Research in Borehole Geophysics is currently ongoing in the following areas:
- Time-lapse full waveform inversion of vertical seismic profiles
- Study of frequency-dependent attenuation of seismic waves from borehole measurements
- Distributed Acoustic Sensing
Time-lapse full waveform inversion of vertical seismic profiles
Full waveform inversion (FWI) is an emerging technology that can significantly facilitate reservoir characterization, as it directly estimates the properties of the subsurface from almost unprocessed seismic data.
Common vertical seismic profile (VSP) processing and interpretation techniques rely on using only transmitted or primary reflected waves. However, the whole wavefield contains information on the structure of the subsurface. FWI attempts to utilize 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 VSP geometry, such as recording of transmitted waves, high signal-to-noise ratio and absence of surface waves on the gather, make VSP particularly suitable for full waveform inversion. On the other hand, the introduction of Distributed Acoustic Sensors (DAS) makes VSP a very convenient tool for permanent reservoir monitoring. This motivates the research conducted in the department. Time-lapse applications are 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.
Study of frequency-dependent attenuation of seismic waves from borehole measurements.
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 characterization of geological objects.
The vertical seismic profiling (VSP), 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 etc.) from borehole seismic measurements. The projects involve 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. One of the ongoing projects focuses on the estimation of the anisotropic attenuation model in Nephrite South area (Cooper Basin) using Q full-wave-inversion (Q-FWI) of 3D VSP data. The project is undergoing with the collaboration of the Center for Wave Phenomena (Colorado School of Mines).
Distributed Acoustic Sensing
Distributed Acoustic Sensing (DAS) is a fibre-optic sensing technology that uses standard fibre-optic cables to acquire seismic data. DAS is recently under highlight within the seismic industry as it offers many advantages towards 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.
The Curtin Borehole Geophysics group has tested DAS acquisition in a variety of field applications. 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.