Natural Gas Distribution | Specialty Gases | Coregas NZ

Natural gas distribution

Natural gas is distributed in a complex grid with various input and withdrawal points. Flow metering and accurate measurement of the heating value are essential for billing.

Sources of natural gas

Natural gas in Australia is derived from four main sources.

1. Conventional natural gas

Natural gas reservoirs are found deep in the ground trapped in underground rock formations and may be found on- or offshore. Gas is drilled from these underground gas reservoirs using similar technology to oil drilling. The 'crude natural gas' is then processed to remove water, H2S and CO2 prior to distribution.

Conventional natural gas in eastern Australia is currently extracted from a number of fields in the Bass, Cooper, Gippsland, Otway and Surat-Bowen basins. The Carnarvon and Perth basins in Western Australia and the Bonaparte basin in the Northern Territory are also rich sources of natural gas.

2. Shale gas

Shale gas is found in shale formations that are often located deep beneath the surface. Extraction has been made economically viable using modern techniques such as horizontal drilling.

3. Unconventional gas

This source is found in coal reserves, closer to the surface than shale gas, where it is held by water pressure in the coal reserves. Coal seam gas is produced in a number of fields in the Surat-Bowen and Sydney basins.

4. Biogas

The majority of biogas plants in Australia – more than 50 – are associated with municipal waste treatment facilities. Commercial biogas production operations include Melbourne Water and Sydney Water, which use sewage as their biogas reactor feedstock.

Natural gas from some sources in Australia is liquefied for export to Asia as LNG. Gas from other sources is distributed through the Australian natural gas pipeline network at a pressure of approximately 3000 kPa (30 bar). The natural gas distribution grid receives inputs from these various sources and operates over 88,000 km of distribution pipelines to distribute the gas to serve more than 4.3 million households and 130,000 business customers.

Natural gas is distributed through high pressure pipeline systems

Billing based on real energy value

Each different source of gas has a slightly different composition. Some are rich in methane while others contain additional higher hydrocarbons, such as ethane (which may typically be present at between 5 and 10%), propane (which may typically be less than 5%) and butane (which might be present at 1 to 2%).

The C2 to C4 hydrocarbons have a higher energy value than methane when burned. For example, ethane yields 70% more heat per cubic metre of gas than methane; propane burns with more than 2.5 times as much energy as methane; and butane with more than three times as much energy as methane. Propane and butane are the main components of LPG.

CO2 may also be present in natural gas, but has no energy value when burned. With inputs from so many different sources and each source having a slightly different composition, the natural gas distribution network is truly a cocktail of different gases.

Some gases burn with a higher energy and flame intensity than others, the Wobbe index is used to compare their burning characteristics

To ensure sources to the grid are remunerated fairly and to ensure customers using gas from the grid are paying the correct price for the energy they consume, there is regular metering of the gas for flow rate and measurement of its calorific value. Calorific value is also known as heating value or BTU value. This measurement is done in many locations in the grid but is especially important at locations where the gas changes ownership, where highly accurate measurements for 'custody transfer' take place.

BTU measurement involves the use of a highly accurate gas chromatograph fitted with a thermal conductivity detector (GC-TCD) or flame ionisation detector (GC-FID). This analyser is calibrated using high precision synthetic natural gas reference materials which are used to determine the BTU value of the natural gas. For custody transfer measurements, it is general practice to use certified and accredited traceable reference materials. The accreditation will be to ISO 17034.

The heating value of the natural gas is calculated from the exact composition of the various hydrocarbons the gas stream contains using the mathematical formulae in ISO 6976:2016. The same standard is used for calculation of the Wobbe index, which is a value representing the heating property of the gas stream at a given supply pressure.

Use of a carrier gas such as Helium 5.0 grade for the chromatography is essential. There is no additional zero gas required because the carrier gas doubles as a zero gas. There are no additional instrumentation gases required for the TCD detector.

Pipeline integrity

Raw natural gas often contains carbon dioxide, which has no value as a fuel and is largely removed prior to distribution to avoid transporting this 'dead weight'. The sources of natural gas are also purified to remove water, sulphurous hydrocarbons (known as mercaptans) and hydrogen sulphide. These chemicals are removed because they can produce corrosive acids which may corrode gas pipelines sufficiently to give rise to dangerous and expensive gas leaks.

To ensure water and H2S levels in the pipeline are at suitably low levels, they are measured using sophisticated analytical instrumentation. The maximum allowable H2S concentration is typically 5 ppm. This instrumentation requires calibration and testing with specialty gases mixtures containing low concentrations of water or hydrogen sulphide and a zero gas such as Nitrogen 5.0 grade.

One of the highest risks for natural gas pipelines is air ingress. The flammable gases are at high pressure inside the pipeline and the addition of oxygen would cause an explosive atmosphere. Furthermore, moisture from the air would be an unwelcome corrosion risk inside the pipeline. The tell-tale sign of a natural gas pipeline leak, which would both result in natural gas escaping and air ingress, would be the measurement of high oxygen levels in the pipeline. So oxygen is effectively used as a leak detection tracer gas in this application.

The analysis of oxygen is done using highly sensitive on-line analytical equipment such as a paramagnetic oxygen analyser. To ensure accurate measurement, this analyser is calibrated using either air or a certified calibration gas mixture containing a known concentration of oxygen in nitrogen. The instrument also requires a zero gas such as Nitrogen 5.0 grade. The desirable measured oxygen concentration would be less than 0.1% and any level approaching 2% would be indicative of a serious failure and would trigger an alarm.

CNG as an automotive fuel

Natural gas is regarded as a relatively clean burning fuel and is becoming increasingly popular for use in commercial vehicle fleets and public cars.

Natural gas is stored as a high pressure gas cars and trucks