PVT (Fundamentals of Petroleum Chemistry)

Petroleum Fluid Properties

Copyright of Core Laboratories (1999 Rev 001) Aberdeen Advanced Technology Centre

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CHAPTER 1

FUNDAMENTALS OF PETROLEUM CHEMISTRY

1-1 INTRODUCTION

Petroleum reservoir fluids are naturally occurring mixtures of hydrocarbons that exist inthe reservoir at elevated temperatures and pressures. Reservoir fluids vary widely inchemical composition and - depending on location and origin of the reservoir - haveentirely different physical and chemical properties.

Reservoir fluids are found in gaseous state as natural gas, in liquid state as petroleum orcrude oil, or in solid state such as bitumen or tars. Generally, crude oil at reservoirconditions contains varying amounts of lighter hydrocarbons which would be gaseous atatmospheric pressure and temperature and it usually contains long chain and heavyhydrocarbons such as waxes, resins and asphaltenes. Its appearance varies from yellowliquid to a dark, often a black coloured, highly viscous material, the variety obviouslybeing a function of composition.

The exact origin of petroleum deposits is not fundamentally certain, but it is generallyconsidered to be derived from the remains of organic organisms such as plant, animal, andmarine life (kerogen).

1-2 CHEMICAL COMPOSITION OF HYDROCARBON RESERVOIR FLUIDS

The composition of hydrocarbon reservoir fluids consist mainly of hydrocarbons with asmall percentage of organic and inorganic non-hydrocarbon compounds.

Hydrocarbons are molecules consisting of carbon and hydrogen atoms. The complexity ofhydrocarbon mixtures results from the extremely large number of unique hydrocarbonmolecules. These molecules have traditionally been characterised by the total number ofcarbon atoms within each molecule. Hydrocarbons are divided into two main classes:aliphatics and aromatics. Aliphatic hydrocarbons are further divided into alkanes (orparaffins); unsaturated hydrocarbons (olefins) such as the alkenes and alkynes; andcyclic aliphatics (naphthenes). The general formula and nature of the chemicalstructures are shown in Figure 1.

In addition to hydrocarbons, the non-hydrocarbons such as nitrogen (N2), carbon dioxide(CO2) and hydrogen sulphide (H2S) are found in petroleum fluids. Finally petroleumfluids may also contain hydrogen (H2), helium (He), and traces of metals such as Sodium(Na), Potassium (K).

Crude oils can be classified according to the type of the hydrocarbons which make up theircomposition. This grouping (Paraffinic, Naphthenic, and Aromatic) is commonly referred toas the PNA distribution (characterising the percentage of each grouping in the crude).



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Hydrocarbons

Aromatics(Arenes)

Naphthenics

Cyclic Aliphatics

Aliphatics

Olefinics

AlkynesAlkenes

Paraffinics

Alkanes

IsoparaffinsNormal

C Hn 2n+2

C Hn 2n

C Hn n

C Hn 2n

C Hn 2n-2

2 2-CH-CH- -CH CH--- -C C----CH

2

CH2

CH2

CH2

CH2

CH2

Figure 1: Petroleum Hydrocarbon Classifications

1-2-1 ALKANE OR PARAFFINIC HYDROCARBONS

The first group in the aliphatic hydrocarbons is the alkanes (or paraffins). Alkanes are thelargest constituent of crude oil and are characterised by relative chemical inertness. Theyconsist of chain hydrocarbon segments with a single (-C-C-) (Carbon-Carbon) bond. Themolecules may be un-branched, ie: normal hydrocarbon, or branched iso-paraffins.

The general formula for the paraffin hydrocarbon series is CnH2n+2 where n varies from 1 to1,000. Methane (CH4) is the simplest paraffin and it is the most common constituent ofpetroleum reservoir fluids. It is gaseous at room conditions. Pentane (C5H12) toPentadecane (C15H32) are liquids; these being the main constituents of gasoline and otherfuel fractions. The higher members of paraffin (n>15) are waxy solids at room conditions.

Table 1 gives some basic physical properties of the more common hydrocarbons of theparaffin series. The properties of the n-alkanes show a relatively smooth trend withincreasing chain length. This is illustrated for several important properties in Figure 2.

Chemical formulae alone, however, do not represent the actual structure of the molecules. Isomers are substances of the same composition that have different molecular structureand therefore different chemical and physical properties. The larger the alkane molecule,the greater the number of structural arrangements. For example, methane throughpropane have no structural isomers, butane has two, pentane has three, hexane has eightwhile n-decane has 75 and C30 has an estimated three billion possible isomers. Sincemost naturally occurring hydrocarbon system contains some C30, and in many cases evenhigher carbon numbers, the complexity of petroleum fluids becomes apparent. Forcomparison, Table 2 presents the physical properties of the four isomers of Hexane.



