Page 111 - Primer on Enhanced Oil Recovery

Gas flooding 101

happens, and difficult-to-recover oil saturated with heavy components is formed
and left in the swiped zones.
At reservoir temperatures above the critical level, carbon dioxide is in a gaseous
state regardless of the pressure. In this case, the development of the field will be
much less effective than in mixing displacement. This is due to the unfavorable
ratio of oil and gas mobility which leads to low sweep efficiency. In order to
achieve technological and economic efficiency in the injection of carbon dioxide, it
is necessary to ensure that it is in a liquid state. Only in this case the overall effi-
ciency of the displacement is at its maximum. For this reason only reservoirs with
temperatures close to critical (25 35 C) will demonstrate the very best results.

Swelling effects. Carbon dioxide injection reduces mobility of water and
increases the mobility of oil. This increases swipe efficiency by improving stability
of the displacement front. In addition, oil volume swelling is one of the most impor-
tant factors for oil displacement by carbon dioxide injection. The exact value of oil
swelling (expansion) is a complex function of the light hydrocarbons content in the
oil, reservoir pressure and temperature,
Bigger oil volume leads to artificial increase in oil saturation and to an increase
in pore pressure. This leads to an effective displacement of residual oil. Just as the
result of this process the oil recovery coefficient may increase by 6 10%.

10.1.3 Applicability criteria
Carbon dioxide injection should be carried out in reservoirs with moderately light
oil (API $ 28), and the reservoir should be sufficiently deep ($ 1500 m) to pro-
vide a sufficiently high pressure to achieve miscibility. Carbon dioxide, when dis-
solved in water, would reduce the interfacial tension between oil and water.
However, produced acidity can also lead to problems with metal corrosion. In this
process, about 20 50% of carbon dioxide slug is displaced by water. Water is usu-
ally pumped jointly with CO 2 in the water alternating gas mode (WAG). This
improves the mobility ratio for the displacing fluids and oil. The criteria for the
applicability of the method are shown in Fig. 10.5.
Carbon dioxide injection is the fastest growing method of enhanced oil recovery
in the United States. The implemented projects continue to demonstrate good addi-
tional oil production. Carbon dioxide injection is used both as a secondary and as a
tertiary method for oil recovery. However, the largest CO 2 injection projects are
EOR implementations on fields that have been in long-term development. As a
rule, this is done in places where waterflooding has been applied for many years.
The CO 2 injection technique has its problems too. The method application chal-
lenges include:

Early CO 2 breakthrough due to its low viscosity;
Metal corrosion in production wells;
The need to use materials withstanding CO 2 environment;
The need to separate CO 2 from the extracted fluids;
The need to further compress CO 2 for re-injection;
High CO 2 demand per unit of oil produced.