Page 304 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 277

conducted to ensure that these values do not change the conclusions
regarding the wettability for each of shale samples.
This table shows that both water and oil contact angles were less than 90
(except two water angles) and the oil contact angles were smaller (zero in most
of the cases) than the water contact angles. By the conventional intuition or
misconception, those shale rocks are believed to be oil-wet. However, as the

table shows, the estimated water wetting angles are less than 90 ,indicating
water-wetness. It seems that more shale samples are water-wet. The author
of this book also observed that when the IFT is low, the liquid drop is gradually
collapsing into the rock or at least the drop is becoming smaller and smaller.
When this observation is made, one may believe the shale samples are oil-
wet; They may not, actually!
The conversion of wetting angle enables some paradoxical data about
wettability. For example, Lan et al. (2015b) observed stronger oil uptake
than water update by their tight siltstone rock samples, indicating strong

oil-wet, but the water contact angle was greater than 37 (supposed to be

higher than 90 if oil-wet). They believed that this phenomenon was caused
by oil sorption onto the organic materials (primarily solid bitumen). Howev-
er, as indicated by the estimated results in the above table, the sample might be
water-wet. Liang et al. (2016) observed the similar behavior for shale samples
from the Lower Longmaxi formation in China, but the water wetting angles

were 12e37 at elevated and normal temperatures; but the estimated results
in the above table indicate that the sample might be water-wet.

9.10 More on wettability of shale and tight
formations

Overall, the wettability of shale rocks is mixed (possibly oil-wet and
water-wet), similar to conventional sandstone rocks. It may not be dominantly
oil-wet as one believes. Adsorption of asphaltenic components controls the
wettability in conventional reservoirs (Kumar et al., 2008). Shale wettability
depends on the total organic carbon (TOC) (Odusina et al., 2011). The mixed
wettability results from the fact that different rock types have different wetta-
bilities, as inorganic shale rocks are more likely water-wet, whereas the organic
parts are more likely oil-wet.
From the measurements of contact angle and zeta potential, Wolfcamp
cores and Eagle Ford cores demonstrated intermediate wet to oil-wet