Chap 3- Composition

8/2/2019 Chap 3- Composition

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IntroductionIntroduction

Vary widely in chemical composition.

Depends on location.

Very complex characteristics.



What is Petroleum ?What is Petroleum ?

Petroleum is a mixture of naturally ocurring

hydrocarbons which may exist in the solid,

liquid or gaseous states, depending on theconditions of temperature and pressure to

which it is subjected. Amyx et al




In gaseous state

Natural Gas

In liquid state

Condensates

Petroleum oil

Crude oil In solid state

Tar, Asphalts




At normal temperature and pressure:

Small molecules will be a gas.

Larger molecules will be a liquid

Larger molecules will be in a solid state.



Classification of Hydrocarbons - H ager, Dorsey

Hydrocarbons

Gaseous

Bituminous

Cereous

Fluid

Viscous

Solid

Petroleum

Marsh gas

Natural gas

Crude oil

Asphaltite

Coal

Bitumous shale

Mineral tar

Brea

Gilsonite

Lignite

Subbituminous

Bituminous

Semibituminous

AnthraciteKerogen

Petroliferous

Mineral Wax



Origins of Petroleum SOURCE

Organic material deposited in sedimentary basins.

Marine life: plants and animals

Land derived: carried by rivers

MECHANISM

Reduction & decarboxylation & thermal cracking petroleum.

Gases

CH4 ( Bacteria)

H2S (Sulphur, sulphates in sediments + sulphates inpetroleum)

CO2 (decarboxylation of organic matter, HCO3 & CaCO3 )

N2 (trapped air, organic matter )



Origins of Petroleum MIGRATION

Primary ( movement of water with oil in solution

Secondary ( buoyancy, capillarity, lithology, earth

movements. )



Origins of Petroleum

TEMPERATURE & PRESSURE

Oil is lighter with depth

100oC all hydrocarbons except CH4, C2H6 &

C3H8 are unstable

200

o

C all except CH4 are unstable



Reservoir Fluid CompositionOrigins

Various hypotheses

Depositional environment

Migration path

Compositions

Vary because of depositional characteristics, age & depth.

Evidence of maturing still taking place.

Within some reservoirs compositional gradient ( e.g. BrentNorth Sea )

Detailed compositional description essential for refining.



Reservoir Fluid Composition

Composition of crude oil mainly organic compounds,

principally hydrocarbons.

Small amounts of inorganic non-hydrocarbons, e.g.

CO2, S, N2 and metal compounds.

Hydrocarbons may include the lightest, CH4 tonapthenes and polycylics with high molecular

weights.

Appearance: gases, through clear liquids, yellow

liquids to a dark often black, highly viscous material. Water is always present in pore space. Original

depositional environment.

Physical properties of oil & gas treated independantly

of water



Reservoir Fluid Composition

In exploration and production detailed compositional

information is not required.

Descriptions are required in order to predict physical

properties and behaviour of the fluids at different

conditions.

Simple descriptions are required to characterise fluids

to predict behaviour.

Two methods:

Black Oil Model

Compositional Model



Compositional Model & Black Oil Model

Compositional model

a multicomponent description in terms of

hydrocarbons.

Black Oil model

a two component description in terms of

produced oil ,stock tank oil , and produced

gas,solution gas.



Compositional Model-Hydrocarbons

Chemistry of Hydrocarbons

Hydrocarbons

Aliphatic Aromatic

Alkanes(Paraffins )

Alkenes Alkynes Cyclic Aliphatics( Napthenes )

Unstable



Chemistry of Hydrocarbons

Alkanes or Paraffinic Hydrocarbons

Largest series with open chain molecules and

saturated bonds.

Carbon a valence of 4

Formula CnH2n+2

Unsaturated hydrocarbons valency of 4

not satisfied- structure is not stable



Chemistry of Hydrocarbons- Isomerism

Above propane there are alternative ways to arrangebranched chains.

They are called isomers.

Isomers. Are substances of the same compositionbut with different molecular structure.

Normal Butane Iso Butane

CH3CH2CH2CH3 CH3CHCH3

CH3B.Pt-31.1oF

B.Pt-10.9oF



Basic Properties of Common Paraffin

Hydrocarbons



State Properties of Common Paraffin

Hydrocarbons



Unsaturated Hydrocarbons

Unsaturated hydrocarbons have double or triplebonds between carbon atoms.

Have the potential to add more hydrogen or other

elements.

Therefore termed unsaturated.

Termed olefins

Two typesalkenes

e.g ethylenealkynes

e.g, acetylene

CH2=CH

2 CH-CH



Stable Hydrocarbons

Only stable types

Paraffins

Napthenes

Aromatics



Napthene Series

Formula CnH2n

Sometimes termedcycloparaffins or alicyclic

hydrocarbons.

Single bonds but carbon chain

is closed and saturated.

Very stable

Important constituents of crude

oil. Properties similar to paraffins.

