Anthony F. Siggins

Constant Q attenuation of subsurface radar pulses

Q is a measure of the energy stored to the energy dissipated in a propagating wave and can be estimated from the ratio of attenuation and frequency. For seismic waves, Q has been found to be essentially independent of frequency. As a result, attenuation is an approximately linear function of frequency and the impulse response function of the earth. Hence, the distortion of a seismic pulse as it propagates can be described by a single parameter. Laboratory measurements show that the attenuation of radio waves in some geological materials can also be approximated by a linear function of frequency over the bandwidths of typical subsurface radar pulses. We define a new parameter Q* to describe the slope of this linear region. The impulse response of the transfer function for a given value of Q* differs from that of the same value of Q only in total amplitude. Thus the change of shape of a radar pulse as it travels through these materials can also be described by a single parameter. The constant Q* model succe...

Geomechanical and ultrasonic characterization of a Norwegian Sea shale

Anisotropy of velocity in shaly overburden is known to cause significant problems for geophysical interpretation, including depth conversion and fluid identification. In addition, mechanical and dynamic elastic shale behavior is not well understood because few tests have been performed on well-preserved samples. Multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the Norwegian Sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation. In addition, standard rock physical properties were characterized as well as composition. The shale microfabric is seen to be strongly laminated, with alternating thick clay-rich laminae and thin silt-rich laminae. Occasional microfractures are also noted parallel to these laminations. The shale has low friction coefficient and cohesive strength, and shows anisotropy of these parameters when the maximum principal stress is oriented parallel to and at 45 � to the microfabric. The orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased. S-wave anisotropy was significantly affected by the increasing stress anisotropy. Stress orientation with respect to fabric orientation was therefore found to be an important control on the degree of anisotropy of dynamic elastic properties in this shale.

Experimental verification of the physical nature of velocity-stress relationship for isotropic porous rocks

The exponential increase of seismic velocities with effective stress has usually been explained by the presence of pores with a broad distribution of aspect ratios. More recently, a stress-related closure of soft pores with a narrow distribution of compliances (e.g. grain contacts) has been suggested to be sufficient to explain such exponential stress dependency. This theoretical interpretation has been verified here using laboratory measurements on dry sandstones. On the basis of these experimental data, linear dependency of elastic compressibility on soft porosity and exponential decay of soft porosity and elastic compressibility with effective stress up to 60 MPa is confirmed. Soft porosity, estimated from the fitting coefficients of elastic compressibilities, is on the same order of magnitude but slightly lower than obtained from strain measurements. The results confirm applicability ofpreviously proposed stress sensitivity models and provide justification for using this approach to model stress dependency of elastic properties for isotropic and anisotropic rocks.

Influence of microheterogeneity on effective stress law for elastic properties of rocks

Understanding the effective stress coefficient for seismic velocity is important for geophysical applications such as overpressure prediction from seismic data as well as for hydrocarbon production and monitoring using time-lapse seismic measurements. This quantity is still not completely understood. Laboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficients less than one and usually vary between 0.5 and 1. At the same time, theoretical analysis shows that for an idealized monomineral rock, the effective stress coefficient for elastic moduli (and therefore also for seismic velocities) will always equal one. We explore whether this deviation of the effective stress coefficient from unity can be caused by the spatial microheterogeneity of the rock. The results show that only a small amount (less than 1%) of a very soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.

Circumferential propagation of elastic waves on boreholes and cylindrical cavities

Elastic waves propagating circumferentially on boreholes in rock are potentially useful in many nondestructive testing applications, including measurement of stress‐induced anisotropy and remote detection of cracks on borehole walls. Although analyses of the far‐field response of the surrounding rock to forces applied within boreholes have been described by several authors, little attention has been focused on the response of the borehole wall to such forces. Fourier‐Bessel and finite‐element methods of analysis are used to derive dynamic compliance transfer functions for a borehole in an elastic rock subject to a localized internal force. The analyses are in overall agreement, and both predict that the borehole behaves as a lossy resonant system when excited by such steady‐state forces. The lossy resonances can be attributed to standing surface waves. For a borehole within a rock with Poisson’s ratio of 0.10, the first three dimensionless wavenumbers k1a at which resonant amplifications occur are 0.288, ...

