Page 76 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

64 Reservoir Engineering

phase saturation) contact angles; receding angles are smaller than advancing
angles [97]. Bartell and coworkers [95-971 were among some of the first
investigators to measure contact angles with crude oil systems that suggested
the possibility that oil reservoirs may not be water-wet. Furthermore, they
concluded that spontaneous displacement of oil by water should occur only when
both advancing and receding angles are less than go", and no spontaneous
imbibition should occur if the two angles are on opposite sides of 90" [97].
A common technique for measuring advancing contact angles using polished
mineral crystals is described in the literature [104]. For many crude oil systems,
a considerable amount of time may be required before an equilibrium contact
angle on a pure mineral is obtained. As shown in Figure 5-45 [104], some
systems that initially appear to be preferentially water-wet become more oil-wet.
Small amounts of polar compounds in some crude oils can adsorb on the rock
surfaces and change wettability fmm preferentially water-wet to more oil-wet [96].
A detailed study on how crude oil components affect rock wettability has been
made by Denekas et al. [105]. Imbibition tests have been described to examine
wettability of reservoir cores [106,107]. A preferred technique of inferring
reservoir wetting from core samples is the Amott method [lo81 which involves
spontaneous imbibition and forced displacement tests; ratios of spontaneous
displacement volumes to total displacement volumes are used as an index of
wettability. Based on the correlation suggested by Gatenby and Marsden [ 1091,
Donaldson et al. [110-1121 developed a quantitative indication of wettability,

I I I
QUARTZ CALCITE ond DOLOMITE
--&.- CRUDE B --+--- CRUDE A
--i;c CRUDE C --+-- CRUDE E
--+--- CRUDE D
" ..... +..- CRUDE 0'
-+- CRUDE F

0. I 1.0 IO 100 IO00
AGE OF OIL - SOLID SURFACE, hm
Flgure 5-45. Change in contact angles with time [104].