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Component Formula Mol Wt Density Normal Melting(g mole-1) (g cm-3) Boiling Point

Point (C) (C)ParaffinsMethane CH4 16.0 0.2997 -161.5 -182.5Ethane C2H6 30.1 0.3558 -88.6 -182.8Propane C3H8 44.1 0.5065 -42.1 -187.6iso-Butane C4H10 58.1 0.5623 -11.8 -159.6n-Butane C4H10 58.1 0.5834 -0.5 -138.4iso-Pentane C5H12 72.2 0.6238 27.8 -159.9n-Pentane C5H12 72.2 0.6305 36.1 -129.7n-Hexane C6H14 86.2 0.6632 49.3 -95.3n-Heptane C7H16 100.2 0.6874 98.4 -90.5n-Octane C8H18 114.2 0.7061 125.7 -56.8n-Nonanes C9H20 128.3 0.7212 150.8 -53.5n-Decane C10H22 142.3 0.7334 165.8 -29.6n-Undecane C11H24 147.0 0.7890 187.2 -25.6n-Dodecane C12H26 161.0 0.8000 208.3 -9.6n-Tridecane C13H28 175.0 0.8110 227.2 -5.4n-Tetradecane C14H30 190.0 0.8220 246.4 5.9n-Pentadecane C15H32 206.0 0.8320 266.1 9.9

Napthenesneo-Pentane C5H12 72.2 0.5968 9.5 -16.6M-C-Pentane C6H12 84.2 0.7533 71.8 -142.5Cyclohexane C6H12 84.2 0.7827 80.7 6.6M-C-Hexane C7H12 98.2 0.7740 100.9 -126.6

AromaticsBenzene C6H6 78.1 0.8820 80.1 5.5Toluene C7H8 92.1 0.8734 110.6 -95.0EthylBenzene C8H10 106.2 0.8735 136.2 -95.0M-Xylene C8H10 106.2 0.8671 139.1 -47.8O-Xylene C8H10 106.2 0.8840 144.4 -25.2T-M-Benzene C9H12 120.2 0.8797 165.8 -25.3Table 1: Physical Properties of Some Hydrocarbons



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1 2 3 4 5 6 7 8 9 10 11 12

melting point

densityboiling point

0.55

0.60

0.65

0.70

0.75

Density(g cm )-3

-200

-100

0

100

200

T / Co

Carbon NumberFigure 2: Some Properties of the n-Alkanes Homologous Series.

IsomerBoilingPoint(oF)

MeltingPoint(oF)

SpecificGravity60o/60o

n-hexane 155.7 -139.6 0.664

3-methylpentane 145.9 -180.4 0.669

2-methylpentane(isohexane)

140.5 -244.6 0.658

2,3-dimethylbutane 136.4 -199.4 0.666

2,2-dimethylbutane 121.5 -147.8 0.654

Table 2: Physical Properties of Hexane Isomers

1-2-2 ALKENES AND ALKYNES

These groups are also classed unsaturated hydrocarbons (or olefins) and have double ortriple bonds between carbon atoms, ie: Alkenes (-C=C-); Alkynes (-CC-). They have thepotential to combine with more hydrogen or other elements and are therefore termedunsaturated. Hence they are generally not found in crude oil except in refined products(as a by-product of cracking).



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1-2-3 CYCLIC ALIPHATICS

These components, sometimes called cyclo-paraffins (or naphthenes), are identified byhaving single (-C-C-) bonds but the carbon chain is closed and is saturated. The generalformula for these rings is CnH2n and - being very stable components - are importantconstituents of crude oil. Their chemical properties are similar to those of the paraffins. The most common naphthenic compounds found in natural petroleum are shown in Table3.

NameBoilingPoint(oF)

MeltingPoint(oF)

SpecificGravity

(g)Cyclopropane -27 -197

Cyclobutane 55 -122

Cyclopentane 121 -137 0.750

Cyclohexane 177 44 0.783

Cycloheptane 244 10 0.810

Cyclooctane 300 57 0.830

Methylcyclopentane 161 -224 0.754cis-1,2-dimethylcyclopentane 320 -80 0.772trans-1.2-dimethylcyclopentane 198 -184 0.750

Methylcyclohexane 214 -196 0.774

Cyclopentene 115 -135 0.774

1,2-cyclopentadiene 108 -121 0.798

Cyclohexene 181 -155 0.810

1,3-cyclohexadiene 177 -144 0.840

1,4-cyclohexadiene 189 -56 0.847

Table 3: Some Physical Properties of Cyclic Aliphatic Hydrocarbons

1-2-4 AROMATICS

The aromatic series have one or more ring structures with aromatic double bonds similarto Benzene (C6H6). These hydrocarbons are unsaturated closed rings with a strongaromatic odour. Naphthalene (C10H8), anthracene (C14H10) and other polycycliccompounds are all commonly found in natural crude oil. Some of the simpler aromaticconstituents of crude oil are illustrated in Figure 3.



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BENZENE

ANTHRACENENAPTHALENE

Figure 3: Some Examples of Petroleum Aromatic Compounds

1-2-5 SULPHUR COMPOUNDS

The most common sulphur compounds found in crude oil Systems are HydrogenSulphide and Mercaptans (in the range of 0.04 - 5 wt%) as well as free sulphur itself. Thenatural gas and crude systems which contain sulphur compounds are called 'sour' whilstthose without sulphur compounds are sometimes called 'sweet'.

