Page 38 - Reservoir Formation Damage

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22 Reservoir Formation Damage

3. Also, fines attached to swelling clays can be dislodged and liber-
ated during clay swelling, the phenomenon of which is referred to
as fines generation by discontinuous jumps or microquakes by
Mohan and Fogler (1997).

Consequently, formation damage occurs in two ways: (1) the perme-
ability of porous formation decreases by reduction of porosity by clay
swelling (Civan and Knapp, 1987; Civan et al., 1989; and Mohan and
Fogler, 1997); and (2) the particles entrained by the flowing fluid are
carried towards the pore throats and captured by a jamming process. Thus,
the permeability decreases by plugging of pore throats (Sharma and
Yorstos, 1983; Wojtanowicz et al., 1987, 1988; Mohan and Fogler, 1997).
Khilar and Fogler (1983) have demonstrated by the flow of aqueous
solutions through Berea sandstone cores that there is a "critical salt con-
centration (CSC)" of the aqueous solution below which colloidally in-
duced mobilization of clay particles is initiated and the permeability of
the core gradually decreases. This is a result of the expulsion of kaolin-
ite particles from the pore surface due to the increase of the double-layer
repulsion at low salt concentration (Mohan and Fogler, 1997). The criti-
cal salt concentrations for typical sandstones are given by Mohan and
Fogler (1997) in Table 2-3.

Mechanism of Clay Swelling

A structural model of swelling clays having exchangeable cations, de-
z+
noted by M , is shown by Zhou et al. (1996, 1997) in Figure 2-12. Zhou
et al (1996) states: "The structure layers are always deficient in positive
charges due to cation substitution, and interlayer cations are required to
balance the negative layer charge. Interlayer cations are exchangeable and
the exchange is reversible for simple cations. The distance between two

Table 2-3
Critical Salt Concentrations in Typical Sandstone

33 34 35 36 37 38 39 40 41 42 43