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33Reservoir Fluids Composition

CONTENTS

1 INTRODUCTION

2 HYDROCARBONS2.1 Chemistry of Hydrocarbons2.2 Alkanes or Paraffinic Hydrocarbons2.3 Isomerism2.4 Unsaturated Hydrocarbons2.5 Napthene Series2.6 Aromatics2.7 Asphalts

3 NON-HYDROCARBON COMPOUNDS

4 COMPOSITIONAL DESCRIPTION FORRESERVOIR ENGINEERING4.1 Definitions of Composition in Reservoir

Engineering

5 GENERAL ANALYSIS5.1 Surface Condition Characterisation5.2 Refractive Index5.3 Fluorescence of Oil

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LEARNING OBJECTIVES

Having worked through this chapter the Student will be able to:

Describe briefly the origin, nature and appearance of petroleum fluids

Be aware that the principal components of petroleum fluids to be hydrocarbons.

Draw a diagram illustrating the classification of hydrocarbons and to identify;paraffins (alkanes ), aromatics and cyclic aliphatics ( napthas).

List the non- hydrocarbon compounds which might be present in small qualitiesin reservoir fluids.

Define the black oil model description of the composition of a reservoir fluid.

Explain briefly what PNA analysis is and its application.

Describe briefly the concept of pseudo components in fluid compositioncharacterization.

Be aware of general analysis descriptors for petroleum fluids e.g. oAPI,refractive index and flourescence.

Be able to calculate the API gravity given the specific gravity

Department of Petroleum Engineering, Heriot-Watt University 3


1 INTRODUCTION

Petroleum deposits vary widely in chemical composition and depending on locationhave entirely different physical and chemical properties. The very complex charac-teristics are evident from the many products which can be produced from oil and gas.

What is petroleum? Petroleum is a mixture of naturally occurring hydrocarbonswhich may exist in the solid, liquid or gaseous states, depending on the conditions oftemperature and pressure to which it is subjected.1

Petroleum deposits occurring as a gaseous state are termed natural gas, in the liquidstate as petroleum oil or crude oil and in the solid state as tars, asphalts and waxes.

For a mixture with small molecules it will be a gas at normal temperature and pressure(NTP). Mixtures containing larger molecules will be a liquid at NTP and largermolecules as a solid state, for example, tars and asphalts.

The exact origin of these deposits is not clear but is considered to be from plant, animaland marine life through thermal and bacterial breakdown.

The composition of crude oil consists mainly of organic compounds, principallyhydrocarbons with small percentages of inorganic non-hydrocarbon compounds.such as carbon dioxide, sulphur, nitrogen and metal compounds. The hydrocarbonsmay include the lightest (C1 methane ) to napthenes and polycyclics with highmolecular weights.

The appearance varies from gases, through very clear liquids, yellow liquids to a dark,often black, highly viscous material, the variety obviously being a function ofcomposition. Although the principal elements are carbon (84-87%), and hydrogen(11-14%), crude oil can vary from a very light brown liquid with a viscosity similarto water to a very viscous tar like material .

Water is always present in the pore space of a reservoir, since the original depositionalenvironment for the rocks was water. This water has subsequently been displaced bythe influx of hydrocarbons but not totaly since surface tension forces acting in the rockpore space cause some of the water to be retained.

For reservoir engineering purposes the description of the composition is an importantcharacterisation parameter for the determination of a range of physical parametersimportant in various reservoir volumetric and flow calculations. It is not the concernof the reservoir engineer to determine the composition with respect to understandingthe potential to separate the material to a range of saleable products. For this reasontherefore simplistic characterisation approaches are used.

The two compositional characterisation approaches used are the compositional modeland the black oil model. The basis of the compositional model is a multicomponentdescription in terms of hydrocarbons and the black oil model is a two componentdescription in terms of produced oil, stock tank oil and produced gas, solution gas. The

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compositional model is the topic covered in this chapter and the black oil model iscovered in the liquid properties chapter.

