Figure 5.4 Simplified diagram of the Sleipner CO2 Storage Project. Inset: location and extent of the Utsira formation.
Box 5.2 The In Salah, Algeria, CO2 Storage Project.
The In Salah Gas Project, a joint venture among Sonatrach, BP and Statoil located in the central Saharan region of Algeria, is the world's first large-scale CO2 storage project in a gas reservoir (Riddiford et al., 2003). The Krechba Field at In Salah produces natural gas containing up to 10% CO2 from several geological reservoirs and delivers it to markets in Europe, after processing and stripping the CO2 to meet commercial specifications. The project involves re-injecting the CO2 into a sandstone reservoir at a depth of 1800 m and storing up to 1.2 MtCO2 yr-1. Carbon dioxide injection started in April 2004 and, over the life of the project, it is estimated that 17 MtCO2 will be geologically stored. The project consists of four production and three injection wells (Figure 5.5). Long-reach (up to 1.5 km) horizontal wells are used to inject CO2 into the 5-mD permeability reservoir.
The Krechba Field is a relatively simple anticline. Carbon dioxide injection takes place down-dip from the gas/water contact in the gas-bearing reservoir. The injected CO2 is expected to eventually migrate into the area of the current gas field after depletion of the gas zone. The field has been mapped with three-dimensional seismic and well data from the field. Deep faults have been mapped, but at shallower levels, the structure is unfaulted. The storage target in the reservoir interval therefore carries minimal structural uncertainty or risk. The top seal is a thick succession of mudstones up to 950 m thick.
A preliminary risk assessment of CO2 storage integrity has been carried out and baseline data acquired. Processes that could result in CO2 migration from the injection interval have been quantified and a monitoring programme is planned involving a range of technologies, including noble gas tracers, pressure surveys, tomography, gravity baseline studies, microbiological studies, four-dimensional seismic and geomechanical monitoring.
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