Abstract

Implementation of carbon dioxide storage in geological media requires a proper assessment of the risk of CO2 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 CO2 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 and Utilities Board (EUB), 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, Canada. The EUB 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 industry.

The evaluation of a leakage pathway through wellbore casing or annuli and what causes these wellbore leaks is a first step in determining what factors may contribute to wellbore leakage from CO2 storage sites. By utilizing available data, major factors which 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, surface casing 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 CO2 storage is intended. This information is useful not only for future operations of CO2 storage in geological media, but also for current operations relating to the exploration and production of hydrocarbons.

Introduction

The possibility of removing CO2 from an industrial emission stream and storing it in deep geological media to reduce the impact on the atmosphere of green house gas is being extensively investigated1. More than 80 CO2 injection schemes have been in operation since as early as the 1970's for tertiary oil recovery as miscible floods2, with the side benefit of CO2 removal from the atmosphere. Other gas injection schemes are also in use within 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 near leak free, to the atmosphere or other geological formations, to justify the costs and to meet safety requirements and greenhouse gas reduction objectives. This paper will focus on human created leakage paths, in particular wellbores that were previously drilled for exploration and production of oil and gas reserves and were subsequently abandoned. The work reported here determines important factors which can be used to predict which wellbores are mosre likely to leak, have future abandonment liablilty and if these wellbores will adversely impact CO2 storage shemes in the future. The analysis is based on data for more than 315,000 wells drilled up to the end of 2004 in the province of Alberta, Canada.

Background
Potential Wellbore Leakage Pathways

Figure 1 illustrates typical wellbore construction and abandonment profiles for Alberta, Canda. From these diagrams one can identify potential leakage pathways from a CO2 storage reservoir or gas-bearing formation. For a leak to occur three elements must exist3:

  1. A leak source

  2. A driving force such as buoyancy or head differential

  3. A leakage pathway

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