Page 63 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 51

Figure 2.42 Continued
mass transfer. However, in their field-scale simulation of real huff-n-puff
gas injection process, inclusion and without inclusion of molecular
diffusion do not lead to a significant difference in oil recovery (less
than 1%).

2.14 Gas penetration depth
It can be understood that gas penetration depth is critically important
to effective huff-n-puff gas injection. A high injection rate and the forma-
tion heterogeneity will promote gas fingering, resulting in a higher penetra-
tion depth. In the case of CO 2 injection, it is easier to inject liquid CO 2 .
However, gaseous CO 2 will penetrate deeper into formation. From this
point of view, preinjection of nitrogen or other dry gas to create a gas
network will help make the subsequent liquid CO 2 penetrate deeper into
the reservoir.
Li et al. (2018) studied gas penetration by simulation. They first did
experiments using a core of 1.5 inches diameter which was saturated
with oil. Methane was used as the gas. One huff-n-puff cycle has
1 h of huff, 7 h of soaking, and 4 h of puff. They used a simulation model
to history match the experiment. Fig. 2.44 shows that methane mole frac-
tion in oil, C 1oil . The model data indicated C 1oil reached 0.525 inches
which was 70% of the core radius (0.75 inches) by the end of puff (1 h).
At this distance, C 1oil is 0.1 which is an arbitrarily selected value. The
experiment indicates the penetration velocity was 1 inch per hour