Fractures exert a strong influence on fluid flow in subsurface reservoirs, and hence an
adequate understanding of fracture properties could provide useful information on how they
may be managed optimally to produce oil and gas or to be used as repositories for carbon
dioxide (CO2) to mitigate climate change. Since fractures are commonly aligned by the stress
field, seismic anisotropy is a key tool in investigating their properties. Velocity anisotropy is
now a well-established technique for determining properties such as fracture orientation and
density, but in recent years, attention has focused on quantifying azimuthal variations in Pwave
attenuation to provide additional information, especially on the fracture size. However,
the practical application of this attribute in geophysical exploration is still limited due to the
uncertainty associated with its measurement and the difficulty in its interpretation in terms of
rock properties. There is still a lack of proper understanding of the physical processes
involved in the mechanisms of attenuation anisotropy. In this thesis, I use the seismic
modelling approach to study the effects of attenuation anisotropy in fractured porous media
using P-waves with the main aim of improving the understanding of these effects and
exploring the physical basis of using attenuation anisotropy as a potential tool for the
characterization of fractured reservoirs.
Fractures with length on the order of the seismic wavelength in reservoir rocks cause
scattering of seismic waves which exhibits characteristic azimuthal variations. I study these
scattering effects using complementary seismic physical (scale-model laboratory
experiments) and numerical (finite difference) modelling approaches. The results of both
approaches are consistent in delineating fracture properties from seismic data. The scattered
energy is quantified through estimates of the attenuation factor (the inverse of the seismic
quality factor Q) and shown to be anisotropic, with elliptical (cos2θ) variations with respect
to the survey azimuth angle θ. The minor axis of the Q ellipse corresponds to the fracture
normal. In this direction, i.e. across the material grain, the attenuation is a maximum. The
major axis corresponds to the fracture strike direction (parallel to the material grain) where
minimum attenuation occurs.
Empirically, the magnitude of P-wave attenuation anisotropy is greater in fluid-saturated
rocks than in dry rocks. I study the influence of fluid saturation on P-wave attenuation
through synthetic modelling and compare the attenuation signature to that of dry fractured
rocks. The results of the analysis show that the relaxation time strongly controls the
frequency range over which attenuation occurs. The magnitude of the induced attenuation increases with polar angle and also away from the fracture strike direction. The attenuation
exhibits elliptical variations with azimuth which are also well fitted with a cos2θ function.
The magnitude of the attenuation anisotropy is higher in the case of the fluid-saturated rocks.
All of these properties of the numerical model are in agreement with the results of empirical
experiments in the laboratory.
The same crack density can result from many small cracks, from a few large cracks, or from
an equal number of cracks of various sizes with varying thicknesses in the same volume of
background material. This makes it difficult to distinguish between the anisotropy caused by
micro-cracks and that caused by macro-cracks. I study the effects of fracture thickness or
aperture on P-wave scattering attenuation through seismic physical modelling, and find that
the induced attenuation has a direct relationship with the fracture thickness or aperture. This
result indicates the potential of using P-wave attenuation to get information which might be
useful in examining the effects of voids in the rocks, and also provides a basis for further
future theoretical development to distinguish the effects caused by thin micro cracks and
large open fractures.
Finally, I study the effects of two types of fluid saturation (brine and CO2 in the supercritical
state) on P-wave attenuation through synthetic modelling, with particular attention to varying
CO2 saturation using the CO2 properties at the Sleipner gas Field in the North Sea. The
presence of CO2 causes more attenuation in the numerical model output than when the rock
is saturated with only brine. The induced attenuation increases with decreasing percentage of
CO2 saturation and has a maximum magnitude at 10 % CO2 saturation. Further work is
needed to quantify the additional effect of fractures on these results