Located in the Algerian Sahara, the In Salah Gas (ISG) Joint Association's Krechba field is the site of an industrial-scale greenhouse gas mitigation operation: the first geological storage of CO2 in a producing gas reservoir.
Operated jointly by Sonatrach, BP and StatoilHydro, ISG produces some nine billion cubic metres of gas annually from Carboniferous and Devonian sands, making it a major contributor to Algeria's growing gas exports. At the Krechba Central Processing Facility the carbon dioxide content of the gas is reduced from between 5% and 10% to 0.3 %. The captured CO2 is compressed to 175 bar and injected into the waterleg of the Krechba Carboniferous reservoir (the Tournaisian formation) through three horizontal injection wells. At the same time, the reservoir is drained by five producers located above the gas-water contact.
Since start up in 2004, more than 2.5 million tonnes of carbon dioxide have been injected. Injection will continue at a rate of approximately one million tonnes annually, facilitating storage of some 14-17 million tonnes in total.
Geological storage of carbon dioxide can be based on injection in water-bearing formations - which is the case for the Sleipner field - or in a hydrocarbon-bearing reservoir such as the Krechba Carboniferous. The former strategy requires geological studies to confirm the quality of the seal. In the latter case, the presence of hydrocarbons is an indication that the cap rock can contain gases for millions of years. The Krechba seal consists of 900 metres of Visean shale that has contained the natural gas for 200 million years.
Once injected, a considerable part of the carbon dioxide will be trapped by capillary forces, dissolution in water and adsorption in sand grains. However, some of the CO2 will remain mobile and will probably move under the action of pressure gradients and gravity. Reservoir monitoring and modelling is necessary to understand these complex dynamic phenomena. A programme was set up in 2004 aimed at verifying the long-term storage of CO2 at Krechba, and to demonstrate industrial-scale geological storage as a viable greenhouse gas (GHG) mitigation option.
The USD 30 million Joint Industry Project (JIP) involves Sonatrach, BP and StatoilHydro as well as several R&D groups in Europe and the USA. The JIP has an active programme in the field ranging from 4D seismics to potable water monitoring wells. Reservoir modelling and simulation is carried out to study the movement of the injected CO2. The intention is to ensure that the carbon dioxide remains within the reservoir and to detect any possible breaches.
The risk of CO2 leaking through the geological seal is seen as exceedingly small, but in June 2007 an ISG well-monitoring team discovered minor amounts of CO2 migrating to the surface through the annulus of an old exploration well (Kb-5). The well was killed and stabilised, and is currently being permanently plugged and cemented using CO2 resistant material.