2 HYDROCARBONS

2.1 Chemistry of HydrocarbonsThe compositional model uses hydrocarbons as the descriptor since hydrocarbonsrepresent the largest proportion in petroleum fluids. It is important to review brieflythe chemistry of hydrocarbons.

The hydrocarbon series is represented in figure 1 below

Hydrocarbons

Aliphatic Aromatics

Alkanes Alkenes Alkynes Cyclic Aliphatics(Paraffins) (Napthenes)

The hydrocarbons divide into two groupings with respect to the arrangement of thecarbon molecules and the bonds between the carbon molecules. The arrangement ofthe molecules are open chain or cyclic and the bonds between the carbon are saturated(single) bonds or unsaturated or (multiple) bonds.

2.2 Alkanes or Paraffinic HydrocarbonsThe largest series is the alkanes or paraffins which are open chain molecules withsaturated bonds. Carbon has a valance of four and therefore the formula for thesecompunds is C

nH2n+2. These saturated hydrocarbons include all the paraffins in which

the valence of the carbon atoms is satisfied by single covalent bonds. This type ofstructure is very stable. Unsaturated hydrocarbons are those where the valence ofsome of the carbon atoms is not satisfied with single covalent bonds so they areconnected by two or more bonds which make them less stable and more prone tochemical change.

The paraffin series begins with methane (CH4), and its basic formula is CnH2n+2.Pentane to pentadecane are liquids and the chief constituents of uncracked gasoline.Its higher members are waxy solids. In a given bore hole the wax may clog the porespace next to the hole as gas expands and cools.

The paraffins are the largest constituent of crude oil and are characterised by theirchemical inertness. Clearly they would not have remained as they are if this were not so.

2.3 IsomerismFrom methane to propane there is only one way to arrange the branched chainshowever above propane there are alternative arrangements and these are calledisomers.

Structural formulae do not represent the actual structure of the molecules. Isomers aresubstances of the same composition that have different molecular structure and

Figure 1Classification ofHydrocarbon.



therefore different properties, for example, normal butane and isobutane.

normal butane CH3CH2CH2CH3 - B.Pt. 31.1F

isobutane CH3CH CH3 - B.Pt. 10.9F

CH3

Pentane has three structures (isomers). Clearly the number of isomers increase as thenumber of carbon atoms increases. Hexane has 5 isomers and heptane 9.

Table 1 below gives some of the basic physical properties of the more commonhydrocarbons of the paraffin series and Table 2 lists the state of the various purecomponents demonstrating that components which might be solid on their owncontribute to liquid states when part of a mixture. Figure 2 gives some structuralformula for three paraffin componds.

Name Chemical Molecular Boiling Point Critical Gas Liquid Formula Weight (C) at normal Temp C (air = 1) (water = 1)

conditionssp.gr.

Methane CH4 16.04 -161.4 -82.4 0.554 0.415 (-614)Ethane C2H6 30.07 -89.0 32.3 1.038 0.54 (-88)Propane C3H8 44.09 -42.1 96.8 1.522 0.585 (-44.5)n-butane C4H10 58.12 0.55 153.1 2.006 0.601 (0)Isobutane C4H10 58.12 -11.72 134.0 2.006 0.557n-pentane C5H12 72.15 36.0 197.2 2.491 0.626Isopentane C5H12 72.15 27.89 187.8 2.491 0.6197n-hexane C6H14 86.17 60.30 228.0 2.975 0.6536

Density

1 Methane Gas2 Ethane Gas3 Propane Gas4 Butane Gas5 Pentane Liquid6 Hexane Liquid7 Heptane Liquid8 Octane Liquid9 Nonane Liquid10 Decane LiquidC5-C17 LiquidC18+ Solid

ALKANES or PARAFFIN HYDROCARBONS Cn H 2n+2

No of carbon Name State (ntp) atoms

Table 1Physical properties ofcommon hydrocarbons.

