Isotopic and molecular weight fractionation of shale gas; the influences of geochemistry
Creators: name: Davey, Ruth Jennifer
Publisher: name: Imperial College London
Year: 2019

description: Shale gas has the potential to be a bridging fuel, facilitating change to a low-carbon economy whilst meting the World’s energy demands. Whilst shale gas is by no means “new” fuel, it’s proliferation in the USA in recent years has brought it to the attention of other countries, including the UK. Many of the shale gas plays in the USA, such as the Bakken, Eagle Ford and Monterrey Shales cover a large areal extent in areas of relative tectonic inactivity. The grid-pattern and infill drilling seen in these plays would not be viable in places such as the UK where the shales have been more heavily deformed and placing well pads are more strictly controlled by legislation. In order to maximise the efficiency of a shale gas well and inform future drilling decisions, many factors must be known; the volume of gas stored within the shale, the percentage of total volume extracted, how the gas is stored and if the well is draining from a large volume of shale or a small volume of shale. Whilst much research has been carried out on shales to determine these factors, often in isolation of one another, there is no consensus in the literature about how they can be calculated. Numerically modelled production decline curve analysis is a commonly used technique but does not factor in the specific geochemistry of a shale which has been shown to influence gas storage and the degassing production rate. There are few geochemical signatures related to shale gas but change in the stable carbon isotope fractionation and changing molecular weight with gas extraction has been seen in production data. Often these changes are attributed to adsorption-desorption processes and are used as a proxy for volume of gas stored in the adsorbed phase. There is however a large body of evidence suggesting that diffusion processes also influence the Δ gas geochemistry thus cannot be used as a direct proxy for adsorbed gas. In order to move away from numerical modelling and work towards empirical studies based on experimental work on shale geochemistry, understanding what the change in gas geochemistry relates to is necessary to develop a geochemical index for degassing shales. Production 4 modelling would be greatly informed by understanding how shale geochemistry and composition influences changing gas geochemistry with gas extraction. This research measures the change in stable carbon isotope fractionation of methane and the change in methane – ethane ratio with gas extraction to see how the degassing profile changes from 100% gas in place to 100% gas extracted, building idealised volume-based production decline curves related to, initially, isolated shale components then whole shales with varying volumes and types of total organic carbon and levels of maturation. The data was collected on a custom-built sample cell coupled with either a Gas Chromatographer – Combustion - Isotope Ratio Mass Spectrometer or Gas Chromatographer – Flame-Ionizing Detector which allowed calculation of the molar volume of gas stored in the sample, the volume of gas extracted and the residual gas that could not be extracted. The cell design allowed for volume-based geochemical curves to be generate for the individual shale components and whole shale samples. The results indicate that changing the shale geochemistry influences the gas geochemistry with gas extraction and the shale composition is a key component for generating a geochemical index for volume-based production decline curves. This technique allows quantitative assessment of not only total GIP but the percentage total volume of gas extracted over given period of time, advancing from current empirical techniques to quantitative analysis. As such, it will replace current generic mathematically calculated decline curve models with a shale/site specific decline curve which is dictated by the mineralogy/TOC content of the shale. This technique facilitates: 1. Verification and calibration of original GIP estimates 2. Generation of expected geochemical fractionation trend for volume of total gas extracted 3. Calculation of gas volume remaining in the reservoir and what percentage has been produced over a given production time period 5 4. Enhancement of the predicative capabilities of a shale gas reservoir production life This will aid economic assessment of a shale gas field, namely, a measurable assessment of production time, volume of gas retained within the reservoir and the economic worth of restimulation of the well and it will no longer be economically viable to keep the well open.