Post on 21-Apr-2018
1
What is petroleum
Petroleum may be defined as a complex mixture of hydrocarbons, which
may be either gas, liquid or solid, depending on composition and the pressure
and temperature at which it is confined.
Petroleum Composition.
Although the composition of petroleum contains many trace elements the key
compounds are:
Element Percentage (%)
Carbon 83 – 87
Hydrogen 10 – 14
Sulphur 0.5 – 4
Nitrogen 0.1 – 2
Oxygen 0.1 – 1.5
Minerals and Salts 0.1
The principal hydrocarbon series found in petroleum are:
1. Paraffins: also called saturated hydrocarbons or alkanes, which
characterized by the rule that the carbon atoms are connected by a single
bond and the other bonds are saturated with hydrogen atoms. The general
formula for paraffins is CnH2n+2 . These are generally straight or branched
chains, but never cyclic (circular) compounds. The branched chains are
called isomers which have different properties than the same numbers of
straight hydrocarbons. Paraffins are the desired content in crude and what
2
are used to make fuels. The shorter the paraffins are, the lighter the crude
is.
The different in chemical structures of straight chains and branched chains is
shown in this figure:
2. Cycloparaffins (Naphtenes): with a general formula of CnH2n . These types
of hydrocarbons have a ring structure as shown below. They are
higher in density than equivalent paraffins and are more viscous.
3
3. Aromatics (benzene series): these compounds having the general formula of
CnH2n-6 . The compounds are chemically active and contain benzene ring.
They are undesirable because burning them results in soot. They have a
much less hydrogen in comparison to carbon than is found in paraffins. They
are also more viscous. They are often solid or semi-solid when an equivalent
paraffin would be a viscous liquid under the same conditions. The simples
member of this compounds is benzene C6H6.
So, crude oil can be classified into six categories according to the percentage of
each type of hydrocarbons.
4
Source of Petroleum
A complete understanding of the origin of petroleum would be great benefit to
exploration operation, but unfortunately this has not yet been obtained. Many
theories of the origin of petroleum have been made. However, the existing
theories of the petroleum formation can be classified into two groups,
inorganic and organic theories.
The inorganic theories attempt to explain the formation of petroleum by
assuming chemical reactions among water, carbon dioxide and various
inorganic substances, such as carbides and carbonates, in the earth. Whereas
the organic theories suppose that petroleum formed from decomposition of
vegetable and animal organism that live during previous geologic ages (millions
years ago).
5
Despite that petroleum can be produced through the reactions of inorganic
materials in the laboratory, Scientists have ignored the inorganic theory as
untenable because the geologic evidence indicates that these inorganic
materials are not present in subsurface in sufficient quantities to produce
petroleum accumulations.
In contrast, the organic theories are supported by much geologic evidence. Oil
and gas are commonly found in sedimentary basins, moreover, these
accumulations are found nearness the source beds. Source bed is a bed which
rich in organic materials.
Certain requirements must be fulfilled for a commercial oil deposit to be
present. These are:
1. Sources: means materials from which oil is formed.
2. Porous and permeable beds (reservoir rocks): in which the oil is migrate
after being formed.
3. Traps: Subsurface condition restricting further movement of oil such
that it may accumulate in commercial quantities.
Traps
In order for petroleum to accumulate in commercial quantities, it must, in its
movement process, face a subsurface rock condition which stops further
migration and causes the accumulation to take place.
In general, traps are classified into:
1. Structural traps.
6
These types of traps are formed as a result of earth’s crust deformation, either
folding or faulting.
2. Stratigraphic traps.
These result from lateral changes that prevent continued migration of
hydrocarbons in a potential reservoir lithology. Many are directly related to
their environment of deposition, but others (particularly carbonates) are
caused by later changes, such as dolomitization. Many large fields are
associated with this kind of trap.
7
One type of stratigraphic trap is the “lenticular trap”. These pinch out or
change permeability on all sides. Lenticular traps are fairly common in
carbonates, usually occurring in the upper part of reef carbonate buildups.
Example of lenticular trap.
3. Combination traps.
As the name implies, a combination trap is where two (or more)
trapping mechanisms come together to create the trap. In reality,
many successful oil traps are combination traps.
8
Petroleum products.
While the average consumer tends to think of petroleum products as
consisting of a few items such as motor gasoline, jet fuel, home heating oils,
kerosine, etc., a survey conducted by the American Petroleum Institute (API) of
the petroleum refineries and petrochemical plants revealed over 2,000
products made to individual specifications. The simplest refinery process is
fractional (atmospheric) distillation where by the constituents in the oil are
separated by their differences in boiling points.
Almost all the refinery processes are shown in figure below.
9
Physical properties of petroleum.
