Page 403 - Petrophysics 2E

EVALUATION OF WE’M’ABILITY 37 1

EXAMPLE

A solution of calcium chloride is used to displace the monovalent
cations from a 600 g sample of sandstone containing a small amount
of clay. Using an atomic adsorption analytic unit to analyze the effluent
from the core, 284 mg of sodium and 162 mg of potassium are found in
the effluent liquid. Calculate the cation exchange capacity (CEC) of the
rock.

SOLUTION

mg x valence 284 x 1.0
meqNa = - = 12.3
-
atomic weight 23
162 x 1.0
meqK = = 4.2
39
12.3 + 4.2 meq
CEC = = 27.5 -
0.6 kg

EVALUATION OF WETTABlLlTY

Evaluation of relative water/oil wetting of porous rocks is a very
important aspect of petroleum reservoir characterization. Wettability has
a decisive influence on oil production rates, the water/oil production
ratio after water breakthrough, the oil production rates of enhanced oil
production technologies, and the residual oil saturation of a reservoir at
abandonment. A large amount of research has therefore been conducted
on wettability, beginning in the 1930s. Several methods for evaluating
wettability have been developed, based on the observable characteristic
interactions of water, oil, and rocks. The direct measurement of
wettability can be made by careful analysis of contact angles. In
addition, several indirect methods provide indexes of the relative wetting
properties: the Amott method, which is based on the amounts of
fluids imbibed by a rock sample under various conditions; the USBM
(U.S. Bureau of Mines) method, which is based on measurements of
the areas under capillary pressure curves obtained using a centrifuge;
the combined Amott-USBM method; and the spontaneous imbibition
method. based on the rates of imbibition.
AMOTT WETTABILITY INDEX

The Amott test for wettability is based on spontaneous imbibition and
forced displacement of oil and water from cores [44]. The test measures