Resolving fingerprints of unconventional gas resources and methane rich groundwaters in the UK

Abstract

IInterest in the utilisation of subsurface geoenergy technologies (such as geothermal or unconventional gas extraction) has increased in recent years, as they are considered key in achieving Net Zero carbon emissions targets. 90% of UK homes used (predominantly gas) fossil fuels for heating, cooking, and hot water in 2020; with heating and cooling accounting for 51% of the energy demand in 2018 globally. However, there are several manageable, but significant techno-societal risks associated with the usage of the subsurface that has to be reduced, such as resource sustainability and efficiency, reservoir quality, operation maintenance, ground gases and environmental change. Determining the presence, magnitude, and origin of subsurface gases, and how their geochemical fingerprints evolve within the shallow subsurface is vital to developing an understanding of how to manage the risk posed by ground gases in geoenergy technology development. One of the risk factors in the development of such technologies is the potential for contamination of potable groundwater resources. As such, it is key to establish whether deep gas that is released by changes to the subsurface can be categorically identified at shallow depths; and distinguished from shallow biogenically derived methane sources. This thesis aims to characterise the subsurface geochemical conditions of potential geoenergy technology sites using a range of geochemical fingerprinting techniques.

The UK Geoenergy Observatory in Glasgow is a unique facility for investigating shallow, low-temperature mine water thermal energy within abandoned and flooded workings. Here, the first CH₄ and CO₂ concentration-depth profiles and stable isotope (δ¹³C꜀ₕ₄, δ¹³C꜀ₒ₂, and δD꜀ₕ₄) profiles obtained from UK mine workings are presented, through analysis of headspace gas samples degassed from cores and chippings collected during construction. These are used to investigate the variability of gas fingerprints with depth within unmined Carboniferous coal measures and Glasgow coal mine workings. Stable isotope compositions of CH₄ provide evidence of a biogenic source, with carbonate reduction being the primary pathway of CH₄ production. Gas samples collected at depths of 63 to 79m exhibit enrichments in ¹³C꜀ₕ₄ and ²H, indicating the oxidative consumption of CH₄ This correlates with their proximity to the Glasgow Ell mine workings, which will have increased exposure to O₂ from the atmosphere as a result of mining activities. CO₂ gas is more abundant than CH₄ throughout the succession in all three boreholes, exhibiting high δ¹³C꜀ₒ₂ values relative to the CH₄ present. δ¹³C꜀ₒ₂ values become progressively lower at shallower depths (above 90m), which can be explained by the increasing influence of shallow groundwaters containing a mixture of dissolved marine carbonate minerals (~0‰) and soil gas CO₂ (-26‰) as depth decreases. These findings provide an insight into the variability of mine derived gases within 200m of the surface and provide an important ‘time-zero’ record of the site.

The Vale of Pickering was first identified as a potential target for exploitation of unconventional gas resources, and whilst such exploration has ceased within the UK at present, the Vale of Pickering site is now being considered for potential geoenergy technology development. Groundwater and gas samples collected from the region provide an opportunity to establish how geochemical tracers, such as stable isotopes and noble gases, can be used to distinguish hydrocarbon source rocks and identify potential migration of deep gas to shallow aquifer bodies. Stable isotope compositions of CH₄ provide evidence of two distinct methane sources, with methane in both the shallow superficial and Corallian groundwater samples being produced biogenically by CO₂ reduction; and methane within the Third Energy gas samples exhibiting a distinct thermogenic gas signature. Both ³He/⁴He and ⁴He/²⁰Ne ratios are elevated in comparison to atmospheric values, and evidence potential mixing between deep gases (KM5) and the atmosphere. From ⁴He generation calculations, it is evident that measured ⁴He concentrations cannot be generated from the in-situ decay of U and Th within the Vale of Pickering stratigraphy alone. Therefore, ⁴He must be sourced from an external radiogenic flux. In order to further investigate how ⁴He noble gas signatures change with depth within the subsurface, a coupled a hydro-chemical model of the Vale of Pickering was developed and is outlined in this thesis. From this, a better understanding of the migration of deep sourced methane to the shallow groundwaters in the Vale of Pickering is resolved, incorporating the likely evolution of gas signatures through migration.

The use of noble gas geochemistry alongside gas composition and stable isotope ratio allows for the better characterisation of geochemical signatures of ground gases, and the identification of different methane sources within the subsurface. This thesis highlights the need for an understanding of the fluid migration pathways and their associated geochemical signatures of ground gases, and establishes key environmental baselines in the event of future geoenergy technology development of the sites.