In practice, it is impossible to obtain the detailed analysis of a crude oil,
therefore, crude oils are classified based on their physical properties. Among
the physical properties commonly considered in various classifications are
color, odor, density, boiling point, freezing point, flash point and viscosity. The
most common used indicator for a crude oil is its API gravity where:
°API =141.5
𝛾− 131.5
10
Where γ is the specific gravity and ⁰API is API (American Petroleum Instituted)
gravity.
The specific gravity of liquids is defined as the ratio of the density of the liquid
to the density of the water (unit less), both at specific conditions of pressure
and temperature. The specific gravity of crude oils ranges from about 0.75 to
1.01.
It may be noted that the API gravity yields numbers greater than 10 for all
materials having specific gravity less than 1. The petroleum industry has
adopted a temperature of 60 ⁰F (15.6 ⁰C) and atmospheric pressure as
standard conditions for calculating specific gravity and API gravity.
Normally, the price which a producer receives for his oil depends on its API
gravity, the less dense oil (higher API gravity) being the most valuable because
it contains higher percentages of the more valuable products such as gasoline.
Example: calculate API gravity of:-
1. 0.8 sp.gr. crude.
2. Crude with density of 7.5 ppg at 60 ⁰F.
Which crude oil contain higher light compounds?
Solution:
1. °API =141.5
𝛾− 131.5
°API =141.5
0.8− 131.5 = 45.375 ⁰
2. Sp. gr. = oil density
𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =
7.5
8.33= 0.9
11
8.33: is the density of water in Ib/gallon at 60 ⁰F.
°API =141.5
𝛾− 131.5
°API =141.5
0.9− 131.5 = 25.722 ⁰
Crude oil No. 1 contains more light compounds than crude oil No. 2.
Basic reservoir rock properties
Porosity
The porosity of a rock is a measure of the storage capacity (pore volume) that
is capable of holding fluids. Quantitatively, the porosity is the ratio of the pore
volume to the total volume (bulk volume). This important rock property is
determined mathematically by the following generalized relationships:
ф = 𝑉𝑝
𝑉𝑏
𝑉𝑝 = (𝑉𝑏 − 𝑉𝑔)
ф = 𝑉𝑏 − 𝑉𝑔
𝑉𝑏
Where:
Ф = porosity
12
𝑉𝑝 = pore volume = the difference between bulk and grain volume.
𝑉𝑏 = bulk volume of the rock.
𝑉𝑔 = grain volume.
As the sediments were deposited and the rocks were being formed during past
geological times, some void spaces that developed became isolated from the
other void spaces by excessive cementation. Thus, many of the void spaces are
interconnected while some of the pore spaces are completely isolated. This
leads to two distinct types of porosity, namely: absolute porosity and effective
porosity.
1. Absolute porosity.
The absolute porosity is defined as the ratio of the total pore space in the rock
to that of the bulk volume. A rock may have considerable absolute porosity
and yet have no conductivity to fluid for lack of pore interconnection. The
absolute porosity is generally expressed mathematically by the following
relationships:
ф𝑎 = 𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑟𝑒 𝑣𝑜𝑙𝑢𝑚𝑒
𝑏𝑢𝑙𝑘 𝑣𝑜𝑙𝑢𝑚𝑒
Where ф𝑎 = absolute porosity.
13
2. Effective porosity
The effective porosity is the percentage of interconnected pore space with
respect to the bulk volume.
ф = 𝑖𝑛𝑡𝑒𝑟𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑝𝑜𝑟𝑒 𝑣𝑜𝑙𝑢𝑚𝑒
𝑏𝑢𝑙𝑘 𝑣𝑜𝑙𝑢𝑚𝑒
Where ф = effective porosity.
The effective porosity is the value that is used in all reservoir engineering
calculations because it represents the interconnected pore space that contains
the recoverable hydrocarbon fluids.
Geologically, porosity has been classified in two types, according to the time of
formation:
A. Primary porosity (intergranular). Porosity formed at the time of
sedimentation.
B. Secondary porosity. Pores formed after the time of sedimentation like
vuges (from solution, fracture, fissures), joints (formed by structural failure
due to folds and faults) and dolomitization (limestone is transformed into
dolomite which have more pores than limestone).
14
The principal factors which complicate intergranular (primary) porosity
magnitudes are:
1- Uniformity of grain size: the presence of small particles such as clay, silt,
etc. which may fit in the voids between larger grains, then porosity is
reduced as a result.
2- Degree of cementation: cementing material deposited around grain
junctions reduces porosity.
3- Packing: geologically young rocks are often packed in an inefficient manner
and are as a result highly porous. There are three main types of packing,
cubic packing, hexagonal packing and rhombohedral packing as shown in
figure.
4- Particle shape.
15