Heavy Oil and Bitumen –Low API’s and High Viscosity
Heavy and extra-heavy crude oils and bitumens are petroleum or petroleum-like liquids or semisolids occurring naturally in porous and fractured media. Bitumen deposits are also called tar sand, oil sand, oil-impregnated rock, and bituminous sand.
These crude oils and bitumens may be characterized first by viscosity and then by density.
In determining the international resource base, viscosity should be used first to differentiate between crude oils, on the one hand, and bitumens, on the other. Subsequently, density should be used to differentiate among extra-heavy crude oils, heavy crude oils, and other crude oils.
Bitumens have viscosities greater than 10,000 mPa-s (cp). Crude and heavy oils have viscosities less than or equal to 10,000 mPa-s. These viscosities are gas-free as measured and referenced to original reservoir temperature.
API gravity is not perfectably correlatable to viscosity, this is because the gravity depends on how the oil was created, reservoir temperature etc. There may be two orders of magnitude variation in viscosity for the same value of °API.
In general, the higher the °API, the higher the price that oil will receive. For heavy oils with low °API, their price is low since the refinery end products yield low quantities of valuable refined products as gasoline and jet fuel and yield large amounts of tar and coke.
Bitumens have densities greater than 1,000 kg/m3 (API gravities less than 10 degrees). Heavy crude oils have densities from 934 to 1,000 kg/m3 (API gravities from 20 degrees to 10 degrees inclusive). These densities (API gravities) are referenced to 15.60°C (60 degrees F) and atmospheric pressure.
Crude oils with densities less than 934 kg/m3 (API gravities greater than 20 degrees) may be classified as medium, light, or other crude oils. These are the definitions as given by the World Petroleum Congress. The UNITAR definition differs from this by defining heavy oil as having a density greater than 922 kg/m3 (22.3 API). The EUB defines heavy oil as those oils having an oil density greater than 900 kg/m3 (25.7º API).
Composition
Most heavy and extra-heavy crude oils and bitumens are composed primarily of hydrocarbons. They are deficient in hydrogen and contain excess of carbon. These crude oils and bitumens generally contain only small percentages of volatile and easily distillable hydrocarbons and frequently contain high percentages of high molecular weight aliphatic and terpenoid hydrocarbons, high percentages of asphaltenes, and significant quantities of oxygen, nitrogen, and sulfur-bearing compounds. The compositions of heavy and extra-heavy crude oils are variable, and the bitumens from different deposits are not necessarily alike.
“Heavy” also refers to high boiling fractions. Distillation is the main refinery operation that separates crude oil into fractions, which are then used as feedstocks to various conversion and upgrading processes. Distillation under atmospheric pressure produces high value distillates and low value residue (or 343 °C+ atmospheric residue) which comprises high molecular weight and carbon number (MW>300, C>20) petroleum molecules. This portion of crude oil is called the “heavy ends”. The heavy ends that were derived from the crude oil in the previous figure influence the viscosity.
The price received for heavy oil is dependent on its composition. A common term used is light oil/heavy oil differential. In Alberta, this is given by:
Light oil Par price at Edmonton minus the Bow River Heavy oil price at Hardisty.
When heavy oil supply is tight relative to demand, the differential decreases and vice versa. Large differentials are good for upgraders. Narrow differentials are good for producers.
The removal of light ends (the low boiling fractions) from these crudes result in a major shift to lower gravity and a dramatic increase in viscosity. The relationship between crude oil gravity and yield of heavy ends (343 degrees C or C >20) is shown in the figure on the bottom page. Crude oils with high yield of “heavy ends” (atmospheric residue) have low gravity and very high viscosity.
The effectiveness of the conversion of petroleum feedstocks into more valuable products is adversely affected by the crude oil tendency to form coke as measured by the carbon residue content (Micro Carbon Residue Test, MCRT, or Conradson Carbon Residue). The figure below shows the relationship between oil gravity and the MCR content. Heavy crudes have a high MCR content and thus are expensive to refine.
Heavy and extra-heavy crude oils and bitumens may contain 3 wt. percent or more of sulfur and frequently contain from several hundred to over 20,000 ppm of vanadium. Nickel and molybdenum are also frequently minor components of these crude oils and bitumens. These metals poison the catalysts used in the conversion process.
There is a general increase of metal content with decreasing oil gravity.
