INTRODUCTION
Petrophysics is the study of rock properties and their interactions with fluids (gases, liquid hydrocarbons and aqueous solutions). Because
petroleum reservoir rocks must have porosity and permeability, we are
most interested in the properties of porous and permeable rocks. The
purpose of this text is to provide a basic understanding of the physical
properties of permeable geologic rocks and the interactions of the various
fluids with their interstitial surfaces. Particular emphasis is placed on
the transport properties of the rocks for single phase and multiphase
flow.
The petrophysical properties that are discussed in this text
include:
• Porosity
• Absolute permeability
• Effective and relative permeabilities
• Water saturation
Irreducible water saturation
• Hydrocarbon saturation
• Residual oil saturation
• Capillary pressure
• Wettability
• Pore size
• Pore size distribution
• Pore structure
• Net pay thickness
• Isothermal coefficient of compressibility
• Mineralogy
• Specific pore surface area
• Dispersivity
PETROLEUM RESERVOIR ROCKS
A petroleum reservoir rock is a porous medium that is sufficiently
permeable to permit fluid flow through it. In the presence of
interconnected fluid phases of different density and viscosity, such as
water and hydrocarbons, the movement of the fluids is influenced by
gravity and capillary forces. The fluids separate, therefore, in order of
density when flow through a permeable stratum is arrested by a zone of
low permeability, and, in time, a petroleum reservoir is formed in such a
trap. Porosity and permeability are two fundamental petrophysical
properties of petroleum reservoir rocks.
Geologically, a petroleum reservoir is a complex of porous and
permeable rock that contains an accumulation of hydrocarbons under a
set of geological conditions that prevent escape by gravitational and
capillary forces. The initial fluid distribution in the reservoir rock, which
is determined by the balance of gravitational and capillary forces, is of
significant interest at the time of discovery.
A rock capable of producing oil, gas and water is called a reservoir
rock. In general, to be of commercial value, a reservoir rock must have
sufficient thickness, areal extent and pore space to contain a large
volume of hydrocarbons and must yield the contained fluids at a
satisfactory rate when the reservoir is penetrated by a well. Any buried
rock, be it sedimentary, igneous or metamorphic, that meets these
conditions may be used as a reservoir rock by migrating hydrocarbons.
However, most reservoir rocks are sedimentary rocks.
Sandstones and carbonates (limestones and dolomites) are the
most common reservoir rocks. They contain most of the world’s
petroleum reserves in about equal proportions even though carbonates
make up only about 25% of sedimentary rocks. The reservoir character
of a rock may be primary such as the intergranular porosity of a
sandstone, or secondary, resulting from chemical or physical changes
such as dolomitization, solution and fracturing. Shales frequently form
the impermeable cap rocks for petroleum traps.
The distribution of reservoirs and the trend of pore space are the
end product of numerous natural processes, some depositional and some
post-depositional. Their prediction, and the explanation and prediction of
their performance involve the recognition of the genesis of the ancient
sediments, the interpretation of which depends upon an understanding
of sedimentary and diagenetic processes. Diagenesis is the process ofphysical and chemical changes in sediments after deposition that convert
them to consolidated rock such as compaction, cementation,
recrystallization and perhaps replacement as in the development of
dolomite.
MINERAL CONSTITUENTS OF ROCKS - A REVIEW
The physical and chemical properties of rocks are the consequence
of their mineral composition. A mineral is a naturally occurring
crystalline inorganic material that has specific physical and chemical
properties, which are either constant or vary within certain limits. Rock-
forming minerals of interest in petroleum engineering can be classified
into the following families: silicates, carbonates, oxides, sulfates
(sulphates), sulfides (sulphides) and chorides. These are summarized in
Table 1.1. Silicates are the most abundant rock-forming minerals in the
Earth’s crust.
ROCKS
A rock is an aggregate of one or more minerals. There are three
classes of rocks: igneous, metamorphic and sedimentary rocks .
Igneous Rocks
These are rocks formed from molten material (called magma) that
solidified upon cooling either:
1. At the earth’s surface to form volcanic or extrusive rocks, e.g.,
basaltic lava flows, volcanic glass and volcanic ash.
or
2. Below the surface, usually at great depths, to form plutonic or
intrusive rocks, e.g., granites and gabbros.
Igneous rocks are the most abundant rocks on the earth’s crust,
making up about 64.7% of the Earth’s crust
Metamorphic Rocks
These are rocks formed by transformation, generally in the solid
state, of pre-existing rocks beneath the surface by heat, pressure and
chemically active fluids, e.g., quartz is transformed to quartzite and
limestone plus quartz plus heat gives marble and carbon dioxide.
Metamorphic rocks are the second most abundant rocks on the
earth’s crust, making up 27.4% of the Earth’s crust.
Sedimentary Rocks
These are rocks formed at the surface of the earth either by
1. Accumulation and consolidation of minerals, rocks and/or
organisms and vegetation, e.g., sandstone and limestone.
or
2. Precipitation from solution such as sea water or surface water,
e.g., salt and limestone.
Sedimentary rocks are the source of petroleum and provide the
reservoir rock and trap to hold the petroleum in the earth’s crust.
Sedimentary rocks are the least abundant rocks on the earth’s crust,
making up about 7.9% of the earth’s crust. Because most reservoir
rocks are sedimentary rocks, they are of particular interest to us in the
study of petrophysics. Therefore, we will examine sedimentary rocks in
more detail than igneous and metamorphic rocks.
classes of rocks: igneous, metamorphic and sedimentary rocks .
Igneous Rocks
These are rocks formed from molten material (called magma) that
solidified upon cooling either:
1. At the earth’s surface to form volcanic or extrusive rocks, e.g.,
basaltic lava flows, volcanic glass and volcanic ash.
or
2. Below the surface, usually at great depths, to form plutonic or
intrusive rocks, e.g., granites and gabbros.
Igneous rocks are the most abundant rocks on the earth’s crust,
making up about 64.7% of the Earth’s crust
Metamorphic Rocks
These are rocks formed by transformation, generally in the solid
state, of pre-existing rocks beneath the surface by heat, pressure and
chemically active fluids, e.g., quartz is transformed to quartzite and
limestone plus quartz plus heat gives marble and carbon dioxide.
Metamorphic rocks are the second most abundant rocks on the
earth’s crust, making up 27.4% of the Earth’s crust.
Sedimentary Rocks
These are rocks formed at the surface of the earth either by
1. Accumulation and consolidation of minerals, rocks and/or
organisms and vegetation, e.g., sandstone and limestone.
or
2. Precipitation from solution such as sea water or surface water,
e.g., salt and limestone.
Sedimentary rocks are the source of petroleum and provide the
reservoir rock and trap to hold the petroleum in the earth’s crust.
Sedimentary rocks are the least abundant rocks on the earth’s crust,
making up about 7.9% of the earth’s crust. Because most reservoir
rocks are sedimentary rocks, they are of particular interest to us in the
study of petrophysics. Therefore, we will examine sedimentary rocks in
more detail than igneous and metamorphic rocks.