Storage of CO2 as hydrate beneath the ocean floor

Abstract

Abstract With the increasing concern about climate change, the public, industry and government are showing increased interest towards reduction of CO2 emissions. Geological storage of CO2 is perceived to be one of the most promising methods that could allow significant reduction in CO2 emissions over the short and medium term. One major concern against geological storage of CO2 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 CO2 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 CO2-hydrate formation–that leads to trapping of CO2 in the solid form–is quite fast, providing the opportunity for secure storage of CO2. In this paper and its companion, two different geological settings that are suitable for formation of CO2 hydrate are investigated. In this paper storage of CO2 beneath the ocean floor is studied, while storage in depleted gas reservoirs is studied in the companion paper. It has been suggested that CO2 may be accumulated in the depressions on the ocean floor, where pressure and temperature conditions are such that either liquid CO2 would accumulate or CO2 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 CO2 at a depth of a few hundred meters beneath the ocean floor, where the CO2 is thought to be of little or no harm to the ocean ecosystem, is studied. Based on density calculations, Schrag and his co-workers have shown that for oceans that are deeper than approximately, CO2 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 CO2 reduces faster than that of water such that at some depth below the ocean floor, CO2 will be lighter than the surrounding water. Injection of CO2 at such depths or deeper intervals will lead to rise of the CO2 until it arrives at a depth where its density becomes heavier than water. The zone above this depth, where CO2 becomes heavier than water is called the negative buoyancy zone. Beneath the negative buoyancy zone, the CO2 is naturally trapped by a gravity barrier. Furthermore, as CO2 is rising towards this depth, it could pass through conditions where CO2 hydrates form. Formation of CO2 hydrate will further reduce formation permeability and introduce a second barrier against CO2 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 CO2 upwards into the negative buoyancy zone, and (ii) the increase in temperature as a result of formation of hydrates. Through a case study, we report numerical simulation studies that indicate that injection of CO2 at a depth of approximately 800 m below the ocean floor leads to the rise of CO2 until a depth of approximately 360 m below the ocean floor, where hydrates will form reducing the formation permeability. Any CO2 that might migrate further upwards could do so for another 135 m before it arrives at the negative buoyancy zone. These simulation studies suggest that total CO2 emissions of large power plants may be stored at such a site.