The fluid which is produced at Christmas tree / well head is termed as ‘WELL FLUID’. Well fluid normally consists of Crude or Oil, Water and Gas. The terms Crude and Oil are often interchangeable. Crude oil is a complex mixture of primarily liquid hydrocarbons with dissolved gasses and trace amounts of suspended water, inorganic sediments.In this section the composition, crude properties, types of crude classification and commonly used units of measures pertaining to crude oil properties shall be discussed.
Composition of Crude Oil
Average crude contains 84% Carbon, 14% Hydrogen, 1-3% Sulphur, 1% Nitrogen, 1% Oxygen and 0.1 % minerals and salts. Crude Oil comprises of both hydrocarbons and non hydrocarbons.
Hydrocarbons
- Paraffins: The paraffinic saturated chain type hydrocarbon (aliphatic) molecules in crude oil have the formula CnH2n+2, and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight chain paraffin molecules are found in gases and paraffin waxes. The branched chain paraffins are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins.
- Aromatics: Aromatics are unsaturated ring type hydrocarbon (cyclic) compounds. Naphthalenes are fused double ring aromatic compounds. The most complex aromatics, polynuclears (three or more fused aromatic rings), are found in heavier fractions of crude oil.
- Naphthenes: Naphthenes are saturated ring type hydrocarbon groupings, with the formula CnH2n, arranged in the form of closed rings (cyclic), found in all fractions of crude oil except the very lightest. Single ring naphthenes (mono-cycloparaffins) with 5 and 6 carbon atoms predominate, with two ring naphthenes (dicycloparaffins) found in the heavier ends of naphtha.
Relatively simple crude-oil assays are used to classify crude oils as paraffinic, naphthenic, aromatic or mixed, based on the predominant proportion of similar hydrocarbon molecules. Mixed-base crudes have varying amounts of each type of hydrocarbon. One assay method (US Bureau of Mines) is based on distillation, and another method (UOP “K” factor) is based on gravity and boiling points. More comprehensive crude assays are conducted to determine the value of the crude (i.e., its yield and quality of useful products) and processing parameters. Crude oils are usually grouped according to yield structure, with high-octane gasoline being one of the more desirable products.
Non-Hydrocarbons
- Sulphur and sulphur compounds: Sulphur is present in natural gas and crude oil as hydrogen sulphide (H2S), as compounds (thiols, mercaptans, sulphides, polysulphides, etc.) or as elemental sulphur. Each gas and crude oil has different amounts and types of sulphur compounds, but as a rule the proportion, stability and complexity of the compounds are greater in heavier crude oil fractions. Sulphur compounds called mercaptans, which exhibit distinct odours detectable at very low concentrations, are found in gas, petroleum crude oils and distillates. The most common are methyl and ethyl mercaptans. Mercaptans are often added to commercial gas (LNG and LPG) to provide an odour for leak detection. The potential for exposure to toxic levels of H2S exists when working in drilling, production, transportation and processing crude oil and natural gas. The combustion of petroleum hydrocarbons containing sulphur produces undesirables such as sulphuric acid and sulphur dioxide.
- Oxygen compounds: Oxygen compounds, such as phenols, ketones and carboxylic acids, are found in crude oils in varying amounts.
- Nitrogen compounds: Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as non-basic compounds which may also include trace metals.
- Trace metals: Trace amounts, or small quantities of metals, including copper, nickel, iron, arsenic and vanadium, are often found in crude oils in small quantities.
- Inorganic salts: Crude oils often contain inorganic salts, such as sodium chloride, magnesium chloride and calcium chloride, suspended in the crude or dissolved in entrained water (brine).
- Carbon dioxide: Carbon dioxide may result from the decomposition of bicarbonates present in, or added to crude, or from steam used in the distillation process.
- Naphthenic acids: Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 232 °C when the acid value of the crude is above a certain level.
- Normally occurring radioactive materials: Normally occurring radioactive materials (NORMs) are often present in crude oil, in the drilling deposits and in the drilling mud, and can present a hazard from low levels of radioactivity.
Types of Crude Oil
- Light crude oil is liquid petroleum that has a low density and flows freely at room temperature. It has a low viscosity, low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions. Light crude oil has an API gravity higher than 31.1° (i.e., less than 870 kg/m3)
- Medium Crude oil is classified as medium according to its measured API gravity. Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m3)
- Heavy crude oil (or extra heavy crude oil) is highly-viscous oil that cannot easily flow to production wells under normal reservoir conditions. It is referred to as “heavy” because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has an API gravity below 22.3° (i.e., 920 to 1000 kg/m3) and Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m3)
- Sweet crude oil contains small amounts of hydrogen sulfide (< 0.42% ) and carbon dioxide. High-quality, low-sulfur crude oil is commonly used for processing into gasoline and is in high demand, particularly in the industrialized nations.
- Sour crude oil is crude oil containing a high amount of the impurity sulfur. It is common to find crude oil containing some impurities. When the total sulfur level in the oil is more than 0.5% the oil is called “sour”.
