document preview

Abstract
Geologic carbon sequestration may involve injection of large quantities of carbon dioxide (CO2) into primarily deep saline aquifers for storage purposes or, where feasible, into oil and gas reservoirs for enhanced oil recovery objectives. The literature and experience from industrial analogs indicates that well-bores (active or inactive/abandoned) may represent the most likely route for leakage of injected CO2 from the storage reservoirs. Therefore, sound CO2 injection well design and well integrity, operation and monitoring are of critical importance in such projects. This paper presents design considerations for (1) the construction of CO2
injection wells including down-hole tubular (casing/tubing/packer) and cements, (2) methods to verify that the wells have mechanical integrity (both internal and external) and monitoring approaches applicable to CO2 geo-sequestration in the U.S. and a short discussion of the risks posed by abandoned wells within a storage field and the safety aspect of CO2 wells.

1.0 Introduction
Geological carbon injection used for enhanced oil recovery (EOR) has a well-established history in the United States and elsewhere. In the United States alone, the oil and gas industry has injected over 600 million tons of CO2 (11 trillion standard cubic feet) over the past 35 years (API, 2007) and the CO2 is believed to remain stable once injected, provided the original pressure of the geological formation is not exceeded. However, the concept of injecting CO2 for long-term geological storage in deep saline aquifers is a direct result of global attention on CO2 as a green-house gas (GHG). This newer practice may compel the construction of fit-for-purpose injection wells explicitly for the purpose of injection and puts more emphasis on the need to assure the long-term stability of stored CO2. Three large-scale projects, the North Sea Sleipner project, Canada’s Weyburn Field project, and the In Salah project in Algeria are generating data that should provide useful information for the future.

Wellbore integrity is important to ensuring the long-term geological sequestration of CO2. NORSOK D-010 defines well integrity as “Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life-cycle of a well”. Wellbore integrity issues can be divided into two types: improper completion and abandonment of the wells (this is particularly true for depleted oil and gas reservoirs which may have thousands of inactive or abandoned wells, with each well representing a potential pathway for the CO2 to reach overlying aquifers or the atmosphere); and the long-term stability of wellbore materials in a CO2-rich environment.

2.0 Regulatory Framework for CO2 injection Wells in the U.S.
The U.S. Environmental Protection Agency (EPA) is mandated under the Underground injection Control (UIC) Program of the Safe Drinking Water Act (SDWA) to protect Underground Sources of Drinking Water (USDW) and the health of persons from underground injection. A USDW is any aquifer or portion of an aquifer that contains water that is less than 10,000 parts per million (ppm) total dissolved solids, or contains a volume of water that is either presently a source or in the future a viable source for a Public Water System. EPA directly implements the UIC program in 10 states, 33 states have primary enforcement authority and EPA and States share UIC program implementation in 7 states.