Crude oil termed napthenic

with high napthene content



Aromatics Aromatic series

unsaturated closed-ring

Formula CnH2n-6

Based on the benzene

compound.

Characterised by strong

aromatic odour.

Various compound found

in crude oil. Closed ring gives greater

stability than open chain

compounds.



Aromatic-napthenic crudes

Associated with limestone and dolomitereservoirs.

Iran, Arabian Gulf and Borneo.

Some crudes described according to relativeamounts of non-paraffin compound.

Paraffinic, Napthenic, Aromatic

Not a reservoir engineering term.





Asphalts

Not a series Highly viscous to semi-solid brown-black

hydrocarbons.

High molecular weight. Usually contain sulphur and nitrogen.

May be present in colloidal suspension and

precipitate due to changes in pressure,temperature and composition.



Non-hydrocarbon compounds Small in volume, less than 1%,can have significant influence on

processing and quality of products.

Sulphur: and associated products 0.04 - 5 wgt%. Include

sulphur and hydrogen sulphide which is very toxic. Mercaptans.

On combustion produce undesirable SO2and SO3.

Oxygen: and compounds. Up to 0.5%wgt. Cause corrosive

products.

Nitrogen: less than 0.1%. Complex compounds. Gaseous

reduces thermal quality.

Carbon Dioxide: Very common. Cause of corrosion. Significantimpact on fluid properties.

Other compounds: Metal in low conc.Gases may contain He,H

& Hg. Non-oil produced fluids -water contain minerals which can

form scales



Composition Description for Reservoir

Engineers

Main issue in reservoir engineering is physical behaviour and properties of the petroleum fluids.

Composition has a significant impact on properties and

behaviour.

Compositional description is the key to unlocking physical

properties.

For the oil refiner the composition is the key to determine

the chemical products which can be extracted or processed

from the material.

Petroleum engineer wants as simple a description as

possible to determine physical properties as a function of

T&P.



Composition Description for Reservoir

Engineers

Two models used to describe composition

Black Oil Model Compositional Model

A 2 component description,where two components are

produced fluids,

stock tank oil and solution

gas.

Associated with this model

black-oil parameters

solution gas-oil ratio

oil formation volume factor .

Compositioanl description based

on paraffin series, CnH2n+2

Described up to a limiting C

number . Components greater than

limiting C number are lumped

together as a C+ component



Compositional Model

Isomers, normal and iso are identified up to pentane.

Non paraffinic compounds assigned to next higher paraffin

according to volatility.

All material above limiting C number are termed C+

fraction. e.g. C7+ for a limiting C6 and C10+ for limiting C9.

C+ fraction is unique and characterised by apparent

molecular weight and specific gravity .

Some fluids complex, Paraffin description may not predictbehaviour, may required to identify Napthenic and

Aromatic compounds. PN A analysis.

Perhaps the case for gas condensates at high T&P.



Compositional Model

Reservoir fluid C1 C2 C3 C4 C5 C6 C7+

Gas at surface conditions

Oil at surface conditions

Distribution of compounds a function

of pressure, temperature and

composition



The Uniqueness of the Reservoir

Reservoir Behaviour

Reservoir Description

Reservoir Development

Plan

Dynamic

Unique & Static

Early agreement to

reservoir descriptionspeeds development

Also includes reservoir fluiddescription



Compositional Model

Improved chemical analysis makes it possible toanalyse up to a C value of C29.

Although this leads to good description,

associated computer effort during prediction

modelling is considerable.

Reduced number of components obtained by

grouping various C number compositions.

Reduced to 4 or 5 components.

These are described as ³pseudo components´.



General Analysis

Since reservoirs are unique they also exist at

different P&T.

Common basis and conditions used for describing

quantities of fluids.

Surface conditions-14.7psia (101.3KPa) and 60oF(298K).

G AS - Standard cubic feet, SCF or standard cubic

meter (SCM). LIQUID - stock tank barrels,STB(cubic metres

(STM3)

Relative gas to oil. GOR SCF/STB



General Analysis - density

Many oil types

API, American Petroleum Institute, classified

oils based on density based on a linear scales

hydrometer.

141.5. 131.5

@60o Degrees API

SpecificGravity F !

Specific gravity relative to water @ 60oF



Classification of Reservoir Fluids



Reservoir Fluid

TypesCrude oil + dissolved gas

Condensate Gas

Heavy hydrocarbons dissolved

Near Critical One Phase Fluid(Light oil or condensate fluid)



General Analysis

Refractive Index

Another indicator of density of produced oils.

Ranges from 1.39 to 1.49.

Heavier crude higher the refractive index.

Fluorescence

Measured by its colour under ultraviolet light

Often used on cuttings during drilling.

2o - 10o API non-fluorescent to dull brown

10o - 18o API yellow brown to gold

18o - 45o API gold to pale yellow

45

o

- above API

blue -white to white.



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Chap 3- Composition