Page 53 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 41

Figure 2.33 CO 2 mole fraction in oil at the end of soaking at different pressures in Cycle
7 for Core 2 (dotted line outlining the core area).

1800 psi, more CO 2 reached the core center. After that, the benefitof
increasing pressure was not significant.

2.10 Effect of diffusion
It is our intuition that in shale and tight formations, diffusion is more
important than in conventional formations as the convection is smaller; and
it may be thought that diffusion plays a dominant role. However, in shale
and tight formations, the diffusion is also smaller than in conventional for-
mations. In the literature, a theoretical or experimental quantification has
not been published regarding the huff-n-puff gas injection process. Re-
searchers generally use simulation models to quantify their roles by
comparing the oil recovery with and without inclusion of diffusion in their
models. Wan and Sheng (2015a) simulated gas flooding in shale formations.
Their simulation results show that the Péclet number in their model is in the
3
order of 10 , indicating a diffusion-controlled flow regime. The Péclet
number (N pe )defines the ratio of the convective term (velocity multiplied
by characteristic length L) to the dispersive coefficient (D). Figs. 2.34 and
2.35 shows the effect of diffusion on oil recovery in gas flooding at different
natural fracture spacings. Diffusion does not show up as there is no gas injec-
tion in the primary production. In the gas injection process, as the fracture
spacing is reduced, the effect of diffusion is enhanced. When the spacing is