Desulphurization and denitrogenation processes must be included in refineries to produce high value fuels. These processes require hydrogen and are expensive. There is a trend of higher sulphur and nitrogen concentrations as the gravity is decreased, as shown on the figures below.
Crude Oils can be described by the general formula:
CnH2n+ZX
where Z is the hydrogen deficiency factor and X are the heteroatoms (sulphur, nitrogen, vanadium,etc).
Heavy oils are hydrogen deficient. The hydrogen deficiency factor affects the crude oil gravity. Hexadecane, a hydrogen rich paraffin has a high gravity of 51.5 °API, and has a Z=+2. Perhydropyrene, a hydrogen deficient naphthene, has a lower gravity of 12.4 °API. It’s Z value is –6. The hydrogen poor aromatic pyrene, which has four unsaturated rings has a Z of minus 22 and an API of –20.2 degrees.
Heavy and extra-heavy crude oils and bitumens are petroleum or petroleum-like liquids or semisolids occurring naturally in porous and fractured media. Bitumen deposits are also called tar sand, oil sand, oil-impregnated rock, and bituminous sand.
These crude oils and bitumens may be characterized first by viscosity and then by density.
In determining the international resource base, viscosity should be used first to differentiate between crude oils, on the one hand, and bitumens, on the other. Subsequently, density should be used to differentiate among extra-heavy crude oils, heavy crude oils, and other crude oils.
Bitumens have viscosities greater than 10,000 mPa-s (cp). Crude and heavy oils have viscosities less than or equal to 10,000 mPa-s. These viscosities are gas-free as measured and referenced to original reservoir temperature.
API gravity is not perfectably correlatable to viscosity, this is because the gravity depends on how the oil was created, reservoir temperature etc. There may be two orders of magnitude variation in viscosity for the same value of °API.
In general, the higher the °API, the higher the price that oil will receive. For heavy oils with low °API, their price is low since the refinery end products yield low quantities of valuable refined products as gasoline and jet fuel and yield large amounts of tar and coke.
Bitumens have densities greater than 1,000 kg/m3 (API gravities less than 10 degrees). Heavy crude oils have densities from 934 to 1,000 kg/m3 (API gravities from 20 degrees to 10 degrees inclusive). These densities (API gravities) are referenced to 15.60°C (60 degrees F) and atmospheric pressure.
Crude oils with densities less than 934 kg/m3 (API gravities greater than 20 degrees) may be classified as medium, light, or other crude oils. These are the definitions as given by the World Petroleum Congress. The UNITAR definition differs from this by defining heavy oil as having a density greater than 922 kg/m3 (22.3 API). The EUB defines heavy oil as those oils having an oil density greater than 900 kg/m3 (25.7º API).
Composition
Most heavy and extra-heavy crude oils and bitumens are composed primarily of hydrocarbons. They are deficient in hydrogen and contain excess of carbon. These crude oils and bitumens generally contain only small percentages of volatile and easily distillable hydrocarbons and frequently contain high percentages of high molecular weight aliphatic and terpenoid hydrocarbons, high percentages of asphaltenes, and significant quantities of oxygen, nitrogen, and sulfur-bearing compounds. The compositions of heavy and extra-heavy crude oils are variable, and the bitumens from different deposits are not necessarily alike.
“Heavy” also refers to high boiling fractions. Distillation is the main refinery operation that separates crude oil into fractions, which are then used as feedstocks to various conversion and upgrading processes. Distillation under atmospheric pressure produces high value distillates and low value residue (or 343 °C+ atmospheric residue) which comprises high molecular weight and carbon number (MW>300, C>20) petroleum molecules. This portion of crude oil is called the “heavy ends”. The heavy ends that were derived from the crude oil in the previous figure influence the viscosity.
The price received for heavy oil is dependent on its composition. A common term used is light oil/heavy oil differential. In Alberta, this is given by:
Light oil Par price at Edmonton minus the Bow River Heavy oil price at Hardisty.
When heavy oil supply is tight relative to demand, the differential decreases and vice versa. Large differentials are good for upgraders. Narrow differentials are good for producers.
The removal of light ends (the low boiling fractions) from these crudes result in a major shift to lower gravity and a dramatic increase in viscosity. The relationship between crude oil gravity and yield of heavy ends (343 degrees C or C >20) is shown in the figure on the bottom page. Crude oils with high yield of “heavy ends” (atmospheric residue) have low gravity and very high viscosity.
