Modern seismic data acquisition and processing methods now enable scientists to extract information on both the stratigraphy and the physical properties of subsurface rocks. Laboratory acoustic measurementsa llow the physical conditions to be precisely measured and controlled. In the present study, P- and S-wave velocities (Vs, VS) and attenuations (1000/Qp, 1000/Qs) were measured in a range of sandstones using the ultrasonic pulse-echo technique, at effective pressures of 5 MPa to 60 MPa. The measurement accuracy is ±0.3 % for velocity and ±0.1 dB/cm for attenuation using this method. Velocities and quality factors( Q) fall with decreasinge ffectivep ressure,a nd the relationships are described by the empirical equationsV =A+KP-B C71' and Q=A-B e7DP , where P is the effective pressure and A, K, B, and D are the regression coefficients (D=0.115±0.016 and 0.048±0.010 for V and Q, respectively). Velocity and Q can therefore be extrapolated to pressures beyond the experimental range. The Biot, Gassmann, and unrelaxed pore-fluid models of seismic wave propagation in porous media fail to explain the pressure-dependenceo f the velocities. The difference between the experimental and Biot model predictions of the rate of change in P-wave velocity with pore fluid salinity (dVýdM) increases with percentage clay content (C) of the rock at the approximately linear rate of 0.95 m/s/mol. There is no clear relationship for dVs/dM. In clean sandstones there is a close agreement between the experimental results and Biot model predictions for dVP/dM, but the agreement breaks down when C>5%. This suggests that changes in the pore-fluid salinity alter the frame bulk and shear moduli of sandstones. Attenuation is generally independent of pore-fluid salinity. Attenuation and velocity are often strongly dependent on the degree of pore-fluid saturation. A study of nine samples shows that 1000/Qp exhibits a resonance peak at midrange saturations (SW av 30 % to SW = 70 %) in most samples, and 1000/Qs shows similar behaviour in several of these. For porosities greater than 13%, the normalised amplitudes of the peaks in P-wave and bulk attenuation are correlated to porosity; the latter increases at a linear rate of 0.98 per percentage increase in porosity. These data suggest that attenuation reaches a maximum when the gas/water mixture is neither too compressible nor too incompressible. The Biot/squirt (BISQ) theory inadequately models the saturation dependence of 1000/Qp and Vp in a sample at low confining pressure. Vp falls with decreasing saturation between SW =100 % to SW - 50 %; below SW = 50 %, the behaviour of Vp is dependent on the confining pressure. Vs generally increases with decreasing saturation over the entire saturation range in all samples. The unrelaxed pore-fluid model of Mavko and Nolen-Hoeksema (1994) describes the Vp data reasonably well in most samples using low wetting fractions (< 15 %), which indicates that the pore fluid is unrelaxed at both the grain and sample scales. The wetting pore fluid becomes unrelaxed at high frequencies and/or low permeabilities. The V. data are poorly described by the model, possibly due to matrix softening by the wetting fluid. The experimental data have indicated significant shortcomings in the mathematical models of seismic wave propagation in reservoir rocks. The data highlight important aspects of wave propagation that must be addressed in revised theories.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:321879 |
| Date | January 1996 |
| Creators | Jones, Simon Mark |
| Publisher | University of Reading |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0026 seconds