Composition of Petroleum

8/2/2019 Composition of Petroleum

1/22

Composition, Classification, &Physical Properties of Crude Oils


2/22

Composition & Classification of Crude OilsMain groups of compounds in crude oils Saturated Hydrocarbons:

Usually the major group, except in degraded heavy oils; Contain normalplusisoalkanes(paraffins) &


3/22

Paraffin-basecrudes constitute only a tiny fraction of modern world crudesupplies (~2%)

The standard is Pensylvania crude; this and most other North Americancrudes are Paleozoic in age (from Michigan, Ohio, Oklahama)

Nearly most of the paraffinic crudes from other continents are much younger:Mesozoic in southern Chile, Brasil, Southern Russia; even Tertiary in someAfrican Basins, Caucasus, Borneo, Chinese basins

Crudes dominated by the naphtheniccomponents are called Asphalt-baseoils,as asphalts are closely associated with the naphthenes and not with theparaffins. ~15% of world crude supplies are truly naphthene based; These are

the

black-oils

of Venezuela, Mexico, parts of California, Gulf coast, and manyRussian crudes The great majority of crude oils are of mixed base (naphthene paraffin); they

include nearly all Middle East, Mid-continent, and North Sea oils Most crude oils contain minor quantities of HC (pristaneand phytane)

belonging to neither the paraffins nor the naphthene series; occurrences ofthese in crude oil and ancient sediments are believed to be derivatives ofchlorophyll, and they constitute useful indicators of an oils origin

The ratio between pristane and phytane content of a crude oil or rock extractindicates the type of organic matter from which the oil originated and/or thelevel of thermal maturation of the source. The ratio is therefore a vital finger-print for any crude oil containing both molecules


4/22

Evolution & alteration change the composition ofcrude oils

Process Initial crude ProductThermalevolution Paraffinic-naphthenic oil Paraffinic oilAlteration(Degradation) Paraffinic &Paraffinic-

naphthenic oilAromatic- naphthenic(1% S)


5/22

Natural Gas Natural gas consists of HC not condensable at 20C (68F) and atmospheric

pressure These are the first four members of the paraffin series (methane through n-

butane) Gas composed almost entirely of methane is dry gas; if the proportion of

ethane (C2H6) and heavier molecules exceeds some arbitrary value(conventionally 0.3 US gallons of vapour / cubic feet of gas, or between 4 and5%, in North America), the gas is called wet gas

Natural gases consisting largely or wholly of methane may have any one of threedistinct origins

Petroleum gas: formed as a byproduct of the generation of petroleum Associated gas: accompanies oil Non-associated gas: no longer accompanies oil; formed by the thermocatalytic

modification of petroleum

Coal gas: formed by the modification of coal, thermocatalytically or otherwise Bacterial gas: formed by the low-temperature alteration of organic matter at ornear the earths surface

The amount of gas in solution increases with increasing reservoir pressure,and exerts great effect upon the oils physical properties. This is alsoresponsible for many oil wells to flow their oil to the surface without artificialassistance

If the gas content is sufficient to saturate the oil under the existing conditions,

the amount unable to go into solution forms a free gas cap above the oil


6/22

Non-HC constituents of oil & natural gases Common non-HC constituents: Heterocompounds(S-N-O compounds) and

Organocompoundscontaining certain heavy metals (principally V, Ni) in crude oil

Natural gas may also contain N, H, He, Ar

S and its compounds(most important heterocompound)

Few are wholly without it Few contain >3% by weight S content is higher in heavy oils (higher MW, BP with more polar fractions i.e.,

resins and asphaltenes) than in light oils Small proportion of S in crude oil is in the form of elemental form in solution

or of H2S; the greater part is bonded with C in organic combination Crude oils containing detectable amount of H

2SSour Crudes

S content as low as 0.1 0.2%Sweet Crudes(Algeria, Angola, Nigeria)Low sulfur crude < 0.6 % SIntermediate crude: 0.6 1.7 % SHigh sulfur crude > 1.7 % S


7/22

High S content are found in reservoirs of dolomite anhydrate faciesMiddle EastArabian heavy fields, Iran, Suez graben 2.8 4.9 % SAthabasca tar sand in Alberta (very heavy degraded oil) 5.5 % SBoscan in Venezuela 5.4 % S

In most of the major exporting countries (Middle East, Mexico, Venezuela) HighS crudes are 5 10 times of sweet crude

H2S Gases high in H

2S occur typically in carbonate-sulfate reservoirs and also

contain higher than normal concentrations of N & CO2 H2S Concentrations exceeding 100 ppm in gas/oil Dangerous as it is highly

corrosive to drilling equipment, particularly in deep well with high temperature North American Basins (the Permian Alberta, Tampico, and Reforma