The effectiveness of the conversion of petroleum feedstocks into more valuable products is adversely affected by the crude oil tendency to form coke as measured by the carbon residue content (Micro Carbon Residue Test, MCRT, or Conradson Carbon Residue). The figure below shows the relationship between oil gravity and the MCR content. Heavy crudes have a high MCR content and thus are expensive to refine.
Heavy and extra-heavy crude oils and bitumens may contain 3 wt. percent or more of sulfur and frequently contain from several hundred to over 20,000 ppm of vanadium. Nickel and molybdenum are also frequently minor components of these crude oils and bitumens. These metals poison the catalysts used in the conversion process.
There is a general increase of metal content with decreasing oil gravity.
Desulphurization and denitrogenation processes must be included in refineries to produce high value fuels. These processes require hydrogen and are expensive. There is a trend of higher sulphur and nitrogen concentrations as the gravity is decreased, as shown on the figures below.
Crude Oils can be described by the general formula:
CnH2n+ZX
where Z is the hydrogen deficiency factor and X are the heteroatoms (sulphur, nitrogen, vanadium,etc).
Heavy oils are hydrogen deficient. The hydrogen deficiency factor affects the crude oil gravity. Hexadecane, a hydrogen rich paraffin has a high gravity of 51.5 °API, and has a Z=+2. Perhydropyrene, a hydrogen deficient naphthene, has a lower gravity of 12.4 °API. It’s Z value is –6. The hydrogen poor aromatic pyrene, which has four unsaturated rings has a Z of minus 22 and an API of –20.2 degrees.
The standard (SARA) analysis of heavy crudes involves determining four fractions:
· Saturates
· Aromatics
· Resins
· Asphaltenes
This analysis involves precipitation of insolubles with an excess of n-heptane (n-heptane asphaltenes). The solubles (called maltenes) are then further separated using adsorption chromatography into the following three fractions: “saturates”, “aromatics” and “resins.” The “saturates” consist of aliphatic hydrocarbons which are readily desorbed from the chromatographic column. The "aromatics" consist both of aromatic hydrocarbons and neutral aromatic heterocompounds (containing S, N and O) which are adsorbed on the chromatographic column stronger than “saturates.” The “resins” and “asphaltenes” both consist of polar predominantly heteroatom containing compounds (N, S and O). The “resins” are soluble in n-heptane while “asphaltenes” are not and precipitate out. *
A 21 °API heavy oil was distilled into a series of progressively higher boiling point fractions (up to a cut point of 621°C, C 65) and a non-distillable resin (743°C, C 115). All fractions were then separated into “saturates”, “aromatics” and “polars” using liquid chromatography. The “polars” consist of both soluble resins and insoluble asphaltenes.
*Asphaltenes are the fraction of crude oil that are insoluble in excess normal alkanes such as n-pentanes or n-heptane but are soluble in benzene or toluene at room temperature. Resins are the fraction of crude oil insoluble in liquid propane but soluble in n-pentane at room temperature. Resins are strongly adsorbed on silica or alumina (surface active materials).
· Saturates
· Aromatics
· Resins
· Asphaltenes
This analysis involves precipitation of insolubles with an excess of n-heptane (n-heptane asphaltenes). The solubles (called maltenes) are then further separated using adsorption chromatography into the following three fractions: “saturates”, “aromatics” and “resins.” The “saturates” consist of aliphatic hydrocarbons which are readily desorbed from the chromatographic column. The "aromatics" consist both of aromatic hydrocarbons and neutral aromatic heterocompounds (containing S, N and O) which are adsorbed on the chromatographic column stronger than “saturates.” The “resins” and “asphaltenes” both consist of polar predominantly heteroatom containing compounds (N, S and O). The “resins” are soluble in n-heptane while “asphaltenes” are not and precipitate out. *
A 21 °API heavy oil was distilled into a series of progressively higher boiling point fractions (up to a cut point of 621°C, C 65) and a non-distillable resin (743°C, C 115). All fractions were then separated into “saturates”, “aromatics” and “polars” using liquid chromatography. The “polars” consist of both soluble resins and insoluble asphaltenes.
*Asphaltenes are the fraction of crude oil that are insoluble in excess normal alkanes such as n-pentanes or n-heptane but are soluble in benzene or toluene at room temperature. Resins are the fraction of crude oil insoluble in liquid propane but soluble in n-pentane at room temperature. Resins are strongly adsorbed on silica or alumina (surface active materials).
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