Geochemical characteristics of unconventional gas resources in the U.K. and the applications for gas tracing
Unconventional gas extraction has caused controversy due to induced seismicity, inadequate disposal of waste by-products, and alleged incidents of shallow groundwater contamination. Determining the origin of shallow gases is problematic because methane and other hydrocarbons have numerous sources that may have overlapping geochemical characteristics, and few baseline measurements were taken prior to drilling. Additionally, hydrocarbons can be fundamentally altered by physicochemical and microbial processes which can mask the original geochemical signatures. This project develops the understanding of natural tracers in UK unconventional gas resources and reports the results of major gas composition, stable carbon and hydrogen isotopes, radiocarbon content of methane, and novel noble gas (He, Ne, Ar, Kr, Xe) measurements from unconventional gas sites across the UK. Characterising different gas sources reveals processes within unconventional gas reservoirs, develops a baseline for future work, and allows the development of a geochemical ‘fingerprint’ which allows more accurate and precise determination of the different sources of methane. The Coal Bed Methane (CBM) field at Airth, Central Scotland, is characterised by a methane-dominated thermogenic gas based on δ13CCH4 and δDCH4 data and elevated ethane and propane contents. Gases show high helium concentrations (1105 ppm – 2984 ppm) with a 3He/4He isotopic ratio (0.18 Ra) which is uniform across the field, indicating a small (<4%) but resolvable mantle helium contribution not previously observed onshore in the UK. Also observed are elevated concentrations of excess 40Ar* (40Ar/36Ar = 371 – 1031), and enrichments of 20Ne/36Ar relative to the ratios normally expected in air saturated water which cannot be explained by simple solubility fractionation or mass-fractionation. A model is outlined where increasing dewatering of the wells as the field is developed for commercial extraction results in lower overall noble gas concentrations, a decrease in the air-derived inventory, and an increase in crustal and mantle components. This is hypothesised to occur via the progressive degassing and removal of dissolved air-derived components from formation waters from the more permeable cleat and fracture networks surrounding wellbores, leaving the remaining gases with a greater contribution from crustal-rich components sourced from within the coal matrix. Abandoned mine gases from Nottinghamshire and South Yorkshire coal mines showed variable methane concentrations (39 – 75%), with the remaining volume being mainly composed of a mixture of nitrogen and carbon dioxide. Correlation of N2 and 36Ar concentrations in some samples showed ingress of atmospheric air as a result of the suction applied to the mine to facilitate gas production, which was subsequently depleted of oxygen in the mine environment. Carbon dioxide was sourced in lower volumes from coal oxidation, variably mixed with higher concentrations formed from the chemical dissolution of carbonates in the acidic mine environment. Coal gases were characterised by a narrow range of δ13CCH4 and δDCH4 values of a primarily thermogenic origin, thermogenic levels of ethane and propane, and high levels of purely radiogenic helium (350 - 1506 ppm at 0.006 – 0.039 Ra). The stable isotope δ13CCH4 and δDCH4 measurements for unconventional gases typically overlapped those of other coal gases, North Sea gases, and landfill gas, which demonstrates this commonly used tool can be an ambiguous for gas source determination. Elevated levels of helium were found to be ubiquitous in all unconventional gases, and up to 3 orders of magnitude above the concentration found in air (5.24 ppm), which shows helium is an excellent tracer for unconventional gases.