Table 2Alkanes or ParaffinHydrocarbons Cn H 2n + 2

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H

Methane

HC HH H

PARAFFINS HC HH

HH

HHHH

Iso-butane

C CCH C CC C CC C HCH HH H H H H H

H HH H H H H H

n-octane

2.4 Unsaturated HydrocarbonsThese are hydrocarbons which have double or triple bonds between carbon atoms.They have the potential to add more hydrogen or other elements and are thereforetermed unsaturated. There are termed the olefins, and there are two types, alkenes, forexample ethylene, CH2=CH2, which have a carbon-carbon double bond and alkynes,for example acetylene,CH=CH which have a carbon carbon triple bond. Bothcompound types being unsaturated are generally very reactive and hence are notfound in reservoir fluids.

2.5 Napthene SeriesThe napthene series (C

nH2n) sometimes called cycloparaffins or alicyclic hydrocarbons

are identified by having single covalent bonds but the carbon chain is closed and issaturated. They are very stable and are important constituents of crude oil. Theirchemical properties are similar to those of the paraffins. A crude oil with a highnapthene content is referred to as an napthenic based crude oil. An example iscyclohexane C6H12. Figure 3 gives the structural formula for two napthenic compounds.

H

CH H

HH

HHH H

HHH

C

C

C

C

C

NAPHTHENES

MethylCyclopentane Cyclohexane

C

H H

HH

HHH H

HH

C

C

C

C

C

H H

2.6 AromaticsThe aromatic series (C

nH2n-6) is an unsaturated closed-ring series, based on the benzene

compound and the componuds are characterisied by a strong aromatic odour. Variousaromatic compounds are found in crude oils. The closed ring structure gives thema greater stability than open compounds where double or triple bonds occur. Figure4 gives the structural formula for two aromtic compounds.

AROMATICS

Benzene

CC C

CCC

H

H

H

H

H

H

Naphthalene

C

C

H

H

H

CC

CC

CC

CC

H

H

H

H H

Figure 2Gives some standardformula for saturatedhydrocarbons

Figure 3Structural formula for twonaphenic compounds.

Figure 4Structural formula for twoaromtic compounds.



The aromatic-napthene based crudes are usually associated with limestone anddolomite reservoirs such as those found in Iran, the Arabian Gulf and Borneo.

Some crude oils used to be described, more from a refining perspective, according tothe relative amount of these non paraffin compounds. Crude oils would be calledparaffinic, napthenic or aromatic. It is not a classification of value in reservoirengineering.

Physical Properties of some Common Petroleum Reservoir Fluid Constituents

Component Formula Melting Point Normal Boiling Point Density (g/cm3) (C) (C) at 1 atm and 15C

Paraffins Methane CH4 -184 -161.5 -Ethane C2H6 -172 -88.3 -Propane C3H8 -189.9 -42.2 -n-Butane C4H10 -135 -0.6 -Iso-Butane C4H10 -145 -10.2 -n-Pentane C5H12 -131.5 36.2 0.626n-Hexane C6H14 -94.3 69.0 0.659Iso-octane C8H18 -107.4 99.3 0.692n-Decane C10H22 030 174.0 0.730Naphthenes Cyclopentane C5H10 -93.3 49.5 0.745Methyl cyclo-pentane C6H12 -142.4 71.8 0.754Cyclohexane C6H12 6.5 81.4 0.779Aromatics Benzene C6H6 5.51 80.1 0.885Toluene C7H8 -95 110.6 0.867Xylene C8H10 -29 144.4 0.880Naphthalene C10H8 80.2 217.9 0.971

2.7 AsphaltsAsphalt is not a series by itself. Asphalts are highly viscous to semi-solid, brown-black hydrocarbons of high molecular weight usually containing a lot of sulphur andnitrogen, which are undesirable components, and oxygen. Asphalts are closelyrelated to the napthene series and because of their high nitrogen and oxygen contentthey may be considered juvenile oil, not fully developed.

