Samuel, Revelation Jacob; (2019) Transient flow modelling of carbon dioxide (CO2) injection into depleted gas fields. Doctoral thesis (Ph.D), University College London.

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Abstract

The internationally agreed global climate deal reached at the Paris Climate Conference in 2015 is intended to limit the increase in global average temperatures to ‘well below’ 2°C above pre-industrial levels. This comes in addition to the European Union ambition for 80% to 95% reduction in the 1990 greenhouse gas emissions by 2050 in order to avoid dangerous climate change. Most scenario studies indicate that Carbon Capture and Storage (CCS) is essential for achieving such ambitious reductions. In CCS operations, depleted gas fields represent prime targets for large-scale storage of the captured CO2. Considering the relatively low wellhead pressure of such fields, the uncontrolled injection of the high-pressure dense phase CO2 will result in its rapid, quasi-adiabatic Joule-Thomson expansion leading to significant temperature drops. This could pose several risks, including blockage due to hydrate and ice formation following contact of the cold sub-zero CO2 with the interstitial water around the wellbore and the formation water in the perforations at the near well zone, thermal stress shocking and fracture of the wellbore casing steel and over-pressurisation accompanied by CO2 backflow into the injection system due to the violent evaporation of the superheated liquid CO2 upon entry into the wellbore. In order to minimise the above risks and develop best-practice guidelines for the injection of CO2, the accurate prediction of the CO2 pressure and temperature along the well during the injection process is of paramount importance. This thesis deals with the development and verification of a Homogeneous Equilibrium Mixture (HEM) model and a Homogenous Equilibrium Relaxation Mixture (HERM) model for simulating the transient flow phenomena taking place during the injection of dense phase CO2 into depleted gas fields. The HEM model assumes instantaneous interface mass, momentum and energy exchange between the constituent CO2 liquid and vapour phases. As such they remain at the same pressure, temperature and velocity, whence the corresponding fluid-flow may be described using a single set of mass, momentum and energy conservation equations. The HERM on the other hand presents an additional equation which accounts for the thermodynamic non-equilibrium thorough the introduction of a relaxation time. It also accounts for phase and flow dependent fluid/wall friction and heat transfer, variable well cross sectional area as well as deviation of the well from the vertical. At the well inlet, the opening of the upstream flow regulator valve is modelled as an isenthalpic expansion process; whilst at the well outlet, a formation-specific pressure-mass flow rate correlation is adopted to characterise the storage site injectivity. The testing of the models is based on their application to CO2 injection into the depleted 2582 m deep Goldeneye Gas Reservoir at Hewett field in the North Sea for which the required design and operational data are publically available. Varying injection scenarios involving the rapid (5 mins), medium (30 mins) and slow (2 hrs) linear ramping up of the injected CO2 flow rate to the peak nominal value of 33.5 kg/s are simulated. In each case, the simulated pressure and temperature transients at the top and bottom of the well are used to ascertain the risks of well-bore thermal shocking or interstitial ice formation leading to well blockage due to the rapid cooling of the CO2. Detailed sensitivity analysis of the most important parameters affecting the CO2 in-well flow behaviour, including the wellbore diameter variations, well inclination, upstream temperature, pressure and time variant injection mass flow rate are conducted. The simulation results obtained for a slow (2 hrs) flowrate ramp-up case using the HEM model produce a minimum wellhead temperature of - 11 oC. The corresponding minimum temperature using the HERM model on the other hand is - 21 oC, demonstrating the importance of accounting for non-equilibrium effects and the model’s usefulness as a tool for the development of optimal injection strategies for minimising the risks associated with the injection of CO2 into depleted gas fields.

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