Effect of Pore Pressure on Compressional Wave Velocity in Coals

G. Yu; K. Vozoff; D. W. Durney

doi:10.1071/EG991475

ISSN: 0812-3985
E-ISSN: 1834-7533

Effect of Pore Pressure on Compressional Wave Velocity in Coals
Authors G. Yu^a, K. Vozoff^a and D. W. Durney^b
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^a Centre for Geophysical Exploration ResearchMacquarie University, North Ryde, NSW, . 2109 ^b School of Earth SciencesMacquarie University, North Ryde, NSW, 2109
Publisher: Australian Society of Exploration Geophysicists
Source: Exploration Geophysics, Volume 22, Issue 2, Jun 1991, p. 475 - 480
DOI: https://doi.org/10.1071/EG991475
- Published online: 01 Jun 1991

Abstract

A system has been developed for determining precisely the compressional wave velocities in coal specimens subjected to independent hydrostatic confining pressure and internal pore pressure. The velocity Vp of compressional waves has been measured, by the pulse transmission technique, on ten water saturated coal specimens in directions perpendicular and parallel to the bedding plane as functions of confining pressure (Pc) and pore pressure (Pp) to 25 MPa.

Velocity changes due to changes in confining pressure were not exactly cancelled by equivalent changes in pore pressure. An effective pressure defined by Pe = Pc- nPp, n < 1, rather than an effective pressure defined simply by the differential pressure Pa =Pc–PP, is found to be the determining factor in the behaviour of Vp. As pore pressure and confining pressure, are increased at constant effective pressure, the value of n increases and approaches 1. But as effective pressure increases at constant pore pressure, the value of n decreases. Our experimental results are consistent with Biot’s theory of the elastic wave propagation in a fluid-saturated porous solid.

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References

Biot, M. A., (1956a). ‘Theory of propagation of elastic waves in a fluid-saturated solid, 1, Low frequency range’. J. Acoust. Soc. Am., 28, 168-178.
Biot, M. A., (1956b). ‘Theory of propagation of elastic waves in a fluid-saturated solid, 2, Higher frequency range’. J. Acoust. Soc. Am., 28, 179-191.
Christensen, N. I., and Wang, H. R, (1985). ‘The influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone’. Geophysics, 50, 207-213.
Gassmann, R, (1951). ‘Elastic waves through a packing of spheres’. Geophysics, 16, 673-685.
Gregory, A. R., (1976). ‘Fluid saturation effects on dynamic elastic properties of sedimentary rocks’. Geophysics, 41, 895-921.
King, M. S., (1966). ‘Wave velocities in rocks as a function of overburden pressure and pore fluid saturants’. Geophysics, 31, 56-73.
Nur, A., and Simmons, G., (1969). ‘The effects of saturation on velocity of compressional waves in low-porosity rocks’. Earth Planetary Sciences Letters, 7, 183-193.
Todd, T, and Simmons, G., (1972). ‘Effects of pore pressure on the velocity of compressional waves in low-porosity rocks’. J. Geophys. Res., 77, 3731-3743.
Yu, G., Vozoff, K., and Durney, D. W., (1990). ‘The effects of confining pressure and water saturation on ultrasonic compressional wave velocities in coals’. Subm. to Int. J. Rock Mech. Min. Sci. & Geomech. Abstr.

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Article Type: Research Article

Keyword(s): coal; laboratory measurement; P-wave velocity; pore pressure; saturation

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Effect of Pore Pressure on Compressional Wave Velocity in Coals

Abstract

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