Page 31 - Reservoir Formation Damage

Mineralogy and Mineral Sensitivity of Petroleum-Bearing Formations 15

Amaefule et al. (1988) point out that there are five primary factors
affecting the mineralogical sensitivity of sedimentary formations:

1. Mineralogy and chemical composition determine the
a. dissolution of minerals,
b. swelling of minerals, and
c. precipitation of new minerals.
2. Mineral abundance prevails the quantity of sensitive minerals.
3. Mineral size plays an important role, because
a. mineral sensitivity is proportional to the surface area of miner-
als, and
b. mineral size determines the surface area to volume ratio of
particles.
4. Mineral morphology is important, because
a. mineral morphology determines the grain shape, and therefore
the surface area to volume ratio, and
b. minerals with platy, foliated, acicular, filiform, or bladed shapes,
such as clay minerals, have high surface area to volume ratio.
5. Location of minerals is important from the point of their role in for-
mation damage. The authigenic minerals are especially susceptible to
alteration because they are present in the pore space as pore-lining,
pore-filling, and pore-bridging deposits and they can be exposed
directly to the fluids injected into the near-wellbore formation.

Mungan (1989) states that clay damage depends on (1) the type
+ + 2+
and the amount of the exchangeable cations, such as K , Na , Ca , and
(2) the layered structure existing in the clay minerals. Mungan (1989)
describes the properties and damage processes of the three clay groups
as following:

1. Kaolinite has a two-layer structure (see Figure 2-3), K + exchange
cation, and a small base exchange capacity, and is basically a
nonswelling clay but will easily disperse and move.
2. Montmorillonite has a three-layer structure (see Figure 2-4), a large
base exchange capacity of 90 to 150 meq/lOOg and will readily
+
adsorb Na , all leading to a high degree of swelling and dispersion.
3. Illites are interlayered (see Figure 2-5). Therefore, illites combine
the worst characteristics of the dispersible and the swellable clays.
The illites are most difficult to stabilize.

Sodium-montmorillonite swells more than calcium-montmorillonite
because the calcium cation is strongly adsorbed compared to the sodium cat-
ions (Rogers, 1963). Therefore, when the clays are hydrated in aqueous