Page 303 - Enhanced Oil Recovery in Shale and Tight Reservoirs

276 Enhanced Oil Recovery in Shale and Tight Reservoirs

The above question 3 is answered by Conclusions 3 and 4. In a water-
oil-solid (liquid-liquid-solid) system, these two types of measurements are
consistent. However, if two systems, air-oil-solid and air-water-solid (gas-
liquid-solid system), are used, these two types of measurements may lead to
different wettabilities.

9.9 Conversion of wetting angles
The preceding section explains why the wetting angles cannot be used
to determine rock wettability if measured in gas-liquid-solid systems; but the
water and oil wetting angles can be compared to determine the wettability
in a water-oil-solid system according to Conclusion 1 discussed earlier.
Logically, if the wetting angles in gas-liquid-solid systems can be converted
to the wetting angles in the corresponding water-oil-solid system, then the
wetting angle measurements can be used to determine the wettability.
From Eqs. (9.51) and (9.52),

s ws ¼ s as s wa cosq wsa (9.62)

s os ¼ s as s oa cosq osa (9.63)
Refer to Fig. 9.35

s os s ws s wa cosq wsa s oa cosq osa
cosq ws ¼ ¼ (9.64)
s wo s wo
Then the water and oil wetting angles can be calculated from the water
and oil contacting angles in dry cores by the following equations:

q ws ¼ cos 1 s os s ws ¼ cos 1 s wa cosq wsa s oa cosq osa (9.65)
s wo s wo

q os ¼ cos 1 s ws s os ¼ cos 1 s oa cosq osa s wa cosq wsa (9.66)
s wo s wo
It is easier to measure wetting angles on dry cores. But they cannot be
used directly to determine wettability. Instead, the water and oil wetting an-
gles in water-oil-solid systems are estimated using the above equations.
Some of the estimated wetting angles are summarized in Table 9.6.
In this table, if the oil/water interfacial tension s wo was not provided by the
references, typical values of 20e30 mN/m are used. When these typical
values are used, cosq is outside the range of 1to þ1 in some cases. So other
values are used. When nontypical values of s wo are used, sensitivities are