Evaluation of the Potential for Gas and CO2 Leakage Along Wellbores

Abstract

Implementation of carbon dioxide (CO 2) storage in geological media requires a proper assessment of the risk of CO 2 leakage from storage sites. Leakage pathways may exist through and along wellbores, which may penetrate or be near to the storage site. One method of assessing the potential for CO 2 leakage through wells is by mining databases that usually reside with regulatory agencies. These agencies collect data concerning wellbore construction, oil and gas production, and other regulated issues for existing wells. The Alberta Energy Resources Conservation Board (ERCB), the regulatory agency in Alberta, Canada, collects and stores information about more than 315,000 oil, gas, and injection wells in the province of Alberta. The ERCB also records well leakage at the surface as surface-casing-vent flow (SCVF) through wellbore annuli and gas migration (GM) outside casing, as reported by the industry. The evaluation of a leakage pathway through wellbore casing or annuli and what causes these wellbore leaks are the first step in determining what factors may contribute to wellbore leakage from CO 2-storage sites. By using available data, major factors that contribute to wellbore leakage were identified. Data analysis shows that there is a correlation between these SCVF/GM and economic activity, technology changes, geographic location, and regulatory changes regarding well completion and abandonment. Further analysis indicates a relationship between low-annular-cement top, external corrosion, casing failure, and wellbore leakage (SCVF/GM). Other factors that could affect the presence of wellbore leakage, such as wellbore deviation, surfacecasing depth, and wellbore density, were also investigated. This paper presents the findings of the data analysis and a method to evaluate the potential for leakage along wells in an area where CO 2 storage is intended. This information is useful not only for future operations of CO 2 storage in geological media, but also for current operations relating to the exploration and production of hydrocarbons.

Numerical investigation of methane and formation fluid leakage along the casing of a decommissioned shale gas well

“…According to several authors, the most frequent failure in a wellbore is casing or well annulus failure [e.g., Cooke, 1979;Cheung and Beirute, 1985;Sepos and Cart, 1985;Schmitz et al, 1996;Brooks et al, 2008;Watson and Bachu, 2009;Gorody, 2012;Muehlenbachs, 2012;Dusseault and Jackson, 2014]. Any failure to seal gasproducing intervals (whether or not targeted as the producing zone) and/or inadequate cementation of the borehole casing annulus (i.e., discontinuities and microchannels in the cement column, between the cement and surrounding rock formations and between the casing and surrounding cement) can create a leakage pathway [Watson and Bachu, 2009] which can lead to upward migration of fluids (in this study including formation fluid and gas) by advective or diffusive processes [Dusseault et al, 2000].…”

“…Any failure to seal gasproducing intervals (whether or not targeted as the producing zone) and/or inadequate cementation of the borehole casing annulus (i.e., discontinuities and microchannels in the cement column, between the cement and surrounding rock formations and between the casing and surrounding cement) can create a leakage pathway [Watson and Bachu, 2009] which can lead to upward migration of fluids (in this study including formation fluid and gas) by advective or diffusive processes [Dusseault et al, 2000]. An example of casing annulus failure was reported in Bainbridge Township (Ohio, USA) [Bair et al, submitted report, 2010], where inadequate cementation of the production casing between the target Clinton sandstone gas reservoir and the overlying Newburg gas-bearing zone allowed gas from both formations to enter the casing annulus and migrate upward along the wellbore.…”

“…An example of casing annulus failure was reported in Bainbridge Township (Ohio, USA) [Bair et al, submitted report, 2010], where inadequate cementation of the production casing between the target Clinton sandstone gas reservoir and the overlying Newburg gas-bearing zone allowed gas from both formations to enter the casing annulus and migrate upward along the wellbore. Watson and Bachu [2009] reported that 4.5% of the 315,000 wells drilled between 1910 and 2004 in Alberta, Canada, had either surface casing vent flow (SCVF, leakage of gas through the annulus between the production and surface casings) or had gas migration (GM) through the soil, from outside the outer casing. As concluded by these authors, uncemented and/or inadequately cemented casings are the main cause of SCVFs/GM.…”

“…With the objective of protecting groundwater resources, most regulations or guidelines for cementation depths are based on the depth of the potable groundwater zone or some minimum distance below it [e.g., AER, 1990]. The overall effectiveness of the seal depends on the amount of cement fill, the class of the set cement, elimination of mud and gas channels within the cement, and the complete bonding of the cement to the casing and the formation [Watson and Bachu, 2009]. Depending on the specific geologic conditions (e.g., temperature, pressure, and presence of sulfates) and the depth of the well, different classes of Portland cement (ranging from A to H) can be used (see American Petroleum Institute [2002] for a detailed discussion).…”

“…However, the potential for leakage in previously abandoned wells still remains a major issue that needs to be better understood [Watson and Bachu, 2009]. Oil and gas wells can develop gas leaks along the casing years after production has ceased and the well has been plugged and abandoned.…”