A novel online desorption GC-C-IRMS system for measuring isotope fractionations of gas released from shales
Creators: name: Wright, Miriam Cláudia Policarpo
Publisher: name: Imperial College London
Year: 2018

description: Shales are known to have very low permeabilities, in the order of the microDarcies. Given that the space available for gas storage and transport is much smaller when compared to conventional reservoirs, gas transport mechanisms such as diffusive transport play a much bigger role in unconventional reservoir evolution compared to pressure-driven flow. It has been shown previously that low-permeability rocks cause the gas desorbed from them to isotopically fractionate in the process. The isotopic fractionation has been observed specifically for carbon-containing species (e.g. CO2 and CH4). 12C-depletion of free methane compared to adsorbed gas species is observed during desorption. The effect is usually attributed to the faster diffusion of gas species that contain the lighter carbon isotope (12C), relative to the 13C-containing species. The effect is documented extensively in studies conducted on coal, but there is a lack of information on sorption and diffusion processes undergone by different isotope species in shale samples. In this research, a new instrument and analytical technique was devised to adsorb/desorb gas onto shales, and then measure changes in stable isotope composition of the desorbed gas. The design process of the new instrumental setup was outlined and initial tests conducted on it. Initial testing was done to ensure that the depressurization chamber was leak free and, more importantly, to confirm that gas could successfully be cycled through it and subsequently analysed. After the initial testing stages, standard samples that were deemed to be suitable proxies for common components in shales (activated carbon, calcite, clay) were successfully analysed using the instrument. A set of Eagle Ford shale samples was then analysed using the novel on-line desorption-Gas Chromatography- Combustion- Isotope Ratio Mass Spectrometry (GC-C-IRMS) system for measuring isotope fractionation of desorbed gas. The data obtained from the experiments conducted on natural samples revealed correlations between a) high organic matter content in the shales and higher amounts of isotopic fractionation and b) high calcite content and lower amounts of isotopic fractionation. The findings highlight the potential that isotopic measurements have in interpreting transport mechanisms in shales and ultimately shale gas reservoirs. Shale gas decline curves have direct relevance to the economic viability of a well. The new measurement system provides the potential to recognise the decline rate of a shale gas well and, therefore, the estimated ultimate recovery. The measurement method will allow confident decisions to be made on refracking frequency. The new experimental apparatus allows for gas desorption to be conducted in a controlled environment, where variables such as temperature and sorbent type, can be controlled. The ability to control experimental variables is useful, as a range of conditions that might be present in real shale gas reservoirs can be replicated. Full desorption curves (δ13C of desorbed gas versus Percentage of gas desorbed) can be plotted, provided that for any given experiment, the full amount of initially adsorbed gas is consequently desorbed and analysed. Within the context of the experimental apparatus presented here, the desorption of 100% of the gas initially adsorbed onto a sample would correspond to the completion of a shale gas reservoir production cycle. The ability to measure the isotopic composition of the desorbed gases opens up the possibility to develop a model to isotopically distinguish free and adsorbed gas phases, at any given production stage, with the aid of the experimental data and insights obtained from the new experimental apparatus.