With the increased concern of global warming, scientists are researching ways to limit the amount of carbon dioxide entering the atmosphere in an effort to mitigate the atmospheric carbon dioxide concentration increase (The United States Environmental Protection Agency 2013). Carbon dioxide has been identified as the primary greenhouse gas emitted as a result of human activities (Benson 2012). Carbon sequestration is the capturing and secure storage of carbon dioxide emitted from the global energy system (West 2013). Capturing CO2 from stationary sources and injecting it into underground geological formations for storage purposes has been considered seriously for reducing CO2 emission.
This essay examines some of the detailed physics associated with the transport of different components in a three-phase system encountered during sequestration of supercritical CO2 into deep saline aquifers (Fuller, Prevost & Piri, 2005). Carbon dioxide at critical temperature and pressure has a density of about 467.6 kg/m3 which allows significantly greater quantities to be sequestered than if it existed in the gas-phase. Conditions that support the existence of supercritical CO2 should be present at depths greater than about 800 m where pressure and temperature would be above the critical point of CO2 (Fuller, Prevost & Piri, 2005). According to Fuller, Prevost and Piri (2005) the phase behaviour of CO2 during sequestration is as below.
The Phase Behaviour os CO2 During Sequestration
The phase behavior during a CO2 flooding is a very complex process with three mechanisms namely; the oil swelling, reduction of oil viscosity, and the acidization of carbonate. The other parameters which effect phase behavior during a CO2 flooding are the temperature, pressure and rock-fluid interactive properties of the reservoir (Rhaman, Nofel & Arshad 2010).
Fuller, Prevost and Piri (2005), describes the behavior of C02 in the following manner. Supercritical CO2 can form its own phase whose plume size and the extent to which it comes in contact with the brine is controlled by relative permeabilities, gravity and heterogeneity of permeability (Fuller, Prevost & Piri, 2005). This is the mechanism by which most of the injected CO2 is stored. When the relevant capillary pressure decreases the brine starts swelling and consequently contributing arc menisci starting from the sharpest corner may hinge and eventually move when the hinging contact angle reaches the advancing value towards the center to meet the other moving or pinned arc menisci. When arc menisci meet the center of the element is dissolved spontaneously by the brine. The trapped clusters of CO2 may shrink and eventually disappear due to dissolution into the brine. CO2 may get dissolved in the brine lowering its pH forming an acidic solution. The dissolved CO2 will travel away from the injection well due to dispersion and molecular diffusion and velocity of the aqueous phase. The concentration of CO2 in the liquid phase is controlled by salinity of the brine, pressure, temperature, and geochemical reactions with primary minerals of the host rock that may also dissolve into the aqueous phase. Homogeneous and heterogeneous chemical reactions of CO2, both free and dissolved in the liquid phase, with the primary minerals of the formation, that also may dissolve in the liquid phase, may produce secondary minerals with low solubility that could precipitate indirectly sequestrating carbon (Rhaman, Nofel & Arshad 2010).
Carbon dioxide is a colorless, odorless, and non-flammable substance, which can be transported as a solid, liquid, gas, or dense-phase liquid (West 2013). The most efficient state of this gas for pipeline transport is in liquid form; in this phase, the fluid has the density of a liquid and is also viscous. In this critical mode, captured CO2 has to be compressed to a pressure above the critical pressure prior to transport (The Pipelines International 2010).
The capturing and storage of carbon dioxide is a very efficient option that could be used to reduce greenhouse gas emissions from the continued use of fossil fuels. Geological storage should be highly embraced as a means to take place in natural underground reservoirs such as oil and gas fields, coal seams and saline water bearing formations utilizing natural geological barriers to isolate CO2 from the atmosphere. The safest mode of transporting carbon dioxide is the pipeline in its liquid form.