COMPOSITIONCOMPOSITION

Adrian C Todd

Heriot-Watt UniversityHeriot-Watt UniversityDEPARTMENT OF PETROLEUM ENGINEERING

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 the conditions of temperature and pressure to which it is subjected. Amyx et al

What is Petroleum ?What is Petroleum ? In gaseous state Natural Gas In liquid state Condensates Petroleum oil Crude oil In solid state Tar, Asphalts

What is Petroleum ?What is Petroleum ?

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.

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 in petroleum)

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

200oC all except CH4 are unstable

Reservoir Fluid Composition Origins 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. Brent North 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 to napthenes 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-HydrocarbonsChemistry of Hydrocarbons

Hydrocarbons

Aliphatic Aromatic

Alkanes(Paraffins )

Alkenes Alkynes Cyclic Aliphatics( Napthenes )

Unstable

Chemistry of HydrocarbonsAlkanes or Paraffinic HydrocarbonsLargest 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 arrange branched chains.

They are called isomers.

Isomers. Are substances of the same composition but with different molecular structure.

Normal Butane Iso Butane

CH3CH2CH2CH3 CH3CHCH3

CH3B.Pt-31.1oFB.Pt-10.9oF

Basic Properties of Common Paraffin Hydrocarbons

State Properties of Common Paraffin Hydrocarbons

Unsaturated Hydrocarbons Unsaturated hydrocarbons have double or triple

bonds between carbon atoms. Have the potential to add more hydrogen or other

elements. Therefore termed unsaturated. Termed olefins Two types

alkenes e.g ethylene

alkynes e.g, acetylene

CH2=CH2 CH-CH

Stable Hydrocarbons

Only stable types Paraffins Napthenes Aromatics

Napthene Series Formula CnH2n

Sometimes termed cycloparaffins 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 dolomite reservoirs.

Iran, Arabian Gulf and Borneo. Some crudes described according to relative

amounts 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. Significant impact 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 modelblack-oil parameters

solution gas-oil ratiooil 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 predict behaviour, may required to identify Napthenic and Aromatic compounds. PNA 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 description speeds development

Also includes reservoir fluid description

Compositional Model

Improved chemical analysis makes it possible to analyse 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 “pseudo 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). GAS - 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 APISpecificGravity F

Specific gravity relative to water @ 60oF

Classification of Reservoir Fluids

Reservoir Fluid Types Crude oil + dissolved gas

Condensate GasHeavy 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 brown10o - 18o API yellow brown to gold18o - 45o API gold to pale yellow45o - above API blue -white to white.