In order for CO2 to remain in a supercritical phase, the pressure in the storage reservoir must be greater than about 68 atmospheres and at temperatures above 31.1°C. (Sminchak et al., 2001). These conditions require that the CO2 be injected at high pressures, which can only be achieved at depths greater than 2,600 feet below the earth’s surface. Unfortunately at this depth CO2 is both supercritical and buoyant. As a result, the buoyant CO2 may migrate back to the shallow subsurface and surface. Keeping the CO2 within the formation forever is the challenge. The effectiveness of geologic storage of CO2 depends on the retention time, reservoir stability, and the risk of leakage which is a huge technical hurdle. Long term the geologic formation may allow the buoyant CO2 to bubble up and leak out of the rocks, reducing the effectiveness of the scheme and potentially contaminating shallow groundwater. Injection sites would have to be monitored for decades if not centuries.
|Image from J.M. Mather presentation|
Another approach for the permanent storage of CO2 is the mineralization of CO2 into stable carbonate minerals such as calcite (CaCO3), dolomite (CaMg (CO3)2), magnesite (MgCO3) and siderite (FeCO3) that would lock up the CO2 in the rocks themselves. It was proposed that this mineralization could be accomplished in-situ in basaltic (silicate) rocks using CO2 fully dissolved in water. Mineral carbonation can theoretically occur in many kinds of rock, but often it is extremely slow. The CarbFix approach accelerates the process by injecting into basalt, a very reactive rock. Basaltic rock is young rock forming where basaltic magma rises from deep within the earth to form new crust. There are many places on earth where this is occurring: the Mid-Atlantic Ridge Mid-Atlantic Ridge, the Pacific Northwest and Iceland are three. Small scale testing of the concept showed that CO2 could be locked in rocks and currently the Big Sky Carbon Sequestration Partnership project in Washington and CarbFix project in Iceland are testing the concept.
Big Sky Carbon Sequestration Partnership relies on existing technologies and is a small scale demonstration of concept project. CarbFix is a pilot project designed to inject 2.200 tons of CO2 per year in Iceland to test the feasibility of in situ mineral carbonation in basaltic rocks in the real world. At the test site in Hellisheidi in the southwest portion of Iceland, the CarbFix project has been in operation for several years. The project was created to optimize in situ mineral carbonation in basalt. The pilot consists of a CO2 pilot gas separation plant, CO2 injection pilot test, laboratory-based experiments, confirmation testing and numerical modeling.
|Image from J.M. Mather presentation|
The CarbFix group developed a new injection system was used to mix the captured and separated CO2 with groundwater in a ratio of one pound of CO2 per 28.5 pounds of groundwater. The injection system delivers the water and CO2 mixture 1,600 feet beneath the surface into the target storage formation. The scientists tagged the CO2 collected with radiocarbon (14C) by adding 14C to the groundwater so that they track and document the mineralization process in basaltic rocks in Icelandic geothermal fields and differentiate between the natural mineral formation and the in-situ CO2 fixing.
Now the scientists have taken a series of core samples to examine the extent of in-situ mineralization of the injected CO2. The project’s implications for the fight against global warming are unknown, though basaltic bedrock susceptive of CO2 injections are widely found on the planet, the costs of capturing the CO2, transporting and mixing with vast quantities of water and injecting into the rock are huge. According to recent Department of Energy estimates, the United States and portions of Canada have enough potential capacity in geologic formations to store as much as 900 years of CO2 emissions (at whatever level of emission they used to estimate that).
Capturing and transporting CO2 from industrial plants is technologically possible but is currently prohibitively expensive, and the additional cost for the CO2 fixation is estimated at $17/ton. World emissions were about 38 billion tons in 2012. Nonetheless, the technology may lead somewhere or be useful in our understanding of our planet or the survival of Homo sapiens on the planet.