Predicting and Monitoring Geomechanical Effects of CO 2 Injection

Predicting and monitoring the geomechanical effects of underground CO 2 injection on stresses and seal integrity of the storage formation are crucial aspects of geological CO 2 storage. An increase in formation fluid pressure in a storage formation due to CO 2 injection decreases the effective stress in the rock. Low effective stresses can lead to fault reactivation or rock failure which could possibly be associated with seal breaching and unwanted CO 2 migration. To avoid seal breaching, the geomechanical stability of faults, reservoir rock, and top seal in potential CO 2 storage sites needs to be assessed. This requires the determination of in situ stresses, fault geometries, and frictional strengths of reservoir and seal rock. Fault stability and maximum sustainable pore fluid pressures can be estimated using methods, such as failure plots, the Fault Analysis Seal Technology (FAST) technique, or TrapTester software. In pressure-depleted reservoirs, in situ stresses and seal integrity need to be determined after depletion to estimate maximum sustainable pore fluid pressures. The detection of micro-seismic events arising from injection-induced shear failure of faults, fractures and intact rock is possible with geophone and accelerometer installations and can be used for real-time adjustment of injection pressures. In the event of injected CO 2 opening and infiltrating extensive fracture networks, this can possibly be detected using multi-component seismic methods and shear-wave splitting analysis.

Dynamic Elastic Tests for Rock Engineering

Publisher Summary Dynamic testing methods conducted on rock samples—generally cylindrical core samples—are well accepted in rock engineering as a rapid means of obtaining estimates of the rock elastic constants. The tests have the added virtue of being nondestructive methods. The method relies on accurate measurement of the elastic wave velocities in the rock specimens. The wave velocities associated with the compressional and shear wave propagation modes are dependent on the ability of the rock to resist compression and shear. Thus, the velocities of the two modes of wave propagation are related to the respective elastic moduli, in particular the low strain moduli, since for most rock materials the deformation moduli are strain dependent and only small strains are generated with the piezoelectric elements commonly used in these tests. Dynamic moduli are determined from the measured wave velocities via well established formulae.

Predicting, monitoring and controlling geomechanical effects of CO2 injection

Publisher Summary This chapter discusses the effects of pore-fluid pressure change on effective stresses in porous reservoir rock. While there is substantial information on pore pressure/stress coupling during pressure depletion in hydrocarbon fields, little is known about such effects during fluid injection. Thus, in cases where CO2 storage in severely depleted reservoirs is envisaged, geomechanical methods that predict depletion-induced faulting, such as by Streit and Hillis, can be useful for estimating whether fault-seal damage was induced. In cases where Biots α can be determined for reservoir rocks in CO2-storage sites, velocity-effective stress relationships should be established to also measure effective-stress changes with seismic techniques in addition to direct fluid-pressure measurement in monitoring wells. To test for injection-related changes in total horizontal stresses in CO2 storage or demonstration projects, extended leak-off tests and hydraulic-fracturing tests can be conducted. In storage sites with an identifiable risk of injection-related fault reactivation to produce events of noticeable magnitude, geophones, and transducers should be installed for the monitoring of induced MS events. The monitoring can be used to detect accidental over-pressurization of the formation as it is likely to allow for real-time adjustment of injection pressures.

The Ultrasonic Response of North Sea Shale to Undrained Loading

A kaolinite-rich shale core from the North Sea was characterized in terms of mineralogical composition and physical properties. Subsampled plugs were then subjected to undrained multi-stage triaxial tests close to failure in order to determine the shear failure envelope. During undrained loading, ultrasonic waveforms were recorded parallel to, perpendicular to and at off-axis angles to bedding, such that the evolution of the full elastic tensor could be monitored with increasing stress anisotropy. Results indicate the importance of fabric elements and their orientation with respect to the prevailing stress field.

The development of synthetic CIPS sandstones for geophysical research

CIPS (Calcite In-situ Precipitation System) is a new method of cementing particulate soils through the injection of aqueous solutions and the precipitation of calcite cement around soil grains and their contacts. The technology was developed at the CSIRO to improve the geotechnical properties of porous sediments and rocks. The process involves the application of a proprietary solution causing the crystallisation of calcite cement in a manner that mimics the cementation process in nature where sediments are transformed into calcarenites and limestone (Figure 1). Laboratory tests have shown that CIPS cemented sands closely reproduce the acoustic and mechanical properties of natural sandstones (Kucharski et al., 1996).