Storage of CO2 as hydrate beneath the ocean floor
Farhad Qanbari, Mehrand Pooladi-Darvish, S. Hamed Tabatabaie, Shahab Gerami
With the increasing concern about climate change, the public, industry and government are showing increased interest towards reduction of CO 2 emissions. Geological storage of CO 2 is perceived to be one of the most promising methods that could allow significant reduction in CO 2 emissions over the short and medium term. One major concern against geological storage of CO 2 is the possibility of its leakage. Carbon dioxide under the pressure and temperature conditions encountered in typical deep
... aquifers remains more buoyant than water. One process that could lead to permanent trapping of CO 2 is one that includes geochemical reactions leading to the formation of solid minerals. However, the time-scale of such reactions is perceived to be centuries to millennia. In contrast, the kinetics of CO 2 -hydrate formation -that leads to trapping of CO 2 in the solid form -is quite fast, providing the opportunity for secure storage of CO 2 . In this paper and its companion, two different geological settings that are suitable for formation of CO 2 hydrate are investigated. In this paper storage of CO 2 beneath the ocean floor is studied, while storage in depleted gas reservoirs is studied in the companion paper. It has been suggested that CO 2 may be accumulated in the depressions on the ocean floor, where pressure and temperature conditions are such that either liquid CO 2 would accumulate or CO 2 hydrates would form. However, there have been significant concerns about the accompanying change in pH and its adverse effect on the ocean ecosystem. In this paper, permanent trapping of CO 2 at a depth of a few hundred meters beneath the ocean floor, where the CO 2 is thought to be of little or no harm to the ocean ecosystem, is studied. Based on density calculations, Schrag and his co-workers [1,2] have shown that for oceans that are deeper than approximately , CO 2 density at the ocean floor is more than the surrounding water. With increased depth below the ocean floor and as a result of increased temperature, the density of CO 2 reduces faster than that of water such that at some depth below the ocean floor, CO 2 will be lighter than the surrounding water. Injection of CO 2 at such depths or deeper intervals will lead to rise of the CO 2 until it arrives at a depth where its density becomes heavier than water. The zone above this depth, where CO 2 becomes heavier than water is called the negative buoyancy zone. Beneath the negative buoyancy zone, the CO 2 is naturally trapped by a gravity barrier. Furthermore, as CO 2 is rising towards this depth, it could pass through conditions where CO 2 hydrates form. Formation of CO 2 hydrate will further reduce formation permeability and introduce a second barrier against CO 2 rise, even before it arrives at the boundary of the negative buoyancy zone. Under dynamic conditions of injection and hydrate formation, the initial state of pressure and temperature is perturbed, affecting the negative buoyancy zone. Simulation studies are presented to investigate (i) the changes in pressure as a result of injection that could push CO 2 upwards into the negative buoyancy zone, and (ii) the increase in temperature as a result of formation of hydrates. below the ocean floor leads to the rise of CO 2 until a depth of approximately below the ocean floor, where hydrates will form reducing the formation permeability. Any CO 2 that might migrate further upwards could do so for another before it arrives at the negative buoyancy zone. These simulation studies suggest that total CO 2 emissions of large power plants may be stored at such a site.