The interfacial tension of partially miscible phases, containing H2O and
hydrocarbons in the presence of CO2 at elevated pressures and temperatures,
has been studied within the context of producing cleaner fossil fuels
by simultaneously tackling greenhouse gas emissions. This is a most relevant
property in
uencing the multiphase reservoir
ows associated with
enhanced oil recovery (EOR), and carbon capture and storage (CCS). The
main core of the thesis focuses on the experimental investigation of the
dependence of interfacial tension on pressure and temperature, for various
mixtures of pure substances relevant to oil- eld conditions and
uids. For
this purpose, a high pressure high temperature (HPHT) apparatus, comprising
a view cell, high pressure capillary tubing connections, and appropriate

uid delivery syringe pumps, was used over an operating temperature
range of (298 to 473)K and at pressures up to 60MPa. The apparatus
implemented the pendant drop method, well suited for the accurate
determination of
uid/liquid interfacial tensions at elevated pressures and
temperatures, linked to a computer-aided drop shape analysis (DSA) system.
Measurements were made over a wide range of conditions for the twophase
systems (H2O+CO2), (n-decane+CO2), (n-dodecane+CO2), (n-
hexadecane+CO2), (H2O+n-decane) and (H2O+[n-decane+CO2]). The
di erent isotherms recorded for each system demonstrated systematic trends
with increasing pressure, while the decrease of interfacial tension with temperature
observed at ambient pressures was usually reversed at elevated
pressures. For the (H2O+CO2) system in particular, the pressure dependence
of interfacial tension demonstrated abrupt changes at certain conditions,
associated with the onset of the liquid or supercritical states, above
which the interfacial tension was less sensitive to changes in both pressure
and temperature. This was not the case for the (n-alkane+CO2) systems,
where the interfacial tension reduced with increasing pressure, vanishing as
the two phases became miscible. This motivated the study of the ternary
vi
system (H2O+[n-decane+CO2]) for which the interfacial tension showed a
positive but comparatively small pressure dependence. Analysis of the values
obtained for di erent amounts of CO2 in the ternary system, provided
surface excess concentrations as well as relevant thermodynamic quantities.
In most cases, the time evolution of the interfacial tension was also examined.
This was largely in line with the time scales required for the equilibration
of a pendant drop by di usion of the, slightly mutually soluble,
associated phases. However, some di erences from the expected time behaviour
for the (H2O+[n-decane+CO2]) systems are discussed with regard
to e ects from the possible presence of amphiphilic impurities. Theoretical
tools based on the statistical associating
uid theory (SAFT) - a molecularbased
equation of state - were used to compare interfacial tension predictions
with the acquired experimental results. This demonstrated the impressive
quality of the ab-initio predictive capabilities of SAFT, as well as indicating
the scope for further improvement of the approach.
The e ect of interfacial tension on the trapping of non-wetting
uids, like
CO2, in porous media was explored at ambient conditions in a separate
study. A water-soluble surfactant, dodecyl trimethyl ammonium bromide
(DTAB), was used to lower the interfacial tension of the ([H2O+DTAB]+n-
decane) system. This was determined for DTAB concentrations ranging
from (0 to 0.0325) mol L 1 using the Wilhelmy plate method. A custommade
sintered-glass porous core was used for conducting
uid displacement
(\
ooding") experiments, which were carried out during a two-month secondment
undertaken at Shell International Exploration and Production BV,
Rijswijk, the Netherlands. The amount of trapped n-decane (the nonwetting
phase) was determined using an in-line micro-CT scanner that required
the addition of a heavy salt, caesium chloride (CsCl), at a xed
concentration of 4 wt%, to create the appropriate density contrast for Xray
imaging. The e ect of CsCl on the interfacial tension of the system
([H2O+CsCl-4wt%+DTAB]+n-decane), was therefore also quanti-
ed; the analysis of the interfacial tension obtained both with and without
CsCl provided values of the surface area per DTAB molecule at the interface
and Gibbs energies of adsorption for the two cases. From micro-CT tomography
scans obtained, 3D spatial distributions of the two immiscible phases
within the core were constructed, providing phase saturations as well as
statistics on the number and size of trapped clusters, for di erent
ooding
vii

uid concentrations and injected pore volumes.
The current work was carried out as part of the Imperial College London
{ Shell Grand Challenge Programme on Clean Fossil Fuels. It covered a
wide range of interfacial tension measurements at conditions relevant to
oil- eld applications, and explored the e ect of interfacial tension on the
trapping potential of a model porous medium. The research represents
a signi cant step in enabling the direct design and optimisation of EOR
and carbon storage processes. Areas in which it might be extended, both
through further experimental studies and improved modelling, have been
identi ed.