Page 484 - Enhanced Oil Recovery in Shale and Tight Reservoirs

448 Enhanced Oil Recovery in Shale and Tight Reservoirs

0.7

0.6
0.5
RF, fraction 0.4

0.3

0.2
80C Air injection
80C N2 injection
0.1
BT @ 0.8 PV
0
0 1 2 3 4 5 6 7 8
PV of gas injection
Figure 13.25 Recovery performance of isothermal air injection test and nitrogen injec-
tion test at 80 C.

more significant. The experimental data did not show this expectation. The
increase in recovery factor as the temperature is increased is due to the
decrease in oil viscosity as the temperature is increased. The oil viscosity
was decreased from 2.7 to 1.1 cP as the temperature was increased from
80 to 120 C.

Fig. 13.25 compares the oil recovery from nitrogen injection and air
injection at 80 C. It is seen that the recovery from nitrogen injection was

higher than that from air injection. Such result was also observed when
live oil was used. In other words, the difference in oil recovery between
nitrogen injection and air injection was not caused by the difference of
gas dissolution of different gases. The difference is probably caused by the
fact that part of the oxygen was consumed. The consumption of oxygen
resulted in less air that actually flooded oil.
During gas injection (either nitrogen or air), gas pushes the oil ahead of it
to the producer. Some oil remains behind the displacement front. After
some PV (e.g., 8 PV) of nitrogen is injected first so that no more oil is
produced, air injection is injected. Then oxidation reactions may occur
between the air and the oil remaining from nitrogen injection. If the
oxidation can significantly improve oil recovery, incremental oil over nitro-
gen injection will be recovered. Fig. 13.26 shows such experimental results
at 80, 100 and 120 C. At any temperature, when air is injected after 8 PV of

nitrogen, no more oil was recovered. Therefore, the LTO thermal effect was
not observed under the laboratory conditions where heat loss deemed more