrease in Ca and increase in TIC as alkaline spring water discharges and flows along the surface, losing its high pH and converting to basic surface water; the values are 1.26 mmol/L Ca and 3.13 mmol/L TIC, respectively. The increase in TIC can be attributed to absorption of atmospheric CO2. In regions with known flow rates, it is possible to determine the total amount of CO2 mineralized annually. For example, near Masibt where the flow rate of a single spring is 3x107 L/yr, the annual loss of Ca is 3.8x104 moles/yr and the amount of CO2 mineralized as CaCO3 by that spring is 0.85 kg/yr. Over 70 alkaline springs have been mapped throughout the Samail Ophiolite3, and doubtless many more exist. At the surface, Ca availability limits carbonate mineral formation; however, in the subsurface, dissolved CO2 must be the limiting species. TIC decreases from 3.24 mmol/L in shallow groundwater to 0.27 mmol/L in alkaline springs. The loss of 2.96 mmol/L TIC likely occurs by magnesite precipitation, meaning that this amount of CO2 is mineralized in the subsurface. If the availability of dissolved CO2 is the limiting factor in mineralization by the Samail Ophiolite aquifer, it may be possible to engineer the system to increase the rate of sequestration by injecting CO2 into the aquifer. To simulate the outcome of such an engineered system, data from the natural system have been incorporated into a reactive transport model. Results of this simulation will be presented. 1Barnes and O’Neil, 1969; 2Bruni et al., 2002; 3Stanger, 1986