Development and application of advanced thermodynamic molecular description for complex reservoir fluids containing carbon dioxide and brines

Abstract

This thesis contains a study of the thermodynamic properties of complex reservoir fluids. The focus of this work is the development of an equation of state and molecular models to describe the phase behaviour of the different components of the reservoir fluids that may be encountered in the context of CO2 injection into geological formations suitable for storage, e.g., saline aquifers or depleted hydrocarbon reservoirs, together with that of mixtures of these components that may be encountered. The major constituents of these reservoir fluids are the injected gas (CO2, which may contain some impurities), alkanes and various other hydrocarbons from natural gas or crude oil, water and salts. The first task is to ensure that the method selected (the SAFT-VR Mie equation of state) to model those fluids can provide an accurate description of the simplest of the fluids encountered, CO2 and hydrocarbons. A crucial aspect of this concerns a detailed examination of the procedure for searching the highly degenerate model-parameter space to obtain the best models for each fluid. The suitability of the method is also assessed by studying other simple fluids, as a means to test the range of validity of the models developed. Once the method has been validated for a wide range of relatively simple fluids, the next step is to study more-complex fluids including, in particular, water. Water is ubiquitous in the systems of interest but is a notoriously difficult fluid to model accurately using simple models of the sort that are tractable for use in the context of equation-of-state modelling. The provision of a good model of water underpins a large part of the work and is accomplished only as a result of further development of the theory upon which the equation of state is based, involving not only its statistical-mechanical foundation but also lengthy numerical procedures to isolate the most physically reasonable application of the theory. Bearing in mind its simplicity, the resulting model for water, within the context of the refined theory, provides for a remarkably good representation of the thermodynamic properties of water and forms a highlight of the thesis. The remaining part of the work is the development of a framework in which to treat the ionic components of reservoir fluids. Following the implementation of a standard method to treat electrolyte solutions, the main goal of the thesis is achieved with the modelling of the phase equilibria of CO2-brine systems, demonstrating that the proposed method is a suitable tool for the study of complex reservoir fluids containing carbon dioxide and brines.