Page 28 - Reservoir Formation Damage

12 Reservoir Formation Damage

!LY PACKED AUTHIGENIC CLAY IN PORES.

DETUTM ClAY AGGREGATE GRAIN! TIGHTIY
DETKITAL CUV MATRIX
NILS PORES
Figure 2-1. Disposition of the clay minerals in typical sandstone (after Pittman
and Thomas, ©1979 SPE; reprinted by permission of the Society of Petro-
leum Engineers).

Composition of Petroleum-Bearing Formations

The studies of the composition of the subsurface formations by many,
including Bucke and Mankin (1971) and Ezzat (1990), have revealed that
these formations basically contain: (1) various mineral oxides such as
SiO 2, A1 2O 3, FeO, Fe 2O 3, MgO, K 2O, CaO, P 2O 5, MnO, TiO 2, Cl, Na 2O,
which are detrital and form the porous matrix, and (2) various swelling
and nonswelling clays, some of which are detrital, and the others are
authigenic clays. The detrital clays form the skeleton of the porous ma-
trix and are of interest from the point of mechanical formation damage.
The authigenic clays are loosely attached to pore surface and of interest
from the point of chemical and physico-chemical formation damage.
Typical clay minerals are described in Table 2-1 (Ezzat, 1990).
However, the near-wellbore formation may also contain other sub-
stances, such as mud, cement, and debris, which may be introduced dur-
ing drilling, completion, and workover operations, as depicted by Mancini
(1991) in Figure 2-2.
"Clay" is a generic term, referring to various types of crystalline min-
erals described as hydrous aluminum silicates. Clay minerals occupy a
large fraction of sedimentary formations (Weaver and Pollard, 1973). Clay
minerals are extremely small, platy-shaped materials that may be present
in sedimentary rocks as packs of crystals (Grim, 1942; Hughes, 1951).
The maximum dimension of a typical clay particle is less than 0.005 mm
(Hughes, 1951). The clay minerals can be classified into three main groups
(Grim, 1942, 1953; Hughes, 1951): (1) Kaolinite group, (2) Smectite (or