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Figure 5.5 Schematic of the In Salah Gas Project, Algeria. One MtCO2 will be stored annually in the gas reservoir. Long-reach horizontal wells with slotted intervals of up to 1.5 km are used to inject CO2 into the water-filled parts of the gas reservoir.

3CO? injection wells

Figure 5.5 Schematic of the In Salah Gas Project, Algeria. One MtCO2 will be stored annually in the gas reservoir. Long-reach horizontal wells with slotted intervals of up to 1.5 km are used to inject CO2 into the water-filled parts of the gas reservoir.

Opportunities for enhanced oil recovery (EOR) have increased interest in CO2 storage (Stevens et al., 2001b; Moberg et al., 2003; Moritis, 2003; Riddiford et al., 2003; Torp and Gale, 2003). Although not designed for CO2 storage, CO2-EOR projects can demonstrate associated storage of CO2, although lack of comprehensive monitoring of EOR projects (other than at the International Energy Agency Greenhouse Gas (IEA-GHG) Weyburn Project in Canada) makes it difficult to quantify storage. In the United States, approximately 73 CO2-EOR operations inject up to 30 MtCO2 yr-1, most of which comes from natural CO2 accumulations - although approximately 3

MtCO2 is from anthropogenic sources, such as gas processing and fertiliser plants (Stevens et al, 2001b). The SACROC project in Texas was the first large-scale commercial CO2-EOR project in the world. It used anthropogenic CO2 during the period 1972 to 1995. The Rangely Weber project (Box 5.6) injects anthropogenic CO2 from a gas-processing plant in Wyoming.

In Canada, a CO2-EOR project has been established by EnCana at the Weyburn Oil Field in southern Saskatchewan (Box 5.3). The project is expected to inject 23 MtCO2 and extend the life of the oil field by 25 years (Moberg et al.,

Box 5.3 The Weybum CO.-EOR Project.

The Weyburn CO2-enhanced oil recovery (CO2-EOR) project is located in the Williston Basin, a geological structure extending from south-central Canada into north-central United States. The project aims to permanently store almost all of the injected CO2 by eliminating the CO2 that would normally be released during the end of the field life.

The source of the CO2 for the Weyburn CO2-EOR Project is the Dakota Gasification Company facility, located approximately 325 km south of Weyburn, in Beulah, North Dakota, USA. At the plant, coal is gasified to make synthetic gas (methane), with a relatively pure stream of CO2 as a by-product. This CO2 stream is dehydrated, compressed and piped to Weyburn in southeastern Saskatchewan, Canada, for use in the field. The Weyburn CO2-EOR Project is designed to take CO2 from the pipeline for about 15 years, with delivered volumes dropping from 5000 to about 3000 t day-1 over the life of the project.

The Weyburn field covers an area of 180 km2, with original oil in place on the order of 222 million m3 (1396 million barrels). Over the life of the CO2-EOR project (20-25 years), it is expected that some 20 MtCO2 will be stored in the field, under current economic conditions and oil recovery technology. The oil field layout and operation is relatively conventional for oil field operations. The field has been designed with a combination of vertical and horizontal wells to optimize the sweep efficiency of the CO2. In all cases, production and injection strings are used within the wells to protect the integrity of the casing of the well.

The oil reservoir is a fractured carbonate, 20-27 m thick. The primary upper seal for the reservoir is an anhydrite zone. At the northern limit of the reservoir, the carbonate thins against a regional unconformity. The basal seal is also anhydrite, but is less consistent across the area of the reservoir. A thick, flat-lying shale above the unconformity forms a good regional barrier to leakage from the reservoir. In addition, several high-permeability formations containing saline groundwater would form good conduits for lateral migration of any CO2 that might reach these zones, with rapid dissolution of the CO2 in the formation fluids.

Since CO2 injection began in late 2000, the EOR project has performed largely as predicted. Currently, some 1600 m3 (10,063 barrels) day-1 of incremental oil is being produced from the field. All produced CO2 is captured and recompressed for reinjection into the production zone. Currently, some 1000 tCO2 day-1 is reinjected; this will increase as the project matures. Monitoring is extensive, with high-resolution seismic surveys and surface monitoring to determine any potential leakage. Surface monitoring includes sampling and analysis of potable groundwater, as well as soil gas sampling and analysis (Moberg et al., 2003). To date, there has been no indication of CO2 leakage to the surface and near-surface environment (White, 2005; Strutt et al., 2003).

