Page 413 - Petrophysics 2E

EVALUATION OF WETTABILITY 381

thermodynamic work required for fluid displacements:

(6.30)

As an example, displacements from a Cottage Grove sandstone core
(Figure 5.16 in Chapter 5) were:

Curve 1 (oil displacing water from S, = 1 .0 to Si,) = 0.165 joules/ml =
24.82 BTU/bbl
Curve 3 (water displacing oil from Si, to - waterflood) =
0.0089 J/ml = 1.34 BTU/bbl
Curve 5 (oil displacing water from h0, to Si,) = 0.771J/ml =
116.25 BTU/bbl
Less energy is required for displacement of water from 100% saturation
(Curve 1) than from S,,, (Curve 5), because a considerable amount
of water is displaced at low pressure after the threshold pressure is
exceeded. A very small amount of energy is required for displacement of
oil because the Cottage Grove Sandstone exhibits a strong water-wetting
tendency; consequently, some oil is displaced by imbibition (at zero
capillary pressure) upon initial contact of the oil-saturated core with
water. Stating this in another way: If the water-oil-rock system is
water-wet, A1 is a large positive value and, therefore, considerable work
must be done on the system to displace the water. On the other hand, the
area under the waterdisplacing-oil curve is a very small positive value;
hence, water will imbibe into the water-wet system spontaneously with
simultaneous displacement of oil.
When a core is strongly water-wet (USBM I, > 0.7), the core will
imbibe water until the water saturation is essentially and the
area under the curve is almost zero; hence, the work required for oil
displacement is almost zero for a strongly water-wet system. The amount
and rate of imbibition depend on a number of simultaneously acting
properties of the water-oil-rock system: the rock and fluid chemical
properties expressed as wettability, interfacial tension, saturation history
of the system, initial saturation, fluid viscosities, pore geometry, and pore
size distribution.
As the system becomes less water-wet, the work required for
displacement of oil increases and, consequently, the amount and rate
of imbibition decreases. Thus, a smaller amount of water will imbibe at a
lower rate as the system becomes less water-wet. At neutral wettability,
water will not imbibe when the water saturation is at Si, and oil will
not imbibe when the water saturation is at S,,,. Thus, a positive initial