Page 129 - Reservoir Formation Damage

Characterization of Reservoir Rock 111

Acoustic Techniques (AT)

The acoustic techniques facilitate acoustic-velocity signatures and
correlations of the acoustic properties of rocks to construct acoustic
velocity tomograms to image the rock damage by deformation, such as
elastic and dilatant deformations, pore collapse, and normal consolidation
processes (Scott et al., 1998). Scott et al. (1998) describe the acoustic
velocity behaviors during compaction of reservoir rock samples. Scott et
al. (1998) show a schematic of a confined-indentation experiment used
and the acoustic velocity tomograms obtained by the indentation tests.

Cation Exchange Capacity (CEC)

The total amount of ions (anions and cations) that are present at the
clay surface and exchangeable with the ions in an aqueous solution in
contact with the clay surface, is referred to as the ion-exchange capacity
(IEC) of the clay minerals and it is measured in meq/100 g (Kleven and
Alstad, 1996). The total ion-exchange capacity is therefore equal to the
sum of the cation-exchange capacity (CEC} and the anion-exchange
capacity (AEC):
IEC = CEC + AEC (6-1)
During reservoir exploitation, when brines of different composition than
the reservoir brines enter the reservoir formation, an ion-exchange process
may occur, activating various processes leading to formation damage (see
Chapter 13). In the literature, more emphasis has been given to the
measurement of the cation-exchange capacity, because it is the primary
culprit, responsible for water sensitivity of clayey formations (Hill and
Milburn, 1956; Thomas, 1976; Huff, 1987; Muecke, 1979; Khilar and
Fogler, 1983, 1987).
The mechanisms, by which aqueous ions interact with the clay minerals
present in petroleum-bearing rock, have been the subject of many studies.
Kleven and Alstad (1996) identified two different mechanisms: (1) lattice
substitutions and (2) surface edge reactions. The first mechanism involves
the ion-exchange within the lattice structure itself, by substitution of A/ 3+
4+
3+
for 57 , Mg 2+ for A/ , as well as other ions to a lesser degree, and does
not depend on the ionic strength and pH of the aqueous solution (Kleven
and Alstad, 1996).
The second mechanism involves the reactions of the functional groups
present along the edges of the silica-alumina units and it is affected by
the ionic strength and pH of the aqueous solution (Kleven and Alstad,
1996). The relative contributions of these mechanisms vary by the clay
mineral types. It appears that montmorillonite and illite primarily undergo