Fluid-mediated seismic properties of some reservoir rocks and analogues

Abstract

Seismological observations, at Hz-kHz frequencies, inform our understanding of hydro-mechanical processes in the shallow crust. These coupled processes are critical not only to explain fluid-dependent crustal changes but also to address several socio-economic issues such as earthquake hazard monitoring, carbon dioxide storage and the enhanced recovery of hydrocarbon resources. For example, the business value in time-lapse seismic monitoring as used in the oil and gas industry relies on the fact that changes in reservoir conditions alter reservoir seismic properties. Therefore, practical calibration and interpretation of time-lapse seismic signatures of reservoir fluid depletion and injection require the laboratory testing of rock physics models of fluid and stress sensitivity in reservoir rocks. However, conventional laboratory measurements of mechanical properties of crustal rocks at ultrasonic (MHz) frequencies probe a different fluid-flow regime - illustrating the frequency-dependent nature of the seismic properties of fluid-saturated porous media. Such frequency dependence makes it difficult to reconcile laboratory ultrasonic measurements with low-frequency field data from the borehole (tens of kHz) and seismic imaging and monitoring (10-100 Hz) surveys. Whereas carefully-executed laboratory broadband measurements appear promising for such reconciliation, the complex nature of rock microstructures constitutes a significant difficulty in the interpretation of such measurements. Such complications limit the rigorous and conclusive testing of theories relating the frequency-dependent seismic properties of the fluid-saturated media to microstructures and fluid flow regimes. Using complementary forced-oscillation (0.001-100 Hz) and ultrasonic techniques (1 MHz) accessible in rock physics laboratories at the Australian National University, Curtin University, and the Ecole Normale Superieure, I investigate the broadband mechanical properties of selected reservoir sandstones and thermally cracked glass specimens of simple microstructure. The specimens were tested both in dry and fluid-saturated conditions, targeted at interrogating the different fluid flow regimes and how microstructure influences the frequency-dependent seismic properties of fluid-saturated porous media. Results from this inter-laboratory broadband testing of reservoir sandstones and thermally cracked synthetic specimens reveal frequency-dependent bulk and shear moduli, along with the corresponding strain-energy dissipation. Bulk, Young's, and shear modulus dispersion and dissipation measured on water-saturated thermally cracked glass specimens, using a combination of forced-oscillation and ultrasonic techniques, show evidence of both draining and squirt transitions at the lowest differential pressures (5 MPa). Furthermore, results on natural reservoir specimens indicate properties of different fluid regimes with different pore fluids. On one hand, an increase of bulk and young's moduli and Poisson's ratio on decane saturation of a reservoir sandstone specimen supplied by Woodside Energy reveals undrained (saturated isobaric) conditions, consistent with Gassmann's theory. On the other hand, a specimen of Wilkeson sandstone shows strong dispersion of both shear and Young's moduli and associated dissipation when tested glycerine-saturated. This behaviour is in marked contrast to the lower and more mildly dispersive moduli and dissipation when the same specimen is tested dry or argon-or water-saturated. These observations reveal the impact of squirt flow responsible for the transition from saturated-isobaric to saturated-isolated conditions with the superimposed influence of lateral fluid flow during flexural oscillation tests. The findings in this study validate predictions from theoretical models of dispersion in porous media and emphasise the need for caution in the seismological application of laboratory ultrasonic data for shallow crustal rocks. Show more