Page 195 - Enhanced Oil Recovery in Shale and Tight Reservoirs

178 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 8.6 Effective gas permeability at different water imbibition times (Shen et al.,
2017).
cells and the gas permeability was quickly measured; after that the shale
samples were put back to the imbibition cells for continued imbibition.
And the shale samples were not confined during imbibition. They also
used sandstone samples and volcanic samples to do the same type of experi-
ments for comparison. The permeabilities from sandstone and volcanic sam-
ples were continuously decreased as more water was imbibed.
Santos et al. (1997a) studied the water-shale interactions and found that
the reaction depended on the moisture of rock samples. The evidence of
reactions (core disintegration or created microfractures) was not noticeable
for preserved shale samples even at atmospheric conditions without confine-
ment, but only noticeable for dry samples. Therefore, they concluded that the
shale instability problem was mainly caused by mechanical failure (mud
weight), rather than by chemical interaction between the rock and drilling
fluid, as opposed to their initial hypothesis. Makhanov et al. (2014) observed
that swelling of clay was not the only mechanism that creates microfractures,
because some microfractures are also created with the imbibition of oil despite
having no affinity for absorption in clays. This suggests that some pore pres-
sure is developed due to imbibition of fluid (water or oil) or called mechanical