Abby L. Harvey
GHG Monitor
1/9/2015
The presence of silicate-rich rock in reservoirs to be used for carbon dioxide storage could hinder the flow of the substance, lowering the efficiency of the process and perhaps increasing the risk of leakage, according to a new study published late last month in Nature Communications. Because of the geochemical reactions between CO2 and silicate-rich rock within potential reservoirs, the CO2 dissolves and falls to depth at a much slower rate than optimal, lead researcher Silvana Cardoso of the University of Cambridge explained to GHG Monitor this week. “What we’re going to get is the CO2 ponding at the top very slowly reacting with the nearby layers of rock but not being transported very efficiently or very fast to depth, it will just be transported by molecular diffusion which is way way slower than the movement of the fluid by convection,” Cardoso said.
This study is unlike others done concerning the movement of CO2 within reservoirs because it takes into account geochemical reactions. Discounting these reactions, researchers “treat [storage] as an inert system,” Cardoso said. “Everyone’s been saying that as the CO2 dissolves in the formation water which is saline water – brine — that it makes it denser and because it makes in denser it moves the carbon to depth,” she went on. “When we add geochemical reactions — which is what we did — we find that for some of them, that’s true. For example, for carbonate rock then the reaction itself even makes the solution denser … and so that carried the CO2 very efficiently to depth, but other rocks like silicate rocks … that’s not true because part of the CO2 is being removed and incorporated into the solid rock, then these currents that would transport the CO2 to depth get very weak and they actually can be completely shut off under some conditions.”
The study looks specifically at reactions with calcium feldspar when considering the mineral makeup of potential storage reservoirs. In her experiment, Cardoso found that “convection is predicted to initiate [one] month after injection of the carbon dioxide, with convection streams as wide as 1m. However, these streams remain weak and completely shut-off after a short period of only [two] months. After this, the carbon dioxide will be transported by much slower diffusional processes. The results presented in this work have important practical implications for storage of carbon dioxide in saline aquifers, enabling informed screening of the most effective sites,” according to the study.
Storage in Silicate-Rich Reservoirs Prolongs Risk of Leak
Because the CO2 in a silicate-rich reservoir would remain pooled at the top of the reservoir in a gaseous state for a longer period of time than in other more efficient reservoirs, the risk of that CO2 leaking in the case of a fracture in the cap rock is somewhat greater. Once CO2 stored underground is dissolved into the brine water and eventually settled to the bottom of the reservoir into a solid form, the risk of leakage is smaller because the substance is denser than its surroundings and thus lacks the buoyance necessary for it to escape, Cardoso said. “What ideally we’d like is the CO2 to go into the water phase because then it becomes heavier than the surroundings and then even if the cap rock of the saline aquifer cracks it won’t escape upwards because it doesn’t have the buoyancy so we’d like it to dissolve and we’d like it to dissolve as much as possible fast and then after that reaction, be incorporated into the solid phase. That’s the ideal scenario. What this says is the CO2 going into the acquis phase is not going to be as fast as we might have thought so there is more risk that if there is a rock fracture, the CO2 may be released because it’s going to stay in the supercritical state or in the gaseous state longer and it’s not going to use the whole reservoir efficiently to eventually be absorbed into solid form. So there might be more risk of leakage, but that risk depends, of course, on the fracture possibility,” she said.
