Gas production and CO₂ Separation

 Under the current phase one of the In Salah Gas project, gas produced from the Tegentour and Reg fields is first dehydrated then piped 120 km north to a central processing facility at Krechba, where it is mixed with gas from the local field. As is usual in the area, approximately 10% of the natural gas in the reservoir is made up of CO₂.  Normal practice of separating the methane is to simply vent this unwanted CO₂ into the atmosphere.

How does In Salah Gas get natural gas to a market from the middle of the Sahara?

 Even with the advanced technology and expertise available, building a processing facility in an area where there is little to no other development while finding a nearly 500km pipeline route to take the processed gas to market took extensive planning.

Construction of the pipeline from the Hassi R’Mel—the distribution port in central Algeria connecting to existing pipelines headed to Tunisia and Europe—began in late 2001. With little infrastructure to transport materials, the joint venture needed to build roads, drill wells and lay pipes through a terrain traversing rock for 115km, then through sand for 100km, and then rock again for the remaining 240km. Aside from the geographic isolation and shifting terrain, workers had to access the site from a narrow, unpaved right-of-way on each side of the pipeline route, and needed to complete tens of thousands of desert journeys to deliver materials.

After the thorough construction, welding, risk assessment and testing process—involving nearly 2200 construction workers and the expertise of engineering partner Bechtel—the pipeline was completed three months ahead of schedule in August 2003. The construction culminated in Bechtel team winning an award for its outstanding safety record and engineering innovation.

To find out more about the economic benefits of gas production for Algeria, and the groundbreaking work done by Sonatrach and other project partners to spur local development, please visit our Economy page in the In Salah Gas Project section.

How does the CO₂ separation at In Salah work?

 The raw natural gas stream at Krechba arrives at a pressure of 86bar and contains 5.5 per cent CO₂ on average by volume.  To reduce this to 0.3 per cent in order to meet export pipeline specifications, the CO₂ is separated from the natural gas in a multi-stage, proprietary aMDEA (activated methyl diethylamine) chemical engineering process licensed by BASF. 

The acid feed-gas (defined as any gas that contains significant amounts of acidic gases such as carbon dioxide (CO₂) or hydrogen sulfide (H₂S) makes contact with an activated amine solution in an absorber column by passing through two parallel process trains. The treatment with the amine, a type of organic chemical compound related to ammonia, removes virtually all the CO₂, leaving the resulting methane gas stream requiring further dehydration in a glycol contactor prior to export.

The amine solution is stripped of the CO₂ and sent back to the absorber to begin the next process cycle. This regeneration process produces a CO₂ stream of over 98 per cent purity at low pressure, around 1.4 bar.

Around 1.4 million standard cubic metres per day of CO₂ are produced from the gas processing facility at Krechba. Before being re-injected into the Krechba reservoir this is compressed to a very high pressure of 185bar to force it into the reservoir’s low permeability sandstone.

Two high-volume, centrifugal compressor trains were purpose-built to meet the In Salah reservoir’s unusually demanding requirements. Each comprises four stages of compression, driven by electric motors that together require 24 megawatts of power, about two thirds of the site’s total power generation output.  Because CO₂ is heavier than natural gas, significant amounts of energy are needed to compress it resulting in high discharge temperatures of more than 250ºC. 

Each stage of compression is preceded by an inlet suction scrubber and followed by an air cooler to regulate temperature.  Between the third and fourth stages the CO₂ is dried by glycol dehydration to avoid wet CO_{2 –} which is mildly acidic and so can react with ferrous metals - corroding the carbon steel flow lines transporting the gas to the injection wells, up to 14km away.

At the discharge pressure of 185bar, the CO₂ is in its dense or supercritical phase where it behaves more like a liquid.  It can quickly shift between the gaseous, liquid and solid states, so it must be kept hot and at high pressure. If it cools and depressurises, it becomes ‘dry ice’. For this reason some areas of the plant are designed to withstand temperatures below -80ºC.

Offsetting emissions generated through re-injection

 The extra energy required by Krechba’s two high power centrifugal compressors creates additional emissions but most of these are offset through efficient heat recovery. In turn, this leads to a corresponding decrease in the fuel needed for the plant’s process heating.

The net increase of around 45,000 tonnes of CO₂ per year emitted by power generation is far outweighed by the positive environmental  benefits of storing around one million tonnes of CO₂ each year in the Krechba reservoir.