Page 277 - Enhanced Oil Recovery in Shale and Tight Reservoirs

254 Enhanced Oil Recovery in Shale and Tight Reservoirs

and the surface roughness. The Amott method (Amott, 1959) or the Amott-
Harvey method (Boneau and Clampitt, 1977) measures the “overall or
average” wettability of a core. Although the formulas vary, the common prin-
ciple is that the wetting fluid will imbibe into the core more than the non-
wetting fluid. For the Amott-Harvey method, at the end of spontaneous
imbibition, forced imbibition is continued. One problem for the Amott
method and Amott-Harvey method is that spontaneous imbibition in shale
or tight cores is very slow; therefore, a long test time is needed. This method
may have its limited use in shale and tight cores.
In the US Bureau of Mines (USBM) method (Donaldson et al., 1969), a
core sample is spun in a water-filled centrifuge tube. After several spin rates,
the sample reaches residual oil saturation, S or , and then it is placed into an oil-
filled tube for another series of measurements. This method consists of two
stages: the primary forced imbibition and the secondary forced drainage.
The centrifuge force represents capillary pressure. If the capillary pressure
in the forced water imbibition process is lower than that in the oil drainage
process, the core is more water-wet, and vice versa. For the currently avail-
able centrifuge instruments, the centrifuge force is not high enough to
displace out the fluids in a shale or tight core. Therefore, this method cannot
be used in shale or very tight cores.
In the USBM tests, spontaneous imbibition is not measured, but it occurs
during initial centrifugation at low pressures. Consequently, the combined
Amott-USBM method (Sharma and Wunderlich, 1987; Anderson, 1986)
is preferred. In the test, spontaneous imbibition is carried out in an Amott
cell, while forced imbibition is carried out in a high-speed centrifuge using
the same multiple speeds as those used for the USBM test.

9.8.2 Capillary rise method and thin layer wicking method
The height h of a wetting phase in a capillary tube is calculated by

p c
h ¼ (9.38)
Drg
The capillary pressure p c is calculated by

2scosq
p c ¼ (9.39)
r
where Dr is the density difference between the wetting and nonwetting
fluids, s is the interfacial tension, q is the contact angle, r is the radius of the
capillary tube, and g is the gravity constant. The contact angle is estimated