Properties of Crude Oil
| Property | Definition | Significance | Measurement & Testing |
|---|---|---|---|
| BS & W | Basic sediment and water (BS&W) is a technical specification of certain impurities in crude oil. When extracted from an oil reservoir, the crude oil will contain some amount of water and suspended solids from the reservoir formation. The particulate matter is known as sediment or mud. | Monitoring crude-oil sediment and water content at the field production site is essential in accurate crude-oil custody transfer operations, as purchasers pay only for the crude oil received and want to minimize the quantity of sediment and water they must handle. | BS & W Measurement is accomplished by manual methods, or on-line devices like capacitance, density, or energy-absorption analyzers. |
| ASTM D-4007-02, Standard Test Methods for Water and Sediment in Crude Oil by Centrifuge Method (Laboratory Procedure). | |||
| For custody-transfer purposes, sediment and water is determined by a test which follows one of the API manuals of petroleum measurement standards (MPMS). | |||
| API Gravity | The density of crude is generally expressed in API Gravity which is an inverse measure of a petroleum liquid's density relative to that of water (also known as specific gravity). | API gravity is a measure of how heavy or light a petroleum liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. Higher API gravity, more paraffinic crude resulting in higher yields of gasoline, conversely low API gravity, lower yields of gasoline. |
API gravity is graduated in degrees on a hydrometer instrument. API gravity values of most petroleum liquids fall between 10 and 70 degrees. |
| To derive the API gravity, the specific gravity (i.e., density relative to water) is first measured using either the hydrometer, detailed in ASTM D1298 or with the oscillating U-tube method detailed in ASTM D4052. | |||
| Viscosity | Quantity expressing the magnitude of internal friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart has unit speed relative to one another. | Crude oil's flow behavior is important for smooth extraction and transport. Every refinery cannot process every type of crude oil. The heavier oil is, the more difficult it is to refine. In case of an accident with subsequent oil spill, the type of oil determines the steps taken to minimize the effect on the environment. |
Dynamic viscosity (also known as absolute viscosity) is the measurement of the fluid's internal resistance to flow. It is expressed in centipoise (cP) or milli Pascal seconds (mPa.s) |
| Kinematic viscosity is the measure of a fluid's inherent resistance to flow, usually measured at 100 deg F when no external force, except gravity, is acting on it. It is expressed in centistokes (cSt) or sqcm/s | |||
| ASTM D 445, Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. | |||
| Reid Vapor Pressure (RVP) | RVP is defined as the absolute pressure exerted by a liquid at 37.8 °C (100 °F) True Vapor Pressure (TVP) is the Equilibrium Vapor Pressure of a Mixture When the Vapor to Liquid Ratio = 0, True vapor pressure is not equal to RVP, it is usually 5-9 % greater than RVP due to presence of air in the mixture. |
Vapor pressure of crude oils is of importance to the crude producer and the refiner for general handling and initial refinery treatment.
Vapor pressure is a useful parameter to measure and control for the purpose of mitigating hazards associated with crude oil volatility. |
ASTM D 6377 – Determination of Vapor Pressure of Crude Oil, method used to report true vapor pressure. |
| ASTM-D-323 – Reid Method, The test method measures the vapor pressure of gasoline, volatile crude oil, and other volatile petroleum products, except for liquefied petroleum gases. | |||
| Pour Point | Pour Point is the temperature at which oil ceases to flow. | In crude oil a high pour point is generally associated with a high paraffin content The production and transportation of oil and gas can be significantly affected by deposition of paraffin and asphaltenes in the equipment, vessels, and pipelines. |
ASTM D 97 is the testing method. The pour point test method establishes the temperature at which oil ceases to flow when the sample is held at 90 degrees to the upright for five seconds. |
| ASTM D5949, Standard Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method) is an alternative to the manual test procedure. | |||
| Flash Point | Flash Point is the temperature above which the oil will spontaneously combust. | The flash point is an indication of how easy a chemical may burn. Materials with higher flash points are less flammable or hazardous than chemicals with lower flash points. | ASTM D93 – Testing by Pensky Martens closed cup method. |
| ASTM D3828 and D3278 – Determination of flash point by the small scale closed cup method. | |||
| Salt Content | Salt content is a measure of Sodium Chloride content in the crude, generally expressed in Parts Per Thousand Barrels (PTB) | The salt content in the crude oil affects the corrosion rate of the wetted parts. Desalting is generally required if the NaCl content is greater than 10 lbs/1000 bbl |
ASTM D 3230 –Standard test method for salts in crude oil (Electrometric method) |
| Sulphur Content | The amount of sulphur, % wt present in crude | The sulphur content is an important property of crude which influences the corrosion of the wetted parts. This property is used to specify the Material of Construction and specify safety measures in the facility. |
Sulphur testing includes ASTM, IP, ISO, GPA and other test methods & standards. |
| ASTM D4294 – Standard test method for Sulfur in Petroleum and Petroleum products by Energy Dispersive X-Ray Fluorescence Spectrometry. | |||
| ASTM D2622 - Standard test method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry. | |||
| Gas To Oil Ratio (GOR) | GOR is the ratio of volumetric flow of produced gas to the volumetric flow of crude oil for crude oil and gas mixture sample. | This is an important ratio used in design of oil and gas processing equipment and pipelines. | The GOR measurement and testing is done as part of Crude Oil Assay tests and PVT anlaysis. |
