Effects of Fluid Saturation on Waves in Porous Rock and Relations to Hydraulic Permeability

Abstract

Detailed and very accurate measurements of compressional Vp and shear Vs wave velocities and their attenuation in dry, partly saturated and fully saturated clay-bearing sandstone were carried out at 20, 145, and 198 degrees C, under variable confining and pore pressures.The velocity results show that water saturated rock has high Vp and relatively low Vs not only at room temperature but also at 145 and 198 degrees C. Varying the pore pressure reveals that the transition from the fully saturated to the dry state involves a gradual transition of Vs and a very irregular transition of Vp - with a minimum in Vp relative to either the dry or the fully saturated cases.The attenuation of S and P waves - Qs(-1) and Qp(-1), respectively - shows similar patterns: in dry rock, both Qs(-1) and Qp(-1) are low. In fully saturated rock Qp(-1) is low and Qs(-1) is high. In partly saturated rock, Qs(-1) changes gradually with saturation, but Qp(-1) shows a very strong maximumsuggesting strong energy dissipation. The same behavior is observed at 145 degrees C, with the steam/hot-water transition. It was also found that attenuation of shear waves increases with permeability in some sandstones, consistent with squirting attenuation mechanism.The results suggest diagnostic characteristics for