A.J. Hartsook Professor Emeritus, Research Professor
Waterflooding recovers only about 35% of the original oil in place. This is because oil
and water does not mix and oil will stop flowing when it is disconnected. At pressures
above the minimum miscibility pressure (MMP), CO2 acts as a miscible solvent for oil
and it can be injected into depleted oil reservoirs with CO2 be trapped in place of oil.
However, this is usually a challenge because super-critical CO2 has the density and
viscosity more like that of a gas and it tends to over-ride and bypass the (more dense
and viscous) oil and water. A method to overcome the low viscosity of CO2 is to add
surfactants to disperse CO2 in water as a CO2-foam. The choice of surfactant was
simple for a West Texas carbonate reservoir with low reservoir temperature and low
salinity. A nonionic surfactant with 12 carbon chain and 22 EO (ethylene oxide groups)
resulted in effective mobility reduction and low adsorption on the carbonate rock
material. The conditions required for the Middle East is more demanding. They are
also carbonate formations but the temperature is over 100 °C and the salinity is
greater than 20% total dissolved solids (TDS). Nonionic surfactants became insoluble
(reached cloud point) below the reservoir temperature. Anionic surfactants (commercially
available) would precipitate and/or chemically degrade under these conditions, in
addition to having high adsorption. The current candidate surfactants are switchable
nonionic-to-cationic surfactants. They are ethoxylated or methyl amines or diamines.
They have limited solubility in supercritical CO2 and are soluble in the high-salinity
brine when equilibrated with CO2 (carbonic acid). They have low adsorption on pure
calcite but can have significant adsorption if silica or clay are present.