3 NON-HYDROCARBON COMPOUNDS

Although small in volume, generally less than 1%, non-hydrocarbon compoundshave a significant influence on the nature of the produced fluids with respect toprocessing and the quality of the products.

The more common non-hydrocarbon consituents which may occur are:sulphur, oxygen, nitrogen compounds, carbon dioxide and water.

Table 3Physical properties of somecommon petroleumreservoir fluid constituents

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Sulphur and its associated compounds represent 0.04% - 5% by weight. Thesecorrosive compounds include sulphur, hydrogen sulphide (H2S ),which is very toxic,and mercaptans of low molecular weight ( these are produced during distllation andrequire special metals to avoid corrosion). Non-corrosive sulpher materials includesulphides. Sulphur compounds have a bad smell and both the corrosive and non-corrosive forms are undesirable. On combustion these products produce S02 and S03which are undesirable from an environmental perspective.

Oxygen compounds, up to 0.5% wt., are present in some crudes and decompose toform napthenic acids on distillation, which may be very corrosive.

Nitrogen content is generally less than 0.1% wt., but can be as much as 2%. Nitrogencompounds are complex . Gaseous nitrogen reduces the thermal quality of natural gasand needs to be blended with high quality natural gas if present at the higher levels.

Carbon Dioxide is a very common consituent of reservoir fluids, especially in gasesand gas condensates. Like oxygen it is a source of corrosion. It reacts with water toform carbonic acid and iron to form iron carbonate. Carbon dioxide like methane hasa significant impact on the physical properties of the reservoir fluids.

Other compounds. Metals may be found in crude oils at low concentration and are oflittle significance. Metals such as copper, iron, nickel, vanadium and zinc may bepresent. Produced natural gas may contain helium, hydrogen and mercury.

Inorganic compounds The non-oil produced fluids like water will clearly containcomponds arising from the minerals present in the rock, their concentration willtherefore vary according to the reservoir. Their composition however can have a verysignificant effect on the reservoir behaviour with respect to their compatibility withinjected fluids. The precipitation of salts, scale, is a serious issue in reservoirmanagement.Many of these salts need to be removed on refining as some generate HC1 whenheated with water.

4. COMPOSITIONAL DESCRIPTION FOR RESERVOIR ENGINEERING

4.1 Definitions of Composition in Reservoir EngineeringIn petroleum engineering, and specifically in reservoir engineering, the main issue isone of the physical behaviour and characteristics of the petroleum fluids. Thecomposition of the fluid clearly has a significant impact on the behaviour andproperties. In petroleum engineering therefore the description of the composition isa key to determine the physical properties and behaviour.

For the oil refiner or chemical manufacturer the composition of the fluid is the key todetermine what chemical products can be extracted or processed from the material.The petroleum engineer is not concerned with the fact that the oil might contain, albeitin small concentrations, hundreds of different components. The petroleum engineerwants as simple a description as possible which still enables the determination of thephysical properties and behaviour under different temperature and pressure conditions.



Two models are used in this industry to describe the composition for physical propertyprediction purposes, the black-oil model and the compositional model.

The black-oil model is a 2 component desription of the fluid where the twocomponents are, the fluids produced at surface, stock tank oil and solution gas.Associated with this model are black-oil parameters like solution gas-oil ratio and theoil formation volume factor. These parameters are discussed in the chapter on liquidproperties.

The compositional model is a compositional description based on the parafin seriesC

nH2n+2. The fluid is described with individual compositions of norman paraffins up

to a limiting C number. Historically C6, more common now to go up to C9, or evenhigher. Components greater than the limiting C number are lumped together anddefined as a C+ component.

Isomers, normal and iso are usually identified up to pentane. Non paraffiniccompounds are assigned to the next higher paraffin according to its volatility. Thematerial representing all compounds above the limiting carbon number are called theC

+ fraction , so C7+ for a limiting value of C6 and C10+ for a limiting value of C9.

