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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

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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

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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

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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

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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.

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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).

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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.

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