Influence of Phase Behaviour in the Well Design of CO2 Injectors

Abstract

The design of CO2 injection wells differs from that of gas and water injectors owing to the effects of the CO2 phase behaviour. Each CO2 injection well requires a design specific to the reservoir conditions, rates and injection stream characteristics just as there is no one well design that fits all hydrocarbon production requirements but there are key themes that emerge from our experience in the design of Carbon Capture and Storage (CCS) wells.In depleted reservoirs, the CO2 injection poses unique challenges when compared to injection for Enhanced Oil Recovery projects or aquifer injection. The potential to generate low temperatures due to CO2 expansion into vapour phase during steady state injection can be a concern. Here one technique is to employ a “reverse velocity string” that is to deploy slim tubing to create back pressure that avoid the production of low temperature during normal injection conditions. Well operations or transient effects can also generate low temperatures, albeit of short duration.During the highly unlikely event of loss of containment leading to flow to atmospheric conditions, there could be a rapid expansion of CO2 in the top part of the well. Such an expansion would be accompanied by a reduction in the temperature due to the Joule Thomson effect as the CO2 travels down the saturation and possibly the sublimation line. This cooling potential is one driver for technological advancement of present day well components. Once the flow is identified, the emergency shutdown system of the well automatically activates (for example closing the tree valves and/or the Subsurface Safety Valve – SSSV). Modelling has been conducted to understand the limitations of the current safety critical equipment for different scenarios. The result of modelling indicates that even in low magnitude releases, the SSSV can be subjected to low temperatures with metal temperatures close to the triple point even when set deep in the well.In addition to the normal requirements of the well equipment in terms of pressure, loads and chemical interactions of the injection fluid; this cooling effect of the CO2 has an impact on the well design. Existing well equipment, including safety critical components (tree valves and SSSV), have limitations in terms of material embrittlement, qualification envelopes and performance of fluids (annuli and power fluids) at this low temperature. The placement of some well components, such as SSSV, also may be affected by the cooling effect, to ensure its ability to close in case of a release.It is crucial to make a distinction between functionality and integrity requirements, whereby components retain integrity under this low temperature excursion even though they might lose operability. This differentiation has to be identified and agreed at an early stage in the well design. Another critical distinction is between the system and component design rating as some components of the system might be exposed to low temperature subject to placement in the well.An area of well design for the lifecycle of the well that requires significant research and development into is around well intervention, both planned routine intervention with wireline and unplanned remedial intervention via platform and/or vessel. Again here the CO2 phase behaviour can pose challenges to current equipment and methods.This paper aims to highlight the effects of CO2 phase behaviour on well design and the considerations for a robust and optimised well design for CCS.