The role of wettability in the flow of injected CO2 through reservoirs and seals.
Trzeciak, Christopher P.
Engineering Honours Degree 2008
University of Adelaide
The geological storage of carbon dioxide generally takes place above the critical temperature and pressure conditions of carbon dioxide such that it will therefore be present in the reservoir in a single and dense state with non-ideal characteristics.
For fluids contacting surfaces in the presence of a continuous phase, Young’s equation details the force balance that exists between the free energies of the fluid phases and the surface whereby a contact angle is made at the triple point (vapour-liquid-solid contact). However, in a three phase environment with supercritical carbon dioxide-water-solid present, theory states that the high pressure and temperature conditions play an important role in the final state of equilibrium between the three phases. At high pressures and thus high carbon dioxide activity, this leads to the work of adhesion being increased between the carbon dioxide and the surface, which (when the solid is of a much lower energy than the carbon dioxide – typically the case) decreases the interfacial free energy value between carbon dioxide and the surface . Therefore the intermolecular relations become more favourable between the continuum and the solid. Increased adsorption of carbon dioxide to the surface of the substrate results (Tripp and Coombes, 1998) corresponding to an increase in the degree of wetting of the surface by carbon dioxide. High pressures also lead to a decrease in the interfacial tension between the carbon dioxide and water (Hebach et al., 2002) and a subsequent increase in the contact angle between the water and solid substrate. Therefore the spreading coefficient for water decreases, promoting the displacement of water by carbon dioxide.
The experimental contact angle data analysed for contact angle measurements in supercritical carbon dioxide/water (brine)/solid (mineral) systems, (where the materials analysed were muscovite mica, silica (quartz), glass and limestone) each displayed tendencies that correspond with the theoretical approach whereby the carbon dioxide increases the contact angle of a brine droplet as the pressure increases irrespective of the temperature conditions. The limestone system was supported by a core flooding and micro-model study which demonstrated the same tendencies for the degree of wettability change that is shown with the contact angle studies.
There are many inconsistencies with the reported data with respect as to how the results can be analogized to the subsurface environments encountered in reservoirs and seals due to hysteresis. The main forms of hysteresis encountered are: surface roughness, surface heterogeneities and immobile surface coatings. Each of these types of hysteresis will most likely enhance the actual influence that carbon dioxide has on the wettability of a surface and as such can be viewed with a positive outlook for reservoir rocks and a negative prospect for sealing formations.
The wettability alteration of reservoir rocks from water wet or intermediate wet to carbon dioxide wet is welcomed by carbon dioxide geosequestration projects as increased storage of carbon dioxide results, however the reduction in water wettability of the granular surfaces comprising a shale seal pose a dramatic reduction in the sealing capacity of the formation. Because the column of carbon dioxide that can be held back by the seal will not be on the same order as a column of hydrocarbon gases held back by the same seal.