| dc.description.abstract | In this thesis we address the problem of determining in situ stress and fracture properties
in reservoirs using borehole logs and surface seismic reflection data. The dissertation
covers four subtopics.
The first is the determination of horizontal stress magnitudes from measurements
in a borehole. Two types of data used are stress-induced rock failures in the borehole,
known as "breakouts," and the dispersions of polarized flexural waves which
propagate along the borehole. Traditionally these data are analyzed to derive stress
orientations but not magnitudes. To determine the magnitude of stresses directly
from breakouts, we use an iterative elastic modeling of stresses around the borehole
and Mohr-Coulomb failure criterion to match the borehole deformation. As a second
method we use dispersion curves of the two polarized flexural waves and their
crossover points. These methods are applied to data from a well in northeastern
Venezuela. The combination of these two techniques provides a complete profile of
stress as a function of depth since the first method is applied at the breakout depths
and the second is applied everywhere else in the borehole. Both borehole methods
agree in the estimation of stress orientation and magnitude. The maximum horizontal
stress is in the NNW-SSE direction, in agreement with a regional stress model
calculated from the relative motions of the Caribbean and South America plates. The
magnitudes of principal stresses are on average, SHmax ≃ 1.1Sv (Sv: vertical stress)
and Shmin ≃ 0.9Sv (Shmin: minimum horizontal stress). This suggests strike-slip
faulting, consistent with earthquake mechanisms in the region.
The in situ stresses play an important role on determining the properties of fractured
formation. The azimuth of SHmax determines the preferred orientation of
open fractures. Surface seismic reflection data provide the means for detecting the
fractures. The second contribution of this thesis is developing a method to detect
discrete fractures, and to determine their orientation and average spacing. We developed
a novel and practical technique, called the F-K method, based on the frequency-wavenumber
(f-k ) domain analysis of seismic coda. The fractured medium targeted
in this study is a network of rather regularly spaced, parallel, sub-vertical fractures, with dimensions similar to seismic wavelength. The seismic response of a fractured
medium is studied by finite difference numerical models for a variety of situations
where orientation, spacing, height, and fracture compliance are varied. In the direction
normal to fractures, scattered waves propagate with slower apparent velocities
than waves propagating along the fractures. The orientation of fractures is well constrained
from the azimuthal dependence of scattering. The spectral characteristics
(frequency, wavenumber and amplitude) of the backscattered waves are related to
fracture properties like spacing, compliance, and height. The dominant wavenumber
is very sensitive to fracture spacing.
We use the F-K method to analyze a data set from the Lynx Field in Canada.
Characterization of fracture properties in this field is important for development plans
to maximize the gas production. In the field data, the acquisition geometry results
in irregular fold, with under sampling of certain azimuths and offsets. We address
the acquisition footprint issue by controlling the azimuth binning of the data and
neglecting the low/irregular fold gathers in the fracture analysis. We also apply the
Scattering Index (SI ) method (Willis et al., 2006) to the same data from the Lynx
Field. The SI method is a robust method to detect fractures and to provide fracture
orientations using multi-azimuth/multi-offset pre-stack data. In the realm of existing
3D seismic surveys, data with such acquisition characteristics are few. The fourth
contribution of this thesis is therefore the conception of a post-stack version of the
SI method that extends the scope of this method to practically every 3D seismic
surface data set. In this version, a scattering index is computed for a fully stacked
trace per CMP gather. As long as the bin contains traces parallel to the fracture
strike, the stacking process of all azimuths and offsets preserves the reverberating
character introduced by the fractures. The post-stack SI at a fractured location has
a large value in comparison to a non-fractured location. The variations of post-stack
SI values across the field reveal the distribution of highly fractured areas. Fracture
strike cannot be determined in this case because it does not include the azimuthal
behavior of the scattering. However, the results from the post-stack SI are helpful to
identify areas of interest to focus the more specialized scattering analysis methods.
We apply the F-K and SI methods to the Lynx Field seismic data and compare the
results. Since spatial resolution of the two methods are different we upscale the SI
maps to match the resolution of the F-K method. The combined analysis of the
Lynx Field indicates that the preferred fracture orientation is N40°E, which agrees
with the regional stress field. The distribution of highly fractured regions appears to
be associated to the geological features, such as folds and faults. The average fracture
spacing, obtained by the F-K method shows that, in the Lynx Field, fracture spacing
decreases in the west side of the field where the structural dips are higher. | en_US |