The physical properties of paraffins up to the limiting C number are well known anddocumented. The C+ component is however unique to the fluid and therefore twoproperties are used to characterise it, apparent molecular weight and specific gravity.

The behaviour of some fluids are complex and the paraffin based description mayhave difficulty in predicting properties under certian conditions. Consideration maybe required to also identify napthenic and aromatic compounds, (PNA analysis),whichcould be contributing to complex behaviour. This is particularly the case for gascondensates existing at high pressures and high temperaures.

Figure 4 illustrates the compositional model and its application as reservoir fluids areproduced to surface. Although the individual components contribute to a single liquidreservoir phase for an oil, when the fluids are produced to surface they produce a gasphase, solution gas, and a liquid phase, stock tank oil. The distribution characteristicsof the individual components is complex and not just a function of temperature andpressure. For reservoir fluids the compostion is also an influence on the distribution.This makes it a difficult task to predict this distribution perspective since reservoirfluid compositions are unique. This topic is further dealt with in the chapter on vapourliquid equilibrium. Improved methods of chemical analysis make it possible todescribe the oil up to a C value of C29. Although such definitions provide a veryaccurate description, the associated computer effort in using such a comprehensivedescription does lead to the use of pseudo components. Pseudo components areobtained by grouping the various C number compositions, thereby reducing thedescription to 4 or 5 "pseudo components". A number of methods exist to group thevarious C values and other components.

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Reservoir Fluid Gas at Surface Conditions

Oil at Surface Conditions

C1 C2 C3 C4 C5 C6 C7+

The relative amounts of C1 - C7+ are afunction of :

Temperature, Pressure, Composition (particularly at high temperature)

5. GENERAL ANALYSIS

5.1 Surface condition characterisationReservoirs as well as having unique compositions also exist at specific pressures andtemperatures. It is important therefore to provide a common basis for describing thequantities of fluids in the reservoir and throughout the production process.

The basis chosen is the fluids at surface conditions, the surface conditions being 14.7psia or 101.3 kPa and 60oF or 298K. These conditions are called standard conditions.For gas therefore this yields standard cubic feet SCF or standard cubic meters SCM.It is useful to consider these expression not as volumes but as mass, the volume ofwhich will vary according to density. For liquids we express surface conditions asstock tank volumes either stock tank barrels STB or stock tank cubic meters STM3.The relative amount of gas to oil is expressed by the gas-oil ratio GOR SCF/STB.

Since there are so many types of oil, each with a wide range of specific gravity, anarbitrary non-linear relationship was developed by the American Petroleum Institute(API) to classify crude oils by weight on a linear-scaled hydrometer. The observedreadings are always corrected for temperature to 60oF, by using a prepared table ofstandard values.

Degrees API = 141.5 -131.5Sp.Gr.at 60F (1)

Sp.Gr = specific gravity relative to water ar 60oF.

Figure 4Compositional Model



The API gravity of water is 10. A light crude oil would have an API gravity of 40,while a heavy crude would have an API gravity of less than 20. In the field, the APIgravity is readily measured using a calibrated hydrometer.

There are no definitions for categorising reservoir fluids, but the following table 5indicates typical GOR, API and gas and oil gravities for the five main types. Thecompositions show that the dry gases contain mostly paraffins, with the fraction oflonger chain components increasing as the GOR and API gravity of the fluidsdecrease.

In chapter 4 we give a classification for the various reservoir fluid types in the contextof phase behaviour.

Type Dry Gas WetGas Gas Condensate Volatile Oil Black Oil

Appearance Colourless Colourless Colourless Brown liquid Black at surface Gas Gas + + significant Some Viscous

clear liquid clear/straw Red/Green LiquidColour Liquid

Initial GOR No Liquids >15000 3000-15000 2500-3000 100-2500 (scf/stb)

API - 60-70 50-70 40-50

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decreases. If whole core is being examined then the whole core should be passed underUV light to determine the fluorescent colour and the pattern of oil-in-place in the coredinterval.

