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.Open Acces
