Properties of Reservoir Rocks
Introduction
Prior to the discovery of petroleum, mankind used coal as the main source of energy to operate their machines. Since the first commercial well drilled in the United States in 1859, the dependence on petroleum as a source of energy has increased tremendously. From that point onwards, petroleum has been and will continue to be the main source of energy for decades ahead due to its availability, efficiency, and low price. In addition, hydrocarbons are not only used as fuel, but also as lubricants and raw materials for many modern industrial products such as plastics, paints, and rubber.
What is Petroleum?
Petroleum is a naturally occurring hydrocarbon (composed of hydrogen and carbon atoms) that can exist as a solid, liquid, or gas. The physical state of the hydrocarbon is a function of the pressure and temperature to which it is exposed as well as its structure (chain length/molecular weight). However, most of the hydrocarbons found within the ground are either liquid or gas, and are referred to as crude oil and natural gas, respectively.
Origin of Petroleum
There are two theories for the origin of petroleum. They are the organic and inorganic theories.
Organic theory: Living matter, particularly carbon atoms, is the source of petroleum. It comes from evolved from the decomposition of animals and plants that lived during previous geological times.
Inorganic: States that petroleum was formed through chemical reactions between water, carbon dioxide, and several inorganic substances such as carbonates in the earth.
The organic theory is the commonly accepted theory.
Petroleum System
A petroleum system consists of
different geological components needed to generate and store hydrocarbons.
These components are source rock, migration path, reservoir rock, trap, and
seal. Source rock is the rock containing organic matter in sufficient quantity,
and is under suitable conditions for the formation of hydrocarbons. Migration
path is the pathway that the hydrocarbons take to move away from the source
rock to the point where they can find a suitable trap.
Reservoir rock is the rock that is
able to store hydrocarbons in its pores. The hydrocarbons will continue migrating
upward until they reach a seal. This is an impermeable layer of rock that
blocks
the hydrocarbons from further
migration. Finally, a trap is a configuration of rocks, ensuring that the
hydrocarbons are stored in it. Traps can be structural, stratigraphic, or a
combination of both. Figure 1.1 shows the components and
processes in a petroleum system.
 |
Figure 1.1: Schematic showing (a)
the process of hydrocarbon formation and (b) the migration of matured
hydrocarbon until it reaches an impermeable seal and attains static equilibrium. |
A petroleum system consists of
different geological components needed to generate and store hydrocarbons.
These components are source rock, migration path, reservoir rock, trap, and
seal. Source rock is the rock containing organic matter in sufficient quantity,
and is under suitable conditions for the formation of hydrocarbons. Migration
path is the pathway that the hydrocarbons take to move away from the source
rock to the point where they can find a suitable trap. The forces driving the
movement of hydrocarbons out of the source rock come from tectonic stresses,
which are coupled with capillarity and buoyancy (density difference); since
hydrocarbons are lighter than water, they move upward. Reservoir rock is the
rock that is able to store hydrocarbons in its pores. The hydrocarbons will
continue migrating upward until they reach a seal. This is an impermeable layer
of rock that blocks the hydrocarbons from further migration. Finally, a trap is
a configuration of rocks, ensuring that the hydrocarbons are stored in it.
Traps can be structural, stratigraphic, or a combination of both. Figure
1.1 shows the components and processes in a petroleum system.
as shown in Figure 1.2.
Natural gas, if present in a reservoir, is always on top because it has the
lowest density, while water is always at the bottom because it has the highest
density among the three reservoir fluids (gas, oil, and water).
 |
| Figure 1.2: Schematic showing typical hydrocarbon distributions in (a) an oil reservoir, (b) a gas reservoir and (c) a gas–oil reservoir. |
The reservoir rock is porous and permeable,
and the structure is bounded by impermeable barriers that trap the
hydrocarbons.
Planning for production from a
hydrocarbon field requires to be know the quantity of hydrocarbons in the reservoirs,
and its properties in the various parts.
Lithology of Petroleum Reservoirs
Lithology is the study of the general physical characteristics of a
rock. Reservoir rocks can be divided into two lithological types, namely,
sandstone and carbonates. Sandstones are formed from grains that have undergone
sedimentation, compaction, and cementation. Carbonates are principally formed
on carbonate platforms by a combination of biogenic and abiogenic processes.
The major characteristics of both sandstone and carbonate rocks are shown in Table 1.1.
|
Table 1.1: Geological comparison between
sandstone and carbonate rocks. |
|
|
Sandstone |
Carbonate |
|
Usually
composed of silica grains (mainly quartz and some feldspar). Consolidated
(the rock is combined as one unit) or loosely consolidated. May
contain swelling clays (clays have negative impact on reservoir quality). |
•
Two
major types are limestone (CaCO3) and dolomite (CaMg(CO3)2). •
Pore
space consists of inter- or intragranular porosity as well as areas of
dissolution (vugs) and fractures. |
The properties, which we will study, include: porosity, rock
compressibility, single-phase permeability, fluid saturation, electrical
properties of reservoir rocks, wettability, capillary pressure, and relative
permeability.
Rock samples are extracted from the reservoir through cuttings
or coring, can be subjected to two categories of laboratory analysis: routine
core analysis and special core analysis.
Routine Core Analysis
Routine core analysis attempts to find the basic properties of the
reservoir rock such as porosity, grain density, permeability, and fluid
saturation, as shown below:
1.
Porosity
2.
Permeability
3.
Saturation
Special core analysis tests
4.
Wettability
5. Surfaceand interfacial tension
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Petrophysics, reservoir, permeability, porosity