This paper discusses a methodology for the design of wells in carbon capture and sequestration (CCS) projects. In addition to carbon dioxide (CO2) injection wells, CCS wells include observation or monitoring wells, as well as utility wells, which are used to reduce pressure by removing formation water.
The paper first outlines the differences between CCS wells and conventional oil and gas wells. These differences include much longer regulatory lifetimes, increasing pressure over well lifetimes, inherently corrosive environment, intermittent operation and large variation of CO2 injection stream properties depending on its impurities. These differences require a different approach to well design for CCS projects. A well design philosophy, which has been developed to address these differences, is presented. The paper outlines the material selection guidelines and tubular load cases. The design philosophy, material guidelines and load cases are illustrated through several example well designs.
For CCS wells, the design should start with the completion size required to achieve the desired CO2 injection rate, and progress outwards. Dual containment is essential; the second barrier must not only be designed for the corrosive environment, but the second barrier and its associated equipment must be periodically inspected or tested. All CCS wells, including injection, monitoring and utility wells, must be designed for potential CO2 exposure. Highest loads may be imposed during transient or upset operations, and may originate from changing thermal conditions. Cement integrity is essential to prevent undetected migration of stored CO2 out of the storage zones. Finally, it is necessary to have pre-prepared contingency plans to detect, shut-in, kill, repair and/or P&A failed wells.
The differences between CCS wells and conventional oil and gas wells require a different approach to well design. If CCS wells were to be designed using conventional methods, the wells might fail to maintain their integrity, thus resulting in the failure to contain injected CO2 in the sequestration zone.