Page 186 - Reservoir Formation Damage

168 Reservoir Formation Damage

The process is called homogeneous or heterogeneous crystal nucleation
depending on the absence or presence, respectively, of some impurities,
seed crystals, or contact surfaces, called substrates (see Figure 9-3 by
Leetaru, 1996). As stated by Schneider (1997), "Nucleation commonly
occurs at sites of anomalous point defects on the grain surface, at
structural distortions caused by edge or screw dislocations, or at irregular
surface features produced by dissolution and etching." Because, Schneider
(1997) adds, "When nucleation occurs at one of these sites, the free
energy of the defect, dislocation, or surface irregularity can contribute
to help overcome any energy barrier to nucleation." Also, the mineral
grain surfaces serve as seed for nucleation if the mineral crystal lattice
structure matches that of the precipitating substance (Schneider, 1997).
The free energy change associated with heterogeneous nucleation at a
surface is expressed by (Schneider, 1997):

(9-4)
AG = AG volume + AG surface + &G strain
where &G strain is the change of the strain volume free energy of shrinking
of a nucleus.

Figure 9-3. SEM photomicrograph of a calcite cement nucleating at a site
in an Aux Vases sandstone sample (after Leetaru, 1996; reprinted by per-
mission of the Illinois State Geological Survey).