Page 27 - Reservoir Formation Damage

Mineralogy and Mineral Sensitivity of Petroleum-Bearing Formations 11

models, including the swelling rate constants, and terminal porosity and
permeability that will be attained at saturation, are determined by corre-
lating the experimental data with these models. The swelling of clayey
rocks is essentially controlled by absorption of water by a water-exposed
surface hindered diffusion process and the swelling-dependent properties
of clayey rocks vary proportionally with their values relative to their satu-
ration limits and the water absorption rate. These models lead to proper
means of correlating and representing clayey rock properties.

Origin of Petroleum-Bearing Formations

As described by Sahimi (1995), sedimentary porous formations are
formed through two primary phenomena: (1) deposition of sediments, fol-
lowed by (2) various compaction and alteration processes. Sahimi (1995)
states that the sediments in subsurface reservoirs have undergone four
types of diagenetic processes under the prevailing in-situ stress, thermal,
and flow conditions over a very long period of geological times:
(1) mechanical deformation of grains, (2) solution of grain minerals,
(3) alteration of grains, and (4) precipitation of pore-filling minerals,
clays, cements, and other materials. These processes are inherent in de-
termining the characteristics and formation damage potential of petroleum-
bearing formations.

Constituents of Sedimentary Rocks
Many investigators, including Neasham (1977), Amaefule et al. (1988),
Macini (1990), and Ezzat (1990), present detailed descriptions of the vari-
ous constituents of oil and gas bearing rocks. Based on these studies, the
constituents of the subsurface formations can be classified in two broad
categories: (1) indigenous and (2) extraneous or foreign materials.
There are two groups of indigenous materials: (1) detrital materials,
which originate during the formation of rocks and have restricted forma-
tion damage potential, because they exist as tightly packed and blended
minerals within the rock matrix; and (2) diagenetic (or authigenic) mate-
rials, which are formed by various rock-fluid interactions in an existing
pack of sediments, and located inside the pore space as loosely attached
pore-filling, pore-lining, and pore-bridging deposits, and have greater for-
mation damage potential because of their direct exposure to the pore flu-
ids. Extraneous materials are externally introduced through the wells
completed in petroleum reservoirs, during drilling and workover opera-
tions and improved recovery processes applied for reservoir exploitation.
A schematic, pictorial description of typical clastic deposits is given in
Figure 2-1 by Pittman and Thomas (1979).