Campeche basin and Texas Panhandle); Permian of Ural Volga region; JuraCretaceous of Aquitaine Basin in France15 16 % H2S

Deep Smackover in Southeastern USA (Jurassic) 30 % routine, 80-90% Deep carbonate reservoirs of Rocky Mt. Foothills60-65% below 4000 m


8/22


9/22

Other elements in oil & gasNatural gas may also contain H, He, or Ar Hydrogen in no case exceeds 0.5 %; usually < 0.1 % Helium Occurs in some dry gases particularly in those with high N content Fields producing both oil & gas more commonly contain 1 % or more He than

dry gas fields In North America, most of the He-bearing reservoirs are over large basement

uplifts Argonrarely exceed 0.1 % in natural gases Ar in gases is enriched in 40Ar to the extent of 6 10 % (derived from the decay

of 40K) Radon occur in many crude oils

Organometallic compounds Porphyrins are HC ring complexes containing N & a metallic (V, Ni) nucleus Porphyrin (traces 400 ppm in crude oils)

C numbers range from 25 36, mostly 30 32 V 30 300 ppm (high S); Venezuela 1100 ppm Ni 20 85 ppm (low S) V : Ni ratio is highest in Paleozoic oils (> 1)

Mesozoic & Cenozoic oils (< 1) There is good correlation between metals, sulfur, & asphaltene content Degraded oils, enriched in asphaltenes, contain also more metals than the

nondegraded oils of similar origin Other metals, may be present are: Fe, Zn, Cu, Pb, As, Md, Co, Mn, Cr, etc.


10/22

Correlation Index Useful means of classifying oils on a qualitative basis. CI is a number whose magnitude indicates certain characteristics of a

crude oil distillationParaffins: CI 0 (zero)Benzene: CI 100

Lower the CIgreater the concentration of paraffins Higher the CIgreater the concentration of naphthenic and aromatic

HC


11/22

Physical properties of Oils

Chemical compositions of crude oilsare the principal determinants of their

physical properties


12/22

Specific Gravity of oil Range 0.73 slightly above 1.0 Paraffin-base oilslight

Asphalt-base oils

invariably heavy

Earlier, gravity was expressed in degrees of the European Beaum scale,read directly on a hydrometer at 15.6C (60F); degree goes up as thedensity decreases; i.e., A high gravity oilis not a heavy oil buta light oil

B = (140 / r) 130

Later, B has been replaced by API scaleAPI value = (141.5 / r) 131.5

So that water under STP conditions becomes 10 API

Relation between API & BAPI value = (1.01071 x B) 0.10714

Presently, the measured quantity isDensity Relative to Water Kgdm-3


13/22

By general convention Oils with API gravities > 30light

30 - 22medium< 22heavy< 10extra-heavy

Worldwide average value is ~33.3 API Most favoured grade of crude oil is ~37 API Oils heavier than 12 API difficult to be distinguished on the basis of gravity;

viscosity provides a more useful discriminant Oils lighter than ~50 API not really oils but rather condensates / distillates

API gravity 30 33 36 LPGDensit 0.876 0.860 0.845 0.570

Relation between API gravity & relative density


14/22

Tansleep sand(Elk Basin,

Pennsylvania)

Rangley field,Colarado

Hawkins pool,(NE Texas)

460-490 cf gas

per barrel of oil0.867/ 31.8 API 0.849/ 35.2 API 0.870/ 31 API

134 cf gas per

barrel of oil

0.892/ 27.1 API 0.869/ 31.3 API 0.960/ 16 API

~37 APIMiddle East, Mid-continent, Appalachian provinces (USA),Alberta, Libya, North Sea, etc. crudes

>40 API large quantities in Algeria, south-eastern Australia,Indonesia, Andes, etc. crudes

Very heavy crudesCalifornia, Mexico, Venezuela, Sicily, etc.


15/22

Viscosity of oilViscosity is the internal friction of a fluid, causing its resistance to change of form. It isthe ratio of stress to shear per unit time; shear in liquid is not constant, but isproportional to time

Viscosity = Force x Distance / Area x VelocityMLT-2L / L2 LT-1 = ML-1T-1

CGS UnitPoise Conventional Unitcentipoise[1 cP = 10-2 Poise, viscosity of water at 20C (68F)]

SI Unit

mPas [1 cP 1 mPas] Saybolt Universal Second(SUS) number of seconds needed for a steel ball to

roll through a standard volume of the fluidSUS = viscosity in cP x 4.635 / relative density

Typical oil viscosities measured in SUS at STP are from ~1000 to 50 Viscosities of light oils: < 30 mPas

Typical values (mPas): between 5.0 & 0.6 (gasoline) mPas Heavy asphaltic oils: 50,000 mPas (Miocene oil in Bolivar coastal field (W.

Venezuela)Very heavy oil ~1,00,000 mPas (Cold Lake, Alberta); >106 mPas (Athabasca) HC having viscosities > 10,000 mPas are known as natural tars


16/22

Viscosity vary directly with densities Hence, viscosities of oil are a function of the number of C atoms

and of the amount of gas dissolved in the oil

Figure: Effect of dissolved gas on viscosity & gravity of crude oil


17/22

Pour PointA useful indicator of viscosity of crude oils; the lowest temperature at which thecrude flows under prescribed, controlled conditions

Pour points > 40C (more than 100F) are relatively common among crudes havinghigh contents of paraffin wax (Middle eastern and African crudes flow at ~ - 36C)

Oils with high pour points because of high wax contents have a shiny appearance&are associated with formation waters with low salinity

When oils with high pour point rise in the traps, their temperatures are lowered andthe waxes crystalise out, forming a residue of high molecular weight paraffins & oilbecomes lighter

Prolific basins in which the original oils were paraffinic and derived oils, in youngerstrata are asphaltic include the Carpathian Basin(Romania), & Niger delta basin(offAfrica)

Both paraffinic & asphaltic crudes may undergo prolific volatilization throughsurface or near surface alteration that they become totally dried-up, so viscous thatthey effectively solids. The drying-up process isINSPISSATION


18/22

Very waxy crudesUinta Basin(Western USA), Anaco trend(Eastern Venezuela) Reconcavo Basin(Brasil), Mendoza Basin(Argentina), Beatrice field(off Eastern Scottland),Mangyshlak field(east of the Caspian Sea), several fields in the Sirte Basin(Libya &

Sudan); Remarkable proportion of the fields in young sandstone reservoirs inEastern Asia& Australia, (many oils from West India& Upper Assam Basinof NorthEastern India contain 10 15 % wax), most crudes from China, Sumatra, and theGippslandoff-shore basin in Australia.

Colours & RI of oilsParaffinic oils are commonly light in colour yellow to brown by transmittedlight, & green(of automobile engine oil) by reflected lightAsphalt-base oils are commonly brown to black;

RI of oils vary with the relative density, between 1.42 and 1.48 for most of the oils

Lighter oilsLower indicesAt lower temperature Lower indicesWithin any one molecular weight range

RI increase from paraffins through naphthenes, to aromatics


19/22

VolumeOil in reservoir contains dissolved gas, and the volume of the solution dependsupon the

formation gas-oil ratio reservoir pressure

The gas that may be dissolved in oil under increasing pressure increases thevolume of the solution until the saturation pressure (bubble point) is reached afterwhich the volume decreases with increased pressure

0.5 0.8 barrel of gas-free oil on the ground (stock tank oil) may represent 1barrel of oil in the reservoirat the saturation pressure

The volume of liquid petroleum at constant pressure increases with increasingtemperature, but at a much lower rate than gases

The solubility of gas in oil increases linearly with pressure in accordance withHenrys law.

Heavy crudes have much less capacity to hold gas


20/22

FluorescenceFluorescence is observed under UV radiation (2537 3650 )

All oils exhibit more or less fluorescence (bloom)Aromatic oilsmost fluorescent

Fluorescent colours of crudes range continuously from yellow through green toblue

Fluorescence is used in the logging of wells to locate oil showings in the cores,cuttings, and drilling mud

Fluorescence rapidly reduces by aging; this helps in differentiating fresh oil fromoil previously caught in the drilling mud


21/22

Optical ActivityMost crudes have optical activity (the power to rotate the plane of polarization ofpolarized light)

Optical activity pertains to the presence of cholesterin (cholesterol), an alcohol(C26H45OH)

Measured by a polariscope in degrees/mm

Average range0 1.2 degreesPlane rotated to the right dextrorotary

leftlevorotary

OdorParaffins, naphthenes, aromatics, S, H2S, N-compounds have characteristics odor

Calorific valueCalorific value of the crude oil decreases as the specific gravity increases


22/22

Flash & Burning PointThese are the measurements of the hazard involved in handling and storingpetroleum and petroleum products

Flash point Temperature at which the vapours rising off the surface of the heatedoil ignites with a flash

Burning point At higher temperature ignition and burning with a steady flame atthe surface

Coefficient of expansionFor an increase in temperature of 1F for crude oils, the coefficient of expansionvaries between 0.00036 0.00096

For most crude oils0.00040 to 0.00065

Average coefficient of expansionPennsylvania crude oils 0.000840

Baku crude oils 0.000817

Heavier crude oils (lower API) lower coefficient & vice-versa

Composition of Petroleum

Documents