Page 81 - Enhanced Oil Recovery in Shale and Tight Reservoirs

68 Enhanced Oil Recovery in Shale and Tight Reservoirs

the puff time was 18 h. The whole injection process is performed isother-

mally at 21 C. Before the huff and puff injection, EF#3 and EF#4 were
saturated with Wolfcamp oil and decane, respectively. Their measured per-
meabilities were 126 nD and 86.7 nD for EF#3 and EF#4, respectively.
After six cycles of huff and puff injection, their remeasured permeabilities
were 78.5 nD and 81.7 nD for EF#3 and EF#4, respectively. The perme-
ability for EF#3 using Wolfcamp oil was decreased by 47.5 nD after huff and
puff injection, while the permeability for EF#4 using decane was almost
unchanged, as shown in Fig. 3.9.
The permeability reduction is consistent with the changed PSD. After
asphaltene deposits onto the rock surface, the pores become smaller or
blocked. Behbahani et al. (2013) found that 60%e80% of the total damage
by asphaltene deposition was caused by mechanical plugging that was recov-
ered by cyclohexane reverse flooding; 20%e40% by adsorption that was
recovered by toluene reverse flooding, but the recovery process took a
long time. In another experimental study, Behbahani et al. (2015) observed
that the carbonate cores experienced more particle plugging compared to
the sandstone cores. In carbonate, the inner surfaces of a core contain
more polar groups that can exert polar interactions with asphaltene surface
group (Hamadou et al., 2008), which results in a high adsorption rate. In
low permeability cores such as carbonate, the plugging mechanism acts

Figure 3.9 Comparison of permeability before and after huff and puff injection.