1 Introduction

Subsurface storage of captured CO2 in depleted oil and gas reservoirs, saline aquifers, or unmineable coalbeds is currently seen as one of the most feasible ways of reducing greenhouse gas emissions (e.g. Bachu, 2000; Wawersik et al., 2001; IPCC, 2005). Key to evaluating this concept and to storage site selection is the assessment of system integrity, with focus on the potential for CO2 leakage. Possible leakage paths include faults and fractures present in the caprock (Pruess, 2005; IPCC, 2005). However, potential migration of CO2 from subsurface reservoirs along wells is generally recognized as the major hazard associated with long-term storage of CO2 in geological formations or CO2-enhanced recovery (EOR, EGR, ECBM) operations (e.g. Gasda et al., 2004; Pruess, 2005; IPCC, 2005; Carey et al., 2007).

With respect to leakage risk evaluation regarding long-term geological storage of CO2, a distinction has to be made between existing wells and future wells (Watson and Bachu, 2007). Existing wells involve all wells that were drilled prior to CO2 storage operations, generally consisting of shallow groundwater wells and deep wells for hydrocarbon production. Future wells comprise wells directly related to the CO2 storage operations, such as CO2 injection or monitoring wells, and wells penetrating or transecting a CO2 storage reservoir aiming at other structures or deeper reservoirs e.g. for production of hydrocarbons or geothermal energy.

Typically, depleted gas and especially oil reservoirs are perforated by large numbers of (abandoned) production/exploration wells (Figure 1.1). Many of these wells also penetrate deep saline aquifers that might be used for geological storage of CO2. Although most of these wells are sealed and plugged with cement at abandonment, the well system may create potential conduits for CO2 leakage.

Figure 1.1 Well density reflecting locations of hydrocarbon-bearing regions. After: IPCC, 2005.

Future wells can be designed, drilled, completed and abandoned taking into account any previous CO2 storage operations. Suitable skills and technology are readily available to effectively seal and isolate the CO2 storage reservoir, using practices that have been developed and employed over decades in oil and gas industry to adequately deal with sour gas (mainly CO2 or H2S) occurrences as well as newly developed advanced techniques and state-of-the-art materials. In contrast, existing wells were designed prior to CO2 storage, not taking into account the presence of corrosive fluids in the storage reservoir. Therefore their configurations may not agree with such purpose. If still operational, existing wells can usually be adapted to fit injection or abandonment of corrosive fluids. Although costly, such operations can be performed using suitable workover materials and techniques that are currently available in the petroleum industry. Consequently, the main leakage risk is associated with previously abandoned wells that may be no longer accessible and therefore cannot be easily improved when needed.

This report focuses on potential hazards to geological storage of CO2 related to previously abandoned deep oil and gas wells. As many prospective CO2 storage projects will be situated in mature sedimentary basins, these operations need to accommodate previously drilled and abandoned wells. The current study aims to provide a high order evaluation of abandoned wells and their suitability to CO2 storage operations. To this purpose a brief explanation of the major well plugging and abandonment techniques is described in Chapter 2. Several case studies are described in Chapter 3, illustrating some typical aspects associated with abandoned wells in the context of geological storage of CO2. In Chapter 4, subsequently, an overview of the current state of knowledge on potential degradation mechanisms of typical well materials (i.e. cement and steel) under influence of aqueous CO2 is presented. Furthermore, in Chapter 5 a geographical overview of numerous well abandonment regulations is provided, reflecting significant differences in regulatory demands around the world. Moreover, both abandonment regulations and practices historically gradually developed to the present high standards. As a consequence, especially older wells are considered to be a potential threat to long-term storage integrity. In Chapter 6 various risk assessment methodologies are described that are tailored to well integrity evaluation for geological CO2 storage. An overview of potential corrective measures and monitoring strategies is introduced in Chapter 7 on recommended practice. Conclusions are presented in Chapter 8.