Page 123 - Enhanced Oil Recovery in Shale and Tight Reservoirs

110 Enhanced Oil Recovery in Shale and Tight Reservoirs

The main condensate zone at the end of primary depletion is
encompassed by the red dotted lines in Fig. 4.26. When the injection
time is 10 days, the pressure in most of the zone is about 1500 psi. When
it is 50 days, the pressure in half of the zone is below 2000 psi. When it is
100 days, the pressure in the entire zone is about 2400 psi, then the average
pressure in the zone is close to the dew point of 2750 psi, and the oil recov-
ery is the highest. From this discussion, it seems that the required injection
time is so long that the pressure in the condensate zone should be above the
dew point pressure. The final injection time should be optimized by
economics, as long as injection requires more gas to be injected. In this
example, it is 50 days. We may extend this statement more generally: the
huff-n-puff injection should be optimized so that the pressure in the main
condensate zone at the end of injection should be above the dew point
pressure.
If a long puff time is used, less cycles will be needed for a fixed project
time, less gas volume is injected, and more production time is obtained.
Based on the above simulation model, Meng and Sheng (2016a) further
studied the effect of puff time. They extended the puff time from 200 to
400 days with 50 days of puff time unchanged. They found the oil recovery
factors for the two cases are almost the same, but the incremental revenues
from the 400-day puff time case are more than two times those from the
200-day puff time case. The gas rate at the end of 200-day puff time is
38% of the rate in the beginning, while the gas rate at the end of 400-day
puff time is 10%. Therefore, they proposed that the puff time should be
determined so that the gas rate at the end of puff period should be 10% of
the gas rate in the beginning. We have to say this 10% is not a general
rule. A longer production is carried out at a relatively low pressure (energy)
mode, which may not be optimal.

4.11 Mechanisms of huff-n-puff injection

Energy supply or pressure maintenance is probably the most impo-
rtant and obvious mechanism in huff-n-puff injection. Mechanisms
re-vaporization, solubilization and improved phase behavior are discussed
in this section.

4.10.1 Revaporization by gas
In Section 4.5, a one-dimensional simulation model is used to simulate gas
and solvent performance. Fig. 4.27 shows the changes in oil viscosity,