When possible, pictures should be taken of the core showing the fluorescence. Theseare very useful when accompanying reports to the head office which may be hundredsif not a few thousand miles away.The degree of flourescence is indicated below for different compositions as reflectedin the API gravity.

2 - 10 API non-fluorescent to dull brown10 - 18 API yellow brown to gold18 - 45 API gold to pale yellow45 - above API blue-white to white

It should be pointed out that most oils increase in API gravity with depth in a givenlithologic column with the reason being that younger juvenile oils, heavier with alower API gravity, have not yet been transformed from the initial formation conditionsto higher petroleum members. Two well-known exceptions to this pattern are foundin the Burgan sands of Kuwait and the shallow sands of the Bibi Eibat field in theUSSR where the high-gravity members are found higher up in the stratified columnthan the low-gravity members.



EXERCISE 1

Calculate the Specific Gravity (SG) of a 38o API oil. What is its density in lbs/cu.ft?(62.32 lbs/cu.ft equals an SG of 1.0 and 43.28 API)Now convert an oil with an SG of 0.744 to Degrees API.

EXERCISE 2

A reservoir oil is quoted as having a Gas Oil Ratio (GOR) of 604 scf/bbl. Convertthis to Standard Cubic Meters (SCM)gas per Stock Tank Cubic Meters (SM3)

1 Foot = 0.3048m1 barrel = 5.615 cu ft.1 barrel = 0.159 M3

EXERCISE 3

A reservoir is said to contain an initial GOR of 11,000scf/bbl. What type of reser-voir is described, and what API oil could be typically expected from such a field?

EXERCISE 4

Define the Black Oil Model and the Compositional Model

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Solutions to Exercises

EXERCISE 1

Calculate the Specific Gravity (SG) of a 38o API oil. What is its density in lbs/cu.ft?(62.32 lbs/cu.ft equals an SG of 1.0 and 43.28 API)Now convert an oil with an SG of 0.744 to Degrees API.

SOLUTION

Convert using the equation 1:

API = (141.5 / SG) -131.5

38= (141.5 / SG) -131.5

Sg= 141.5 / (131.5 + 38)

SG = 0.835

Similarly, to convert SG into API:

API = (141.5 / 0.744) -131.5API = 58.7o

EXERCISE 2

A reservoir oil is quoted as having a Gas Oil Ratio (GOR) of 604 scf/bbl. Convert thisto Standard Cubic Meters (SCM)gas per Stock Tank Cubic Meters (SM3)

1 Foot = 0.3048m1 barrel = 5.615 cu ft.1 barrel = 0.159 M3

SOLUTION

604 scf/bbl = 604 * 0.30483 STM/bbl = 17.09 SCM/bbl= 107.48 SCM/STM3

EXERCISE 3

A reservoir is said to contain an initial GOR of 11,000scf/bbl. What type of reservoiris described, and what API oil could be typically expected from such a field?

SOLUTION

A reservoir with a GOR of 11,000 scf/bbl would be typically termed a GasCondensate Reservoir. The API gravity would probabally be in the low 50s.



EXERCISE 4

Define the Black Oil Model and the Compositional Model

SOLUTION

Black Oil Model.Two component description of the reservoir fluid consisting of stock tank oil andsolution gas. Compositional changes with varying pressure and temperature areignored. Terms such as Gas Oil Ratio and Formation Volume Factor are blackoil model terms.

Compositional Model.The compositional model is based on the parafin series C

nH2n+2. To keep the number

of components in the model manageable, long chain members are grouped togetherand given an average property. These compounds are termed collectively as the C+fraction. Typically this covers the hydrocarbons above Heptane and therefore iscalled the C7+ fraction, which is characterised using the terms Apparent MolecularWeight and Specific Gravity.

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REFERENCES.

1. Amyx,J.W.,Bass,D.M., and Whiting,R.L."Petroleum Reservoir Engineering",McGraw-Hill Book Company, New York 1960

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