Page 44 - Reservoir Formation Damage

28 Reservoir Formation Damage

Clay particles

Swollen
clay Water
absorption

Clayey matrix

-. ^ .^ absorption _ —
^-^-r^ r- - _ -r f-o^sc^.
^
Swollen,
matrix

Figure 2-17. Clay particle expansion and pore space reduction by swelling
(after Civan, ©1999 SPE; reprinted by permission of the Society of Petro-
leum Engineers).

between the clay particles is higher than that in the aqueous pore fluid,
the water in the pore fluid diffuses into the double-layer to dilute its ion
concentration. This phenomenon creates an osmotic repulsive pressure
between the clay particles. As a result, the interparticle distance increases
causing the clay to expand and swell. Therefore, the driving force for
osmotic pressure is the difference of the total ion concentrations between
the clay double-layer, c c, and the surrounding pore fluid, cy, as depicted
by Figure 2-18 of Ladd (1960).
For only very dilute aqueous solutions, the van't Hoff equation given
below can be used to estimate the osmotic pressure (Ladd, 1960):

P osm = RT(c c-c f) (2-1)
Non-ideal models are required for concentrate solutions.

Water Absorption Rate
Consider Figure 2-19 (Civan, 1994, 1999) showing swelling of a solid
by water absorption. Civan et al. (1989) assumed that water diffuses