A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The integrity of this " bounding seal" system is affected by various mechanical, chemical and thermal forces that act during initial exploration, development and oil production operations, during CO2 injection operations, and during the subsequent CO2 storage phase. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Equations are given which are helpful for identifying the key parameters that govern these geomechanical factors, and further enable firstorder estimates of the risks that they pose to bounding seal integrity. The results of this review are compiled into a table that summarizes key geomechanics-related risks, the mechanisms associated with these risks, and approaches to assess and mitigate them. Where possible, examples are given where these mechanisms have affected oil and gas field operations.
In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO2) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO2 sequestration in geological media in terms of volume, duration and minimum or null environmental impact1. The first commercial scheme for CO2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 106 tonnes of CO2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1000 m below the sea bed2.
In light of the economic benefits of enhanced oil recovery (EOR) derived from CO2 injection in oil reservoirs3, these types of reservoirs will be attractive CO2 injection targets and, most likely, CO2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO2 storage in aquifers. An advantage of CO2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada4 is an example of a large-scale application of EOR operations using anthropogenic CO2, in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone.
A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The initial integrity of this " bounding seal" system is governed by geological factors. A considerable amount of effort has been devoted to the development of procedures for assessing fault seal capacity in potential hydrocarbon reservoirs (e.g., reference 5). The emphasis of this paper is not on the initial fault seal properties, which are assumed to have been good for reservoirs that have proven to be effective oil or gas producers.
