The evaluation and study of offshore renewable energies to reverse oil and gas exploitation

Abstract

Methane is traditionally produced from highly emitting processes like steam methane reforming. Alternatively, this process can be upgraded using depleted oil and gas reservoirs used as carbon capture and storage sites and introducing wind-hydrogen to produce methane at the natural reservoir conditions. These high pressure and relatively lower temperature conditions are the opposite of industrial methanation conditions but are thermodynamically favourable. This thesis thus introduces the novel process of enhanced carbon capture and storage (ECCS), investigating all parts of this scheme, including green hydrogen production from wind and low-temperature methanation.

The first part of this thesis investigates hydrogen production from wind using commercially available desalination and electrolysis units. The intermittency of wind is examined and compared against grid-bought electricity. The hydrogen selling price required to ensure profitability over a 10-year period was determined through techno-economic analysis. Firstly, where hydrogen is produced using grid energy, with electricity purchased when below a specified price point or between specific hours. Secondly, where a wind turbine owner pays an initial CAPEX but does not purchase electricity. Hydrogen prices are closely linked to the CAPEX, with the equipment size, space and safety identified as limiting factors.

As the offshore energy landscape transitions to renewable energy, thousands of offshore structures worldwide are reaching the end of their operating lifetimes and are a liability for the gas and oil industry. Oil and gas platforms offer the opportunity for hydrogen production by desalination and electrolysis using offshore wind power. The industry must avoid stranded assets, liabilities and decommissioning write-downs. GIS mapping was used to investigate the potential of offshore gas and oil infrastructure for green hydrogen production and carbon dioxide abatement by examining the North Sea and the co-evolution of offshore wind and fossil fuels. This work found that the North Sea could produce over 3 million tonnes of green hydrogen while mitigating 6.4 million tonnes of carbon dioxide annually. Globally, projected offshore wind capacity could produce almost double current world hydrogen production.

A novel thermodynamic analysis of the Goldeneye reservoir in the North Sea in Aspen Plus was completed to investigate methanation at reservoir conditions. The Goldeneye reservoir is in proximity to wind farms in operation and planning. The study used the RGibbs reactor model that utilises with Gibbs free energy minimisation method. This analysis highlighted the lack of literature on the kinetics of low-temperature and high-pressure methanation and was to show the thermodynamic maximum of the reaction. Low temperatures were highlighted as more favourable and a stoichiometric feed ratio of hydrogen and carbon dioxide provides the highest conversion of carbon dioxide and methane selectivity. Goldeneye can store 30 Mt of CO2 and if connected to a 4.45 GW wind farm, could produce 2.10 Mt of methane annually and abate 4.51 Mt of CO2 from wind energy in the grid based on the thermodynamic model.

Low-temperature carbon dioxide methanation was investigated using a shale rock-supported nickel catalyst in an unstirred batch reactor for the first time. Shale rock is present in the geological formations associated with reservoirs in areas of interest, such as the North Sea. The temperature range of 200-250°C and a pressure of 50 bar were used. Catalyst characterisation used BET, SEM, FT-IR, H2-TPR, XRD and ICP. A higher temperature increases the rate of reaction and the catalyst may be reused, with deactivation due be metal species oxidation . The shale support is complex, comprising of calcite and quarts with other smaller impurities and the methanation mechanism is difficult to determine. The presence of carbonate species on the surface of the catalyst support was hypothesised to favour methanation via the formate pathway. Additionally, the presence of CaO and Ca(OH)2 and their decrease in intensity in FTIR results post reaction show the potential for carbonation to occur via a competing reaction. With slower kinetics at reservoirs temperatures, this reaction will occur over long periods, mimicking the current oil and gas model and providing a transitionary fuel.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	David Rooney (Supervisor), Peter Robertson (Supervisor) & Aoife Foley (Supervisor)