Borehole geophysics

Seismic attenuation directly affects seismic amplitude and waveform shape, which impact attributes used for reservoir characterisation (such as those derived from AVO analysis and inversion). As a result, a better understanding of attenuation can reduce uncertainty in reservoir characterisation. CRGC is developing multiple approaches to estimate attenuation using vertical seismic profile (VSP) data.

When seismic waves propagate through the earth subsurface, they usually experience scattering by small-scale subsurface heterogeneities and loss of energy due to the irreversible internal friction in rocks (absorption). Both scattering and absorption are frequency-dependent phenomena that induce velocity dispersion and frequency-dependent amplitude loss of propagating waves. Knowledge of the factors that control seismic attenuation and dispersion in the subsurface may be useful for seismic imaging and quantitative characterisation of geological objects. These phenomena can only be significant if there is a strong contrast in elastic properties between the layers. The same stack of high-contrast layers can also cause significant anisotropy. This opens a possibility to use anisotropy parameters to predict anomalously high attenuation areas and vice-versa.

The vertical seismic profiling, or borehole seismic measurements, has always been a choice method for estimation of frequency-dependent seismic attenuation. The Curtin geophysics team are involved in a number of projects that focus on the study of scattering and intrinsic absorption parameters in various exploration fields of Australia (Cooper Basin, Bonaparte Basin, Otway C02CRC geosequestration site) from borehole seismic measurements. The projects involve modelling, processing of VSP datasets and estimation of attenuation using conventional methods (spectral-ratio, centroid frequency shift) and more advanced inversion approaches. Recently, the Curtin team have developed a new robust approach for estimation of Q-factors using waveform inversion of VSP and well log data. The inversion allows separation of layer-induced scattering and absorption in the horizontally layered subsurface.

Researchers: Prof. Roman Pevzner, Prof. Andrej Bona, Prof. Boris Gurevich, Dr. Konstantin Tertyshnikov

Most sedimentary rocks are anisotropic, yet it is often difficult to accurately incorporate anisotropy into seismic imaging workflows because analysis of anisotropy requires knowledge of a number of parameters that are difficult to estimate directly from surface seismic data. The CRGC team has developed an approach to infer azimuthal P-wave anisotropy from S-wave anisotropy that is calculated from vertical seismic profile data.

In the CRGC approach we first compute the azimuthal P-wave anisotropy in the dry medium as a function of the azimuthal S-wave anisotropy using a rock physics model, which accounts for the stress dependency of seismic wave velocities in dry isotropic elastic media subjected to triaxial compression. Once the P-wave anisotropy in the dry medium is known, we use the anisotropic Gassmann equations to estimate the anisotropy of the saturated medium. We tested this workflow on the log data acquired in the North West Shelf of Australia, where azimuthal anisotropy is likely caused by large differences between minimum and maximum horizontal stresses. The obtained results are compared to azimuthal P-wave anisotropy obtained via orthorhombic tomography in the same area.  This methodology could be useful for building the initial anisotropic velocity model for imaging, which is to be refined through migration velocity analysis.

Researchers: Prof. Andrej Bona, Prof Roman Pevzner

Full waveform inversion (FWI) is an emerging technology that can significantly facilitate reservoir characterisation, as it directly estimates the properties of the subsurface from almost unprocessed seismic data. CRGC is focused on time-lapse FWI applied to vertical seismic profile (VSP) data obtained using conventional geophones and distributed acoustic sensing (DAS).

Common VSP processing and interpretation techniques rely on using only the transmitted or primary reflected waves. However, the entire wavefield contains information on the structure and properties of the subsurface. FWI attempts to utilise all the events on the seismic gather in order to construct an accurate model of the elastic properties of the studied medium.

Particular properties of the VSP geometry, such as the recording of transmitted waves, high signal-to-noise ratio and absence of surface waves on the gather, make VSP data particularly suitable for full waveform inversion. In addition, the use of Distributed Acoustic Sensors (DAS) makes the VSP a very convenient tool for permanent reservoir monitoring. This motivates the research conducted by the CRGC team. Time-lapse applications are one the main focus of the research. Different time-lapse FWI strategies are studied, with applications to synthetic data and field data acquired on the CO2CRC Otway site, with both traditional geophones and DAS.

Researchers: Prof. Roman Pevzner, Prof. Andrej Bona, Dr. Sinem Yavuz

Distributed Acoustic Sensing (DAS) is a fibre-optic sensing technology that uses standard fibre-optic cables to acquire seismic data. The Curtin Borehole Geophysics group has tested DAS acquisition in a variety of field applications in order to develop best practices and better understand the method’s capabilities and limitations.

Distributed Acoustic Sensing can offer many advantages over conventional seismic receivers. DAS requires no point sensors and it acquires acoustic data along the entire cable simultaneously at small spatial intervals. DAS can be deployed on the surface, but the best results are seen when using DAS to acquire VSP data. In case of VSP acquisition, the fibre-optic cable can be deployed cemented behind the well casing, on the tubing, or coupled to the borehole fluid.

At the CO2CRC Otway Project, DAS data has been acquired for surface seismic, offset VSP, walk-away VSP, and 3D VSP. The VSP data acquired using cemented fibre-optic cables present a high signal to noise ratio, outperforming the geophone VSP data. 3D VSP with cemented DAS, acquired in a 1600 m vertical well, also resulted in high-quality datasets, imaging reflections beyond 2 km deep. Surface seismic acquired with DAS still requires further developing as DAS lacks in broadside angle sensitivity for incident waves close to a right angle. Additionally, at Curtin University campus, we have drilled testing well where we installed a fibre-optic cable along the entire length of the well. At the well, we have tested different deployments of DAS, different designs of fibre-optic cables, as well as testing DAS against conventional geophone and hydrophone tools.

As DAS can be installed permanently in the well, this tool is particularly applicable to permanent reservoir monitoring. At the Otway Project, we combine permanently installed DAS with permanently installed surface orbital vibrators to come up with a continuous remotely accessible seismic monitoring program to image the development of the injected CO₂.

Researchers: Prof. Roman Pevzner, Prof. Andrej Bona, Dr. Konstantin Tertyshnikov, Dr. Sinem Yavuz