1. The Origin of Natural Gas

[GLOSS]Natural Gas[/GLOSS] is a colourless, highly combustible gas, one of a group of primary fuels (CF62) known as [GLOSS]fossil fuel[/GLOSS]s.  Like other such fuels, it is formed over millions of years of geological time from buried plant and animal debris.  Natural gas is not a single substance but is a complex mixture of [GLOSS]hydrocarbons[/GLOSS] such as [GLOSS]methane[/GLOSS] and [GLOSS]ethane[/GLOSS], together with other gases such as nitrogen and carbon dioxide.  The composition of natural gas reflects its origins and its value as a fuel or chemical feedstock.

The main constituent of commonly distributed natural gases is methane (CH4). Methane will generally represent more than 80% by volume of the hydrocarbons present in natural gas.

2. The Formation of Natural Gas (and Oil)

The formation of these natural products is controlled by a great number of factors including the nature of the original [GLOSS]organic material[/GLOSS], the environment under which it was deposited and its subsequent burial history.  

As the organic debris is deposited in warm, shallow seas or the deltas of major rivers, it undergoes many chemical changes.  Under the right conditions when only a limited amount of oxygen is present, significant quantities of the organic material are preserved within the sediments.  With further sedimentation, the organic matter is buried and subjected to increasing temperature and pressure to produce organic-rich strata known as [GLOSS]source rock[/GLOSS]s.  Typical source rocks are [GLOSS]shales[/GLOSS], where the organic content can range from a few percent to more than 30%, and [GLOSS]coal[/GLOSS]s that frequently have a higher organic content.

Initially, the organic chemicals in the developing source rock react to form complex, high molecular weight substances.  In turn, and over many million of years, these substances slowly break down to form oil and/or natural gas.  Source rocks where the material has been derived from terrestrial plants tend to produce more gas, whilst those that are rich in marine material derived from the small animals and plants that make up plankton, will be more oil prone.  However, the situation is complex, and many source rocks can produce both oil and gas, with the ratio between the two changing as the source rock matures.

Once formed, the oil and gas are ejected from the source rock, and as both are lighter than water, they will rise upwards, with the path taken being controlled by the relative permeability of the strata through which the fluids pass.  Eventually, either they reach the surface – and are essentially lost – or they are trapped within a porous rock such as [GLOSS]sandstone[/GLOSS] or fractured [GLOSS]limestone[/GLOSS], beneath an impermeable rock stratum.  When this happens, an oil or gas reservoir has been formed deep within the earth.  It is not unusual for the hydrocarbons within a relatively small reservoir of a few hundreds of square metres to have derived from a much larger area of many hundreds of square kilometres.

A reservoir may contain mostly [GLOSS]black oil[/GLOSS] with dissolved gas or even a smaller layer or cap of gas above it.  When the gas is produced with the oil, it is referred to as [GLOSS]associated gas[/GLOSS].  Examples of such reservoirs include many Middle East fields, and the oil fields of the northern North Sea.  At the other end of the spectrum, when only gas is present, it is classified as [GLOSS]dry gas[/GLOSS]; the large structures in the southern North Sea are good examples of dry gas reservoirs.  The stark difference between the fields of the northern and southern North Sea results largely from the nature of the source rock.  The hydrocarbons from the northern fields have been generated from a shale rich in marine material, the Kimmeridge Clay, formed in turn in the [GLOSS]Jurassic[/GLOSS] geological period.  The southern North Sea gas derives from older coals that were formed from terrestrial plants in the [GLOSS]Carboniferous[/GLOSS] period.

3. The Production and Treatment of Natural Gas

Once released from the reservoir through one or more drilled wells, natural gas must generally be treated before it can be transported to its customers by pipeline or (in liquefied form) by ship.  The composition of the raw gas can vary greatly, from gases with over 95% of methane to less valuable gases that contain high levels of contaminants such as carbon dioxide, nitrogen and hydrogen sulphide.  For example, many gas fields in SE Asia such as the Natuna field in Indonesia, contain carbon dioxide levels up to 70%.  Undesirable components must be removed because they are non-flammable, corrosive or toxic. Natural gases containing significant quantities of sulphur compounds are known as [GLOSS]sour gas[/GLOSS].

Gas treatment will always include drying to remove water, separation of any solids and control of the [GLOSS]dew point[/GLOSS] to prevent liquid drop out during transportation.  In addition, a variety of chemical engineering processes can remove or reduce undesirable components present to acceptable levels.  Treatment may take place offshore where the gas is being produced, at an onshore location, or a combination of the two depending on costs and the availability of treatment facilities. Sour gases are preferably treated to remove the sulphur at or near the production facility, but for smaller offshore fields, it is more economic to transport the untreated fluids to land-based facilities via pipelines built with corrosion resistant materials.

4. Natural Gas Characterisation and Quality

Pipeline companies will generally specify strict criteria for delivered gas before it will be accepted for transportation, including ranges for temperature, pressure, volumes, composition and combustion characteristics.  A typical set of composition acceptance criteria is shown below:

q         Hydrogen sulphide:  not more than 3.3ppm

q         Total Sulphur:    not more than 15ppm

q         Hydrogen:  not more than 0.1 mole%

q         Oxygen:  not more than 10ppm

q         [GLOSS]Hydrocarbon dew point[/GLOSS]:  not more than –2 degree C

q         [GLOSS]Water dew point[/GLOSS]:  not more than –10 degree C

q         Carbon dioxide:  not more than 2.0 mole%

q         Nitrogen:  not more than 5.0 mole%

q         Inert gases: not more than 7.0 mole%

q         Contaminants:  no solid or liquid material

The composition of raw, treated and delivered gas is carefully determined instrumentally, with the majority of components being measured by [GLOSS]gas chromatography[/GLOSS].  In addition, dedicated instruments will be used to measure a few components such as water and sulphur containing compounds.

The pipeline companies will also frequently set criteria on the combustion properties of the delivered gas, for example:

q         [GLOSS]Gross Calorific Value[/GLOSS]:  36.9 – 42.3 MJ/m³(st)

q       [GLOSS]Wobbe Index/Number[/GLOSS]:  48.14 – 51.41 MJ/m³(st)

Further discussion of the way in which natural gases are characterised may be found in a linked Combustion File (CF230).

Until the mid 1980s, combustion properties were measured by specialised facilities such as [GLOSS]calorimeter[/GLOSS]s for determining gross calorific value.  However, with the improvement of chromatographic measurement techniques, the combustion properties can be derived from the composition of the gas.  Gas chromatographs for determining gas quality and combustion properties have been made more rugged for use on operational sites, and automated to improve accuracy and reliability.  These advances have changed the way gas and its energy content is charged to customers.

5. Liquefied Natural Gas

Liquefied natural gas ([GLOSS]LNG[/GLOSS]) is a cryogenic fluid used for:

·         long distance transportation of natural gas by sea when a pipeline is uneconomic or unfeasible

·         local gas  storage, in particular to meet winter peak day demands

·         fuel for road vehicles

The main constituent of natural gas (methane) will not liquefy at any pressure when at ambient temperature. Natural gas becomes a liquid at a temperature of circa minus 165^oC and at atmospheric pressure.

Most contaminants and inert gases are removed when the natural gas is liquefied. LNG contains mainly methane and ethane with trace amounts of propane and butane. This gas must be odorised when it is re-evaporated for distribution in low-pressure pipelines.