Hydrogen sulphide

Hydrogen sulphide (H2S) is a colourless gas with a boiling point of -59.5 oC and anextremely unpleasant odour. Hydrogen sulphide is highly poisonous and its presence isdetrimental to the petroleum refiner since sulphur deteriorates (poisons) the metalliccatalysts used in the refining process. It is usually removed from natural gas byabsorption with Ethanolamines. However, some natural gases with high concentration ofH2S are used in the production of sulphur as a by-product.

Mercaptans

The mercaptans or thiols have the general formula R-S-H in which R represents anyorganic group such as alkyl or cyclic aliphatics, ie:

H - S - CH3 R' - S - HMethanethiol Thiophenyl

where: R' = Benzene ring

In general the mercaptans have a more disagreeable odour than hydrogen sulphide andmust be processed and removed from crude during the refining process.



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Alkyl Sulphides

The alkyl sulphides or thio ethers have the general formula of (R-S-R). Their distinctiveodours are not as disagreeable as H2S or mercaptans, but they cause much the sameproblems.

CH2=CH-CH2-S-CH2-CH=CH2 Alkyl sulphide

1-2-6 OTHER COMPONENTS CONTAINING NON-HYDROCARBONS

Compounds incorporating nitrogen and oxygen are also found in limited amounts incrude oil. Organic acids contain oxygen (up to 0.5 wt%) while nitrogen occurs (up to 0.1wt%) in the form of bases and neutral nitrogen compounds (Figure 4).

PYRIDINE

H

H

CARBAZOLES

PYROLLES

QUINOLENES

Figure 4: Typical Nitrogen Compounds found in Crude Oil

Free elemental nitrogen can also occur in petroleum, however free oxygen does not due tooxidative reactions.

1-2-7 WATER, ELECTROLYTES AND METALS

Water, inorganic salts (eg: Magnesium Chloride (MgCl2), Calcium Chloride (CaCl2), SodiumChloride (NaCl)) in aqueous solution and metals such as Copper (Cu), Zinc (Zn),Iron (Fe),and Vanadium (Va) are also found in crude oil in small quantities. Many of these saltsneed to be removed before refining as some generate Hydrochloric Acid (HCl) when heatedwith water.



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1-3 ANALYSIS OF RESERVOIR FLUIDS

Since the number of compounds in a crude oil are virtually innumerable, no crude oil hasbeen separated into all of its individual components. The compositional analysis ofreservoir fluids is usually reported by paraffin fractions (groups of components falling intothe boiling ranges - see Reservoir Fluid Analysis) which is sufficient for most applications. In general, all isomers are reported individually up to n-pentane, but above n-pentanethey are lumped together as groups designated C6, C7, .... Cn+ where n+ is everything abovea specified carbon number n. Examples of field compositions for Natural Gas, Separatorgas and oil, and gas condensates for the North Sea are shown in Table 5.

Crude oils can also be classified chemically according to the structure of the largermolecules in the mixture. As already mentioned, classification may be expressed byParaffinic, Naphthenic, Aromatic, and Asphaltic.

Paraffinic oils yield very fine lubricating oils and also paraffin wax from the distillationresiduum, while Asphaltic oils produce heavier fractions which are used for pitch, roofing,and paving asphalt.

More generally, oil may be classified according to :

i) The American Petroleum Institute (API) gravity number (see Chapter 3)

ii) The general Refractive Index (range for oils is 1.39 to 1.49)

iii) Fluorescence of oil measured under UV light.

API GRAVITY No.RANGE

COLOUR

2 - 10 Non-Fluorescent(Dull)

10 - 18 Yellow-Brown to Gold

18 - 45 Gold to Pale Yellow

45 - Above Blue- White to WhiteTable 4



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Component GasGas

CondensateVolatile

OilBlackOil

N2 0.30 0.71 1.67 0.67

CO2 1.10 8.65 2.18 2.11

C1 90.00 70.86 60.51 34.93

C2 4.90 8.53 7.52 7.00

C3 1.90 4.95 4.74 7.82

C4 (i+n) 1.10 2.00 4.12 5.48

C5 (i+n) 0.40 0.81 2.97 3.80

C6 (i+n) 6+ : 0.30 0.46 1.99 3.04

C7 0.61 2.45 4.39

C8 0.71 2.41 4.71

C9 0.39 1.69 3.21

C10 0.28 1.42 1.79

C11 0.20 1.02 1.72

C12 0.15 12+ : 5.31 1.74

C13 0.11 1.74

C14 0.10 1.35

C15 0.07 1.34

C16 0.05 1.06

C17 17+ : 0.37 1.02

C18 1.00

C19 0.90

C20 20+ : 9.18

Total: 100.00 100.00 100.00 100.00

Table 5: Typical Compositions of Fluids

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PVT (Fundamentals of Petroleum Chemistry)