Establishing the geochemical baseline of the CMC FieldResearch Station, Alberta, Canada, prior to CO₂ injection andresolving the source and fate of natural CO₂ in the Morecambe and Rhyl Fields, East Irish Sea Basin, UK

Abstract

Carbon capture and storage (CCS) is an industrial scale, cost-effective mitigation strategy to reduce anthropogenic CO₂ from entering the atmosphere. Successful deployment of CCS is critically dependent on the safe and secure storage of CO₂ injected into the subsurface for storage.

Geochemical monitoring and modelling are an important tool in tracking the migration and fate of injected CO₂ throughout the entire life cycle of a CCS project, from baseline to post-closure. In this thesis, a wide range of geochemical tools are utilised to understand the geochemical baseline of the CMC Field Research Station, Alberta, Canada prior to CO₂ injection, and to identify the source and potential trapping mechanism of CO₂ residing within the Morecombe and Rhyl Fields within the East Irish Sea Basin.

The CMC Research Institutes Field Research Station (FRS) is a purpose built test site for developing and demonstrating the monitoring of subsurface fluids following subsurface CO₂ injection. This thesis presents the first multi-well gas and groundwater characterisation of the geochemical baseline of the site.

The work highlights that gases sampled from a range of depths exhibit low CO₂ concentrations, and that biogenic methane occurs pervasively in the shallow (<550 m) succession.

Furthermore, the presence of a minor thermogenic component that increases with depth is also established, correlating with elevated levels of radiogenic 4He. 4He generation and expulsion has been mathematically modelled, highlighting that concentrations in some samples are above the level that could be generated from in-situ radioactive decay of U and Th within the FRS stratigraphy. This research shows that a resolvable radiogenic contribution to fluids and gases at the site, indicating a fluid connection to a deeper hydrocarbon producing formation in the Western Canadian Sedimentary Basin. The inherent geochemical fingerprints within the injected CO₂ were also determined, and found to be depleted in He, Ne and Ar, yet elevated in ⁸⁴Kr and ¹³²Xe relative to ³⁶Ar. This implies that inherent noble gas fingerprints could be used as an effective geochemical tracers of the injected CO₂ at this site.

The Rhyl Field, located within the East Irish Sea Basin, is a producing gas field, with a recorded CO₂ concentration of 37%. Prior to this work the source of the CO₂ was unconstrained and no knowledge existed on the effect that this elevated concentration may have had on the mineralogy of the reservoir units.

Identification of mantle derived noble gas isotopes confirms the presence of magmatic volatiles within the Rhyl field, however the stable isotopic profile of the produced gas does not exhibit a magmatic signature. The ratio of -36.2‰ suggests a highly organic source. Integrating the history of the basin with previously unpublished well reports and data relating to a hydrothermal event within the basin, allows the CO₂ source to be better constrained. Paleogene dyke emplacement likely heated coaliferous Type II/III source rock underlying the basin, causing the fractional generation of CH₄, CO₂ and N2 -providing the Rhyl field with its distinct bulk gas signature.

Additionally, detailed mineralogical studies into the Rhyl field show no evidence of late stage CO₂- rock interaction resulting in the precipitation of known carbonate minerals which have previously been linked to CO₂ sequestration (including siderite, dolomite, calcite, dawsonite etc). Importantly, the Rhyl field was found to contain up to 0.22% tunisite (NaCa₂Al₄(CO₃)₄(OH)₈Cl) cement, a rare carbonate bearing mineral. Through studying formation water data, obtained from the exploration of hydrocarbons, the formation mechanism of tunisite has been constrained. This study highlights that this fluid chemistry, combined with extensive K-feldspar dissolution and CO₂ ingress in the early Paleogene, is the most likely means of tunisite precipitation in the Rhyl field.

This work shows, for the first time, that tunisite can be a resolvable sink of CO₂ within the subsurface.