Preprint E47/2017
The effect of carbon dioxide in oil recovery from calcite reservoirs Part II.
A. C. Alvarez, W. J. Lambert, J. Bruining,D. Marchesin
Keywords: Riemann problem | oil recovery | Conservation laws

We are interested in the effect of carbon dioxide in water flooding of light oil at high pressure in a calcite reservoir, representative for the presal
formations off the coast of Brazil. We study this process initially in a 1-D setting in an oleic phase that includes carbon dioxide and an aqueous phase
that contains all the ionic substances and dissolved mineral salts. By assuming chemical equilibrium we can describe the motion of all dissolved
compounds in a limited number of transport equations of \emph{master species}. Indeed, from the concentrations of the master species, chemical equilibrium
determines the quantities of relevant solutes inclusive carbon dioxide, which is the only substance that is considered present in both phases. We use this
model to determine the oil recovery when water is injected as a secondary recovery technique.To study this mechanism we formulate the conservation equations of hydrogen, twice oxygen minus hydrogen, chloride and decane. Therefore, we solve analytically and numerically these equations elucidating the effects of the injection of low salinity carbonated water into a reservoir containing oil equilibrated with high salinity carbonated water. We use PHREEQC (acronym of pH-REdox-Equilibrium C-program) to obtain the accurate  equilibrium partition of neutral species that are soluble both in the oleic and the aqueous phase by application of the Krichevsky-Ilinskaya extension of Henry's law for solubility of gases in liquids. Using Gibbs phase rule it can be shown that the phase behavior only depends on the pH and the chloride concentration. The above mentioned equilibrium relations use Pitzer's activity coefficients to extend the validity up to 6M. We obtain the saturation, composition and the total Darcy velocity profiles. The significant new insight obtained is that by changing only the salinity in carbonated waterflooding the oil recovery can be enhanced.