2003; Law, 2005). The fate of the injected CO2 is being closely monitored through the IEA GHG Weyburn Project (Wilson and Monea, 2005). Carbon dioxide-EOR is under consideration for the North Sea, although there is as yet little, if any, operational experience for offshore CO2-EOR. Carbon dioxide-EOR projects are also currently under way in a number of countries including Trinidad, Turkey and Brazil (Moritis, 2002). Saudi Aramco, the world's largest producer and exporter of crude oil, is evaluating the technical feasibility of CO2-EOR in some of its Saudi Arabian reservoirs.

In addition to these commercial storage or EOR projects, a number of pilot storage projects are under way or planned. The Frio Brine Project in Texas, USA, involved injection and storage of 1900 tCO2 in a highly permeable formation with a regionally extensive shale seal (Hovorka et al., 2005). Pilot projects are proposed for Ketzin, west of Berlin, Germany, for the Otway Basin of southeast Australia and for Teapot Dome, Wyoming, USA (Figure 5.1). The American FutureGen project, proposed for late this decade, will be a geological storage project linked to coal-fired electricity generation. A small-scale CO2 injection and monitoring project is being carried out by RITE at Nagoaka in northwest Honshu, Japan. Small-scale injection projects to test CO2 storage in coal have been carried out in Europe (RECOPOL) and Japan (Yamaguchi et al., 2005). A CO2-enhanced coal bed methane (ECBM) recovery demonstration project has been undertaken in the northern San Juan Basin of New Mexico, USA (Reeves, 2003a) (Box 5.7). Further CO2-ECBM projects are under consideration for China, Canada, It2aly and Poland (Gale, 2003). In all, some 59 opportunities for CO2-ECBM have been identified worldwide, the majority in China (van Bergen et al., 2003a).

These projects (Figure 5.1; Table 5.1) demonstrate that subsurface injection of CO2 is not for the distant future, but is being implemented now for environmental and/or commercial

5.1.3 Key questions

In the previous section, the point is made that deep injection of CO2 is under way in a number of places (Figure 5.1). However, if CO2 storage is to be undertaken on the scale necessary to make deep cuts to atmospheric CO2 emissions, there must be hundreds, and perhaps even thousands, of large-scale geological storage projects under way worldwide. The extent to which this is or might be, feasible depends on the answers to the key questions outlined below and addressed subsequently in this chapter:

• How is CO2 stored underground? What happens to the CO2 when it is injected? What are the physico-chemical and chemical processes involved? What are the geological controls? (Sections 5.2 and 5.3)

• How long can CO2 remain stored underground? (Section 5.2)

• How much and where can CO2 be stored in the subsurface, locally, regionally, globally? Is it a modest niche opportunity or is the total storage capacity sufficient to contain a large proportion of the CO2 currently emitted to the atmosphere? (Section 5.3) 2

• Are there significant opportunities for CO2-enhanced oil and gas recovery? (Section 5.3)

• How is a suitable storage site identified and what are its geological characteristics? (see Section 5.4)

• What technologies are currently available for geological storage of CO2? (Section 5.5)

• Can we monitor CO2 once it is geologically stored? (Section 5.6)

• Will a storage site leak and what would be the likely consequences? (Sections 5.6 and 5.7)

• Can a CO2 storage site be remediated if something does go wrong? (Sections 5.6 and 5.7)

• Can a geological storage site be operated safely and if so, how? (Section 5.7)

• Are there legal and regulatory issues for geological storage and is there a legal/regulatory framework that enables it to be undertaken? (Section 5.8)

• What is the likely cost of geological storage of CO2? (Section 5.9)

• After reviewing our current state of knowledge, are there things that we still need to know? What are these gaps in knowledge? (Section 5.10).

The remainder of this chapter seeks to address these questions.

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