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Rising demand, technology gains and a spate of new projects have put sour gas in operators’ sights. The niche product has the potential to grow.
Natural gas accounts for a growing share of the total energy mix, due to massive discoveries, advances in hydraulic fracturing, pressure for cleaner-burning fuel, and technological advances in extraction methods and end uses.
Not all natural gas is equal, however. Sour gas, which contains significant amounts of hydrogen sulphide and carbon dioxide (CO2), is considerably more difficult and expensive to produce than so-called sweet gas. Hydrogen sulphide (H2S) is highly toxic and flammable, and sour gases can damage drilling equipment and corrode standard piping.
Sour gas makes up a substantial portion of total natural gas reserves worldwide, especially in the Middle East, where it accounts for about 60% of total reserves, and where demand is growing for gas to power electricity grids and desalination plants.
The United Arab Emirates has the distinction of being both a net exporter of liquefied natural gas (LNG) and a major gas importer. According to the World Energy Council, the country imported 18 billion cubic metres of natural gas from neighbouring Qatar in 2014.
"Sour gas will play an increasingly important role in satisfying indigenous energy demands."
Abu Dhabi, which contains most of the nation’s proven gas reserves, has several initiatives under way to increase production of sour gas, most notably the Shah project being developed by a partnership between Abu Dhabi National Oil Company (Adnoc) and Occidental Petroleum.
The project reached full capacity of 1 billion cubic feet per day in late 2015. An expansion plan is in the works that will increase processing capacity at the plant by 50%, to 1.5 Bcfd.
Hydrogen sulphide is very toxic at levels as low as 0.05%. Gas with 10% or more H2S content is deemed “very sour”. The Shah fields produce gas with an average H2S content of about 23%.
Last November, operator Wintershall and its field partners Adnoc and OMV began drilling the second of a planned three appraisal wells at the Shuwaihat field, a sour gas reservoir in Abu Dhabi’s west that lies partly below the mainland and partly below the Persian Gulf.
The condensate-rich Schuwaihat gas contains about 20% H2S and 7% CO2. Producing such highly toxic and corrosive gas will require rigorous safety measures and leading-edge technology, says Klaus Langemann, director of technology at Wintershall.
Such characteristics have long kept sour gas mostly in the ground.
“Due to the toxic components of sour gas, which have to be removed before selling the gas to the market, sour gas is costlier to produce,” Langemann says.
“This, in combination with the fact that there are still vast resources of available non-sour natural gas in the world, will keep sour gas a niche fuel in the worldwide energy mix for the foreseeable future.”
Moving gas over long distances, however, requires “huge infrastructure investments” in pipelines and LNG chains, he says.
“As a result, in regions where non-sour gas is a scarce resource, as in the Middle East, sour gas will play an increasingly important role in satisfying indigenous energy demands.
The costs for lifting and purification are still cheaper than investment into new gas transportation infrastructure and, since the sour gas fields are domestic, the security of supply is much higher.”
The safety risks involved in sour gas production demand a thorough and well-planned health, safety and environment (HSE) programme, Langemann says.
During the appraisal drilling stage, Wintershall is using fixed and portable gas detection equipment to identify any H2S release, placing breathing systems on the rig and outfitting workers with personal breathing apparatuses.
Similar equipment will be used during production, along with emergency shut-off valves on the plant to prevent H2S from leaking.
Of utmost importance is “a well-functioning integrated HSE management system in order to prevent any exposure to hydrogen sulphide”, Langemann says.
“On the production technology side, frequent depositions and scale of sulphur-related components occur along the production stream, so it is very important to have preventive measures in place,” he continues.
“Most importantly — and most energy consuming — is the removal of sour gases from the hydrocarbon gas in order to deliver in-spec sales gas.”
Several technologies for purification are available, he says. Wintershall will use OASE technology from parent company BASF to remove H2S.
The methane produced from the process can then be transported for domestic and industrial purposes, while the hydrogen sulphide recovered from the process is first converted into sulphur dioxide and then into pure sulphur, which can be sold to the chemical industry.
The OASE technology “ranks at top in terms of energy efficiency and reliability”, Langemann says.
Wintershall has a history with sour gas that goes back to the early 1960s. The company has developed 16 fields in Germany, produced about 30 billion cubic metres of sour gas and built four processing plants in its home country.
That experience makes the company a good fit for the Shuwaihat partnership, Langemann says.
“A lot of the German gas fields are sour gas fields, so Wintershall possess a very long history and highly qualified experience in dealing with sour gas, even in the heart of a densely populated country like Germany.”
Collaboration with BASF brings added expertise in gas purification, material selection, sulphur marketing “and the ability to deal with toxic substances in general in a safe and responsible way”, he says.
In 2013, BASF and the Petroleum Institute of Abu Dhabi launched a research collaboration to develop new processes for removing sulphur compounds from sour gases.
The research focusses on methods that consume less energy, including the use of adsorbents - porous substances that bind with certain other molecules through physical interactions.
Adnoc has set a production target of 3.5 million barrels per day in 2018, some 400,000 barrels per day higher than 2016’s production rate. The company said it would achieve the target with a combination of enhanced oil recovery and sour gas production.
Wintershall expects results from the second appraisal well at Shuwaihat later this year. If the project moves forward, Shuwaihat will be the Abu Dhabi’s first offshore field that contains very sour gas, but almost certainly not the last.