Page 109 - Primer on Enhanced Oil Recovery

Gas flooding 99

decreases the solubility of carbon dioxide. Pressure increase has the opposite effect
and the solubility increases. Under reservoir conditions, the solubility of gaseous
3
3
carbon dioxide in water is 30 60 m /m (3 5%).
The dissolution of carbon dioxide in water by 20 30% increases water viscosity.
When carbon dioxide is dissolved in water some carbonic acid is formed. The acid
etches carbonates and clays. This etching opens and widens throats between forma-
tion grains and the permeability of carbonate rocks increases by 6 75%, and sand-
stone rocks by 5 15%. The acidic environment also reduces swelling of clays. This
has a significant effect on increasing reservoir permeability.
Dissolution in oil. At the optimal conditions carbon dioxide has an excellent sol-
ubility in oil. Compared to water, oil can uptake 4 10 times more of carbon diox-
ide at the optimal conditions. This high solubility also ensures CO 2 significant
transfer of carbon dioxide to oil from an aqueous solution in oil-water contact. This
transfer reduces the interfacial tension between oil and water, and the oil displace-
ment becomes almost miscible. The highest mixing of carbon dioxide and oil
occurs when the pressure of full mixing is exceeded, regardless of the CO 2 concen-
tration. The specified pressure strongly depends on the physicochemical properties
of the oil and is somewhere in the region 8 30 MPa. At the same time, for heavy
oils, high temperature and gas saturation pressure, the pressure of complete misci-
bility is significantly higher.
At pressures below the mixing pressure, carbon dioxide and oil separate, forming
gaseous and liquid phases. In this case, the gas phase is formed by carbon dioxide
with the light fractions of oil. The remaining liquid oil is stripped of light fractions.
In this case it is possible that the liquid oil further separates into fractions and
asphalt-resin-paraffin deposits (ARPD) start to precipitate and accumulate.
It should be noted that the viscosity of the oil is significantly reduced when car-
bon dioxide is dissolved in it. Separation of oil and carbon dioxide leads to signifi-
cant increase in the reformed oil density and viscosity. This reformed oil is then left
behind the front of propagating carbon dioxide slug.
Full mixing of carbon dioxide and oil at the beginning of carbon dioxide injec-
tion due to the above phenomena does not occur immediately. However, in the pro-
cess of displacing oil, carbon dioxide is enriched with hydrocarbons, and the
displacement becomes miscible. Therefore, the mixing pressure for carbon dioxide
is substantially lower than for hydrocarbon gases, nitrogen and flue gases. For
instance, the pressure required to displace light oil by mixing with hydrocarbon
gases is almost two times higher than for carbon dioxide.
It should be noted that oil swelling (volume increase) with the dissolution of car-
bon dioxide has a significant effect on increase of oil recovery. When this occurs, a
significant decrease in the viscosity of the oil is observed. The volume of oil
increases 1.5 1.7 times, while the increase in density is negligible (2 3%)

10.1.2 Mechanism of the process

Mixing oil displacement. In the case of a miscible displacement, the oil is dis-
placed by carbon dioxide like it is by a conventional solvent. In this case, three