Research in Experimental Rock Physics is currently ongoing in the following areas:
- Broadband acoustic measurements: from seismic to ultrasonic frequencies
- Compaction behaviour of clay sediments
- Digital Rock Physics Under Stress
- Laboratory measurements of intrinsic and stress-induced anisotropy
- Quantitative microstructure characterisation from micro-CT and nano-indentation
- Experimental and Theoretical Study of Partial Saturation Effect on Elastic Properties of Sedimentary Rocks
Rock Physics Laboratory
Broadband acoustic measurements: from seismic to ultrasonic frequencies
Theoretical rock physics models need to be tested and calibrated using laboratory measurements.
To this end, we are performing comprehensive experimental testing of these theories using broadband measurements of dynamic elastic properties (elastic moduli) and attenuation of rock samples.
Experiments are conducted at reservoir conditions using a combination of patent protected forced-oscillation ultra-sensitive strain gauge measurements (0.01 Hz–200 Hz) and ultrasonic testing (1 MHz). The results are being compared with theoretical predictions computed using numerical simulations.
Seismic frequency measurements
Compaction behaviour of clay sediments
Mechanical compaction–one of the main rock-forming processes–occurs in the upper parts of the sedimentary basins, where the weight of overlying deposits compacts sediments below.
During this process, initially unconsolidated sediments lose water and porosity, experience significant changes in microstructure and become stiff rock with different physical properties.
In this experimental work, we study how changes in microstructures affect elastic properties of quartz-kaolinite mixtures during mechanical compaction. Uniaxial stress is applied to the samples progressively to achieve distinct levels of porosity, at which transit times of ultrasonic P- and S-waves in the samples are measured. Velocities and anisotropy parameters are then calculated. The microstructure of the samples is characterised by micro-CT data and SEM image analysis, as well as neutron diffraction experiment and compared with microstructures of real shales.
Digital Rock Physics Under Stress
Modelling of physical properties of rocks from their X-ray microtomographic images (known as digital rock physics) is becoming an important technology in geophysical rock characterisation.
Most commonly, X-ray microtomographic images are obtained at ambient conditions (i.e. at room pressure and temperature). However, reservoir rocks are at such depth that they experience high stresses and temperatures. The thermodynamic properties of the fluids inside the reservoir are pressure and temperature dependent, therefore transport properties are also temperature and pressure dependent. Moreover, it is well established that elastic rock properties of rocks are strongly affected by stress and/or fluid distribution. Thus in order to acquire realistic pore network structures and fluid distributions (including but not limited to residual saturation), and to reliably estimate transport and elastic properties from micro images, rocks have to be imaged at reservoir pressure and temperature conditions.
We thus have developed X-ray transparent pressure and temperature (P-T) cells capable of holding 5 mm to 38.5 mm diameter samples. The P-T cells are mounted inside a high-resolution 3D X-ray microscope VersaXRM-500 (XRadia-Zeiss), and 3D images with nominal voxel size resolutions from (0.3 μm)3 to (40 μm)3
Laboratory measurements of intrinsic and stress-induced anisotropy
Laboratory measurements of elastic properties of rocks are important for calibration of seismic data and for corroboration of theoretical models of rocks.
The most common way of determining the elastic properties of rock samples in laboratory settings is to estimate the velocities of the ultrasonic waves propagating in different directions. We are using an innovative approach to implement the Laser Doppler interferometer (LDI) for measuring the displacement on the sample surface upon arrival of waves into it. LDI can measure the particle velocity of a small (0.01 mm2) element of the sample’s surface along the direction of the laser beam.
By measuring the particle velocity of the same surface element in three independent directions and transforming them to Cartesian coordinates, we can obtain three components of the particle velocity vector. Therefore LDI can be used as a localised three-component (3C) receiver of acoustic waves, and, together with a piezoelectric transducer or a pulsed laser as a source, can simulate a 3C seismic experiment in the laboratory. Performing such 3C measurements at various locations on the sample’s surface produces a 3C seismogram, which can be used to separate P and two S waves, and to find polarisations and traveltimes of these waves.
A ‘walk away’ laboratory experiment demonstrates high accuracy of the method. The measured data matches very well with the results from analytical modelling. From our results, we can conclude that it is possible to characterise elastic properties of materials from the described measurements. In particular, we are able to determine: 1) the angle between the particle movement and the direction of the wave propagation, i.e. the polarisation; 2) the type of waves; and 3) the arrival times of the wave at the point and thus the wave velocities.
Quantitative microstructure characterisation from micro-CT and nano-indentation
Rocks exhibit a complex microstructure, which is the result of both transport and deposition of various minerals and diagenetic alterations.
This complexity in microstructure will translate into complex relationships between geophysical observables and rock parameters (e.g. elastic moduli vs. porosity, permeability vs. porosity) and thus makes reservoir quality prediction difficult. The Rock Physics laboratory is equipped with a 3D X-ray Microscope Versa XRM 500 (Zeiss – XRadia), which allows imaging of rocks and sediments with a resolution down to 0.6 um. In-house built flow cells enable us to reproduce reservoir conditions of temperature and pressure and to circulate various fluids though the rock (single or multiphase conditions). Various rock parameters such as porosity, connectivity, saturations, etc., can then be obtained with the use of visualisation and computation software accessible through the Pawsey Supercomputing Centre. Within our discipline and through collaborative projects, we are developing methods to compute rock properties, such as elastic wave velocity or electrical conductivity, from the 3D X-ray images.
We also own a nano-indentation system (IBIS Fisher-Crips Laboratories Pty. Ltd.). This technique provides static and dynamic Young’s moduli at the micro-meter scale as well as various mechanical parameters such as hardness, fracture toughness or creep behaviour. We have used nano-indentation tests to map and quantify mechanical weakening due to exposure of the rock to CO2-rich fluids as well as to map heterogeneities in Young’s moduli of carbonates and relate it to microstructure heterogeneities acquired by SEM. Through a statistical analysis of the data, and with the use of rock physics models and micro-CT images, we are currently developing techniques to upscale these data at the core- and reservoir-scale.
Experimental and Theoretical Study of Partial Saturation Effect on Elastic Properties of Sedimentary Rocks
One of the most important problems in rock physics analysis of seismic and acoustic data is the characterisation of a fluid saturating a rock formation from elastic wave velocities.
Of special interest is the case when a small amount of fluid is absorbed in thin compliant pores. In particular, characterisation of fluid effects from seismic data often require knowledge of dry velocities, but these are usually measured in room conditions and hence may strongly depend on room humidity.
The adsorption of water between clay particles in shales and quartz grains in sandstones leads to elastic weakening of the rocks. We have developed the laboratory facilities to study the effect of varying hydration on the elastic properties of rocks. Hydration of rock samples is changed using sorption processes by maintaining the samples in an atmosphere with controlled air humidity. The stress-dependent elastic properties of the samples at different hydration states are measured using the ultrasonic testing technique. The measurements of the elastic properties are complemented with the characterisation of changes in microstructure and concurrent deformation using X-Ray micro-CT and strain gauge measurements.
A number of rock physics theories is applied to the laboratory data to understand the driving mechanisms of the elastic weakening phenomenon. The theoretical research includes the study of the dependencies of the elastic properties of rocks on 1) the composition of the rock and the pore-filling fluid, 2) the stress-dependent compliant porosity, and 3) the orientation of pores and particles. These dependencies are modelled with the effective media theories, the Sayers-Kachanov non-interactive approximation, the orientation-distribution function data, and other rock physics theories and methods.
The results have the potential to improve interpretation of the near-surface seismic data as well as seismic and well-log characterisation of shaly formations.