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Impurities, and their implications for natural gas

While natural gas deposits are mostly methane, they also often contain useful ethane and other closely related hydrocarbon molecules, which make up natural gas liquids. A few are even fortunate enough to contain some helium, now highly sought after. However, a large portion gas-fields also include a host of other less welcome constituents, ranging from harmless but diluting nitrogen, to carbon dioxide and worst of all, sulphur dioxide and hydrogen sulphide. We take a look at a few examples, trends and statistics from around the world.

Apart from helium, most impurities raise the cost of developing a gas field, often to the point where it becomes uneconomic, even for giant fields, such as Natuna in the South China Sea. Located in largely Indonesian waters, (parts are disputed, but that hasn’t stopped southeast Asian nations developing other fields jointly), the field contains 35 trillion cubic feet (Tcf) of gas, but 65% of that is carbon dioxide (CO2).

If it had been destined for LNG, methane feedstock from Natuna would also have had to have been very pure. The process for liquefying natural gas for transport requires extremely low concentrations of CO2 – less than 50 parts per million (ppm). This is because when the gas is cooled for liquefaction (down to -160 degrees Celsius), CO2 will freeze, causing blockage of flow lines and other operational problems.

Even if it can be separated economically, emitting the CO2 into the atmosphere is increasingly controversial, although it takes place widely. But now some operators are tackling this by storing the CO2 underground. Chevron has managed to make its giant Gorgon LNG project in Australia commercially viable even with the cost of separating and sequestering its 14-16% CO2 (300 million tonnes (MMt)). It cost an extra $2 billion, and is the world’s largest commercial-scale CO2 injection facility, preventing emissions of 3.6 MMt/yr.

Sour and costly

Gas that contains CO2 or other acidic gases such as hydrogen sulphide, is referred to as sour gas, and always puts up the cost of field development. According to the International Energy Agency, about 43% of the world’s natural gas reserves (2,580 Tcf), excluding North America, are sour. The Middle East has the world’s biggest sour gas reserves, with 60% of reserves classified as sour gas, while in Russia the figure is 34%.

Over 30 large gas fields worldwide have been identified to contain more than 5 MMt of CO2. The world’s largest gas field, South Pars/North Field in the Gulf, is estimated to contain 400 MMt of CO2. Some of the gas is used for reinjection to help support the field pressure, the rest is emitted. In the case of the giant sour Kashagan oil and gas field in Kazakhstan, the very high acidity of associated gas led to a dramatic overrun in costs. A major by-product from this field will be pure sulphur.

The trend toward development of increasingly sour gas suggests that more reservoir CO2 will be liberated into the atmosphere unless geological storage becomes widely adopted.

Bad impurities, good impurities.

In 1984, when dealing with different characteristics of Norwegian and British gas, Mr B. C. Dutton developed a method of comparing gases from different fields, represented by an equivalent gas comprising only three components; methane, propane and nitrogen. The classification suited northwest Europe’s sweet fields, where the biggest gas-field - Groningen – has a high nitrogen content, which means its gas has a lower calorific content, and is distributed largely along a separate network to customers with appliances designed to cope. A conversion process is required for it to enter the main network.

Better than nitrogen is helium. Until recently, practically all of the world's helium reserves had been derived from the US southwest, which historically has had fields with the highest helium concentrations. More recently, the Qatar Helium 2 Project has made the tiny Gulf state the world’s second largest helium producer and operator of the world’s largest helium production facility, with capacity of 1.3 billion cf/yr.

Qatar’s North Field had been thought to contain the world’s largest proven helium reserves, but in 2016 the discovery of a much purer helium gas field in Tanzania's East African Rift Valley may have displaced it, helping ease fears of a global helium shortage. Worldwide demand for helium currently stands at around 6 billion cf/yr, a figure expected to increase by about 30% by 2020.

Of course, the biggest gas-field impurity of all is oil. Associated gas (where gas occurs in oilfields) now accounts for about a quarter of worldwide proven reserves, down from about 35% in the 1970s. This is largely because gas-fields are now more legitimate exploration targets in their own right than had been the case in the ‘70s, when oil was the only target in many areas. Now some explorers are specifically after gas, although hitting oil is still unlikely to disappoint – finding helium, however, now beats them all!

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