Evaluation of the Potential for Gas and CO2 Leakage Along Wellbores

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 106817, "Evaluation of the Potential for Gas and CO2 Leakage Along Wellbores," by Theresa L. Watson, SPE, T.L. Watson & Assocs. Inc., and Stefan Bachu, Alberta Energy and Utilities Board, prepared for the 2007 SPE E&P Environmental and Safety Conference, Galveston, Texas, 5–7 March. Implementation of carbon dioxide (CO2) storage in geological media requires proper assessment of the risk of CO2 leakage from storage sites. Leakage pathways may exist through and along wellbores that penetrate or are near the storage site. One method of assessing the potential for CO2 leak-age is by mining databases that usually reside with regulatory agencies. Introduction CO2-injection schemes have been in operation since as early as the 1970s for tertiary oil recovery as miscible floods, with the indirect benefit of CO2 removal from the atmosphere. Other gas-injection schemes also are used in the oil and gas industry, such as natural-gas storage and acid-gas disposal. In the case of CO2 sequestration, the storage unit must be almost leak free to the atmosphere or other geological formations to meet safety requirements and greenhouse-gas-reduction objectives. The full-length paper focuses on human-created leak-age paths—in particular, abandoned wellbores that were drilled for oil and gas exploration and production. The analysis is based on data collected by the Alberta Energy and Utilities Board (EUB) for more than 315,000 wells drilled until the end of 2004 in the province of Alberta, Canada. EUB also records well leakage at the surface as surface-casing vent flow (SCVF) through wellbore annuli and gas migration (GM) outside casing. Background Abandonment Methods. Wells Drilled and Abandoned. For a typical openhole abandonment scenario in Alberta, regulations require that any porous zone be isolated or covered to prevent cross-flow between geological formations. In addition, useable groundwater must be covered with cement and isolated from potential hydrocarbon-bearing zones. After the downhole cement plugs have been set, the well must remain open for inspection for a minimum of 5 days. After this time, the well is checked for static fluid level or other indications of plug leakage before the casing can be cut and capped below grade level. Wells Drilled, Cased, Completed, and Abandoned. There are three main types of zonal isolation and abandonment for a typical cased-hole well after reservoir depletion.Bridge plug capped with cement above perforations.Retainer and cement squeezed into perforations.Cement plug set across perforations. Regulations since 2003 require zonal isolation behind casing and that useable groundwater be protected. In many cases, older wells were constructed with low annular cement tops, allowing many zones to be in communication behind casing. Under the current regulations, a cement squeeze would be required to achieve isolation before final abandonment. Wellbores must be abandoned with inhibited fluid inside the casing and be pressure tested to a minimum of 7000 kPa. Before cutting and capping the production and surface casing, the well must be checked for SCVF and

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