Properties of Crude Oil

INTRODUCTION

Crude oils are made up of liquid paraffin hydrocarbon compounds ranging from pentane to pentadecane (C₅ – C₁₅). These hydrocarbon compounds consist of different groups such as the normal paraffins, iso-paraffins (branched chain paraffins), alkyl paraffins, naphthenes (or cycloparaffins), alkylbenzene and nuclear aromatics. The normal paraffins are the saturated, low molecular weight hydrocarbons. The associated gaseous phases are within this group. The naphthenes (or cycloparaffins) are highly bonded, high molecular weight hydrocarbons. All crude oils contain some appreciable amount of the naphthene compounds, (10% by composition).

Crude oils also contain a great variety of heteroatomic chemical constituents, comprising of sulphur, oxygen, carbon dioxide, nitrogen and trace metals. Nitrogen varies from 0.01 to 2% as dissolved gas in the crude oil (Levinson, 1974). Oxygen occurs in different forms in oxygen-bearing resinous substances.

Crude oils accumulate in geologic structures called 'traps'. A trap can be stratigraphic, paleogeomorphic or a combination of these. Paleogeomorphic traps includes structural folds and stratigraphic traps are those caused by lateral changes in reservoir rock properties within a stratum.

Physical Properties of Crude Oils

The physical properties = quantitatively measurable characteristics of crude oils. They vary according to the composition of the oil, the relative abundance of the groups of hydrocarbons, and essentially depend on reservoir temperatures and pressures.

           

Specific (or A.P.I) Gravity

            This is the weight of a given volume of crude oil. It is measured in two gravity scales, as stated below:

            i. A.P.I.

            ii. Baume gravity

•      The A.P.I. (American Petroleum Institute) gravity scale is more commonly and widely used than the European Baume gravity scale.

•      The A.P.I. gravity of a crude is influenced by the composition of the oil. Crude oils characterized by high amount of dissolved gases, are less dense thus, light in weight and therefore, possess high A.P.I. gravities, while denser crude oils of low amount of dissolved gases are characterized by low A.P.I. gravity values.

•      The group of hydrocarbons predominating in a crude oil also influences the A.P.I. gravity.            

           

For example, paraffin crude oils (45-60% paraffin hydrocarbons and less amount of naphthenes and aromatics) are light, thus, high A.P.I. gravities. But naphthenic base crudes (consisting predominantly of naphthene hydrocarbons 60-75%, with lesser amount of paraffins and aromatics) are heavy and have low A.P.I gravities.

•      The A.P.l. gravities of crude oils usually increase with depth. This is because a combination of source and reservoir maturation processes associated with slow but continuously increasing geo temperatures, cause the generation of lighter (or High A.P.I gravity) oils at greater depths of burial.

Viscosity

            This is the measure of resistance to flow in crude oils due to internal friction. It is expressed in 'poise' or ‘centipoise’.

•      The viscosity of a crude oil is influenced by the amount of dissolved gases at the prevailing temperature. Crudes characterized by high amount of dissolved gases have high A.P.I gravities and low viscosity or moderately high fluidity.

•      At high temperatures molecular agitation (or velocity) of the crude increases, making for a volumetric expansion and reduction in internal molecular friction, thus, reducing the viscosity of oil.

•      The greater the quantity of a high-molecular weight hydrocarbon group in a crude, the denser and more viscous it is.

Refractive Index

            The refractive index n, of a crude oil is measured from n = sin i / sin r

where; i = incidence angle

                                            r = angle of refraction

            It depends on the density of the oil. Heavy crudes (of low A.P.I) have high refractive indices. This is because a dense crude would create a dense medium for a passing ray of light, which is refracted towards the normal at a low angle (r). On the other hand, light oils have low refractive indices.

Optical activity

            It is the power of crude oils to rotate the plane of polarization of a polarized light. It is commonly expressed in degrees per millimetre. If any crude oil causes the plane of polarization to rotate to the right, it is called a 'dextrorotary', but if is to the left it is known as a 'levorotary’.

•      This property is destroyed at high temperatures (250 - 300°C).

•      Optical activity is also exhibited by some organisms that contain cholesterine substances (such as cholesterol C26 H45 OH).                                                         

According to Amosov (1951), the amount of optical rotation shown by a crude oil depends mainly on its sterane- pentacyclic and triterpane content. And these are hydrocarbon compounds derived from the microbial decarboxylation of organic cholesterine substancestical activity.

           

Cloud and Pour points

The pour point

            The temperature at which a crude oil will no longer flow, when a tube containing it is first heated in a bath, in order to dissolve all its wax content and then gradually cooled. At this temperature, the crude oil is in semi-solid to solid form, and thus loses its fluidity.

            If the pour point of a crude is above the surface temperature, it will precipitate its paraffin waxes on approaching the surface of the ground will only flow on heating.

The Cloud point

             The temperature slightly above the pour point, with an appearance of cloudy substances in the crude: which is due to the settling out of the solid paraffin waxes contained in the crude oil.

            This property determines the influence of low temperatures on crude oils. It provides information about the amount of solid paraffin waxes contained in the oil. This property is common in paraffin base crudes but wax-free naphthenic oils do not show cloud point.

Volume

            The volume of a crude oil in its reservoir rock differs from the volume it occupies in the surface.

            This is due to formation gas-oil ratio and reservoir pressures.

            The formation gas-oil ratio expresses the volume of gas contained in one barrel of a crude oil as it comes from the reservoir rock.Under high reservoir pressure, the volume of oil in the reservoir increases because of the influence of dissolved gases. But on release of the reservoir pressures, the dissolved gases escape, leading to the shrinkage of the volume of the crude oil at the surface.

