Geological storage has emerged as a leading candidate because of the potential wide spread availability of these storage sites and their ability to handle large quantities of CO2. Currently, three geologic storage options emerge as most feasible:
Other promising candidates may exist onshore or offshore, but in every case CO2 is injected into deep subsurface geologic formations.
Potential of Geologic Formations for Storage of Carbon
The International Energy Agencys (IEA) Greenhouse Gas R&D Programme and the Intergovernmental Panel on Climate Change have explored the potential of the three primary storage options discussed
The storage options presented above are not the only geologic formations under consideration. Other possible storage sites include basalt formations, organic rich shales, salt caverns, and abandoned mines. These options may not be suited for large scale CO2 storage and/or require extensive additional research to assess their viability as storage sites.
The storage potential of types of geologic formations located within the U.S. has been under active investigation and their storage resources have been recently re-evaluated. The range of storage potential for each major reservoir type is summarized below:
Depleted Oil and Gas Fields 138-152 Gton CO2
Un-minable Coal Seams 157-178 Gton CO2
Deep Saline Reservoirs 3,297-13,909 Gton CO2
Existing Megatonne-Scale CCS Projects Site Selection Criteria for Geologic Storage of Carbon
The crucial part in carbon storage is the selection of site for storing the captured carbon. Site selection of storage reservoirs must take many factors into consideration. Various properties of the storage rock and seals, or traps, must be considered including:
Porosity - the measure of the space available for storing the CO2 (acting as a fluid)
Permeability - the measure of the ability of the rock to allow fluid to flow; and
Injectivity - the rate at which the CO2 can be injected into the site
Accessibility the location should be economically accessible to the CO2 source
Capacity the formation should have adequate total storage volume to serve the intended purpose
Storage Security- well defined trapping mechanism should exist within the storage formation. Cap rock should be adequately impermeable and thick to prevent upward migration of CO2.The geological environment should be adequately stable, free from faults or uncapped penetrations/wells into the storage site
Costs of storage include capital expenditures (CAPEX) that cover site evaluation and development costs, drilling costs, surface facilities, and monitoring costs such as seismic and operational expenditures (OPEX), which include operational and maintenance items as well as other monitoring activities. Costs for drilling oil and gas wells can be used to approximate CO2 injection well costs since the major capital costs for CO2 geological storage are drilling wells, infrastructure and project management. The cost of installing and running CO2 monitoring equipment is generally small compared to storage costs. Operating costs include manpower, maintenance and fuel. The costs for licensing, geological, geophysical and engineering feasibility studies required for site selection, reservoir characterization should also be considered. The cost of injection increases with the depth of the well. Offshore wells cost significantly more than onshore wells, as a function of water depth and well complexity, and can be more than four times higher even in shallow water environments. Deep-water wells are much more expensive. The cost of oil and upstream operations (drilling, completion and production) has risen significantly over the past few years due to increases in the price of materials and a shortage of resources (e.g. drilling rigs and crews, engineering expertise, etc). It is more costly to inject and store other gases (NOx, SOx, H2S) with CO2 because of their corrosive and hazardous nature, although the capture cost may be reduced (Allinson et al., 2003).
Overall storage costs were estimated in the IEA in 2008. Because of the cost escalation, storage costs have been updated with the cost increase factored in. In Europe (onshore and offshore), 30 Gtons of saline aquifer capacity could be used at a cost of USD 10-20/t; and 5 Gtons of depleted oil and gas field capacity could be used at USD 10-25/t.
An assessment of storage costs in US, Australia and Europe done by IPCC in 2005 found that the costs for storage excluding transportation and compression ranged from a low of 1-2 $/ton to a high of about 12-15 $/ton of CO2. Taking into consideration inflation and increase in drilling and injection costs since 2005, the current cost of storage are about 10-20 $/ton.