Fluorescence

            It may be yellow, green or blue.

            For example, when a paraffin base crude oil (gasoline-rich) is exposed to ultraviolet fluorescence light, it emits yellow colour, while naphthenic oils emit brownish colour.

            This property is important in testing for cutting, core and drilling mud samples and in well-logging interpretation, for location of different oil horizons.

Colour

            This is the light transmitted through crude oils. It is yellowish to red for light oils and dark or even opaque for heavy (or low A.P.I gravity) oils.

           

  

Some other Physical Properties

           

Odour

           

            This varies greatly in crude oils. High content of light hydrocarbons (paraffins and naphthenes) in a crude gives rise to a gasoline-like odour. A pleasant odour is produced if the crude has abundant aromatic hydrocarbons. But with high amount of unsaturated hydrocarbon compounds, sulphur and nitrogen compounds in the oil, it produces a repugnant odour.

Coefficient of Expansion

            This is the measure of volumetric increase of a crude under thermal influence. It increases with increase in A.P.I gravity. Oils containing high amount of dissolved gas and possibly high A.P.I gravities possess high values of coefficient of expansion. Heavy crude oils (low A.P.I gravity) have lower coefficients of expansion.

Aqueous Solubility

            The aqueous solubility of crude oil and its fractions increases linearly with temperature.

            The rate of solubility becomes significant at temperatures of about 100°C. At temperatures above 180°C, crudes occur as molecular solutions in mixed phase with water. According to Cartmill and Dickey (1970), at such high temperatures, the nature of the phase enhances primary migration of oil by molecular solution mechanism. And salinity of about 150,000 ppm of sodium chloride results in the separation of liquid hydrocarbons from the aqueous phase.

Surface tension Effect

            Crude oils possess some intermolecular forces of cohesion, expressed as force per unit peripheral outline. Because of this force, oil in dispersed state cannot move through water- wet sand, much less, fine-grained shales.

            The small forces created by natural hydrodynamic gradients do not overcome those created by surface tension. Consequently the oil is dispersed in the form of globules.

Flash Point

            This is the temperature at which the volatiles rising off the surface of heated oil will ignite with a flash, on passing a flame over the surface. This provides some clue about the gaseous content of the crude oil.

Chemical Properties

           

            The chemical properties of crude oil deal with the chemical nature and the changes in composition in relation to temperature and pressure variations occurring at all times within the oil pool. Some of the chemical properties are related to the origin, migration, and accumulation of the crude oil.

Hydrogenation of crude oils

            During the early stages of crude oil formation, there is a remarkable thermal cracking of the organic materials into decomposed complexes, and the coupling activity of anaerobic bacteria processes, and the catalytic influence of such available trace metals as vanadium and nickel, lead, to the transformation of the complex organic matter into alkene rich paraffinic oil.

            And according to Zobel (1947), reservoir catalytic chemical reactions lead to the dissociation of avail sulphides into free sulphur and hydrogen. The elemental hydrogen would convert the alkene rich paraffinic crude oil into an accumulation of gaseous paraffinic oils (of high A.P.I gravity), in relatively close association with the kerogen (or organic source rock).

Paraffin wax content

            Paraffin waxes in crude oils are semi-solid to solid forms of hydrocarbons, consisting mainly of normal paraffins. These n-paraffins range from about C₅ – C₃₀. Hedberg (1968), described waxes as complex petroleum substances whose complexity is caused by molecular mixture of branched chain and n-paraffin hydrocarbons, with molecular weights, high enough to be solids at ordinary temperatures.

            The amount of wax in crude oils varies very greatly. High content of paraffin waxes in oils may lead to the clogging of pores of the reservoir rocks. Such oils congeal at atmospheric temperatures and exhibit high pour points.

Odd carbon Chain Lengths

            Chemical analysis shows that some crude oils exhibit a detectable predominance of n-paraffin of odd number carbon chains over those of even Members. The odd carbon chains range from C₁₇ to C₃₃. The ratio of the sum of the mole percentages of odd carbon n-paraffin to the sum of even Carbon n-paraffin in a specific molecular weight range, serves as an index of odd carbon preference.

Welte (1965) believed that crude oils from different environments

Paraffin waxes in crude oils have melting points above 30 degree Celsius possess correspondingly different odd carbon preference indices.

Porphyrins in Crude Oil

            Porphyrins are complex hydrocarbon compounds that originate from living organic matters such as chlorophyll and hermins.

            According to Hodgson et al (1967), porphyrins have high affinity for trace metals of vanadium, nickel, and iron. Chemical investigations have shown that paphyrins are one of the important constituents of crude oils.

            According to Hunt (1968), these hydrocarbons are derivatives from marine phytoplankton.

            Porphyrin substances are of moderately low temperature origin, usually destroyed at temperatures of about 200°C. Thus, their presence in crude oils is an indication that the

 crude was formed at temperatures below 200 C

Trace Metals in Crude Oils

            Crude oils contain varying amounts of trace elements some of which include, iron, aluminium calcium, magnesium, copper, lead, tin, astemum, antimony, zinc, silver, nickel, chromium, molybdenum and vanadium. But the most important of these trace elements are vanadium, nickel and iron. The concentration of any of these trace metals is so small that the value is expressed in parts per million. The concentration of trace metals in any crude oil is found to be inversely proportional to the A.P.l gravity of the oil.

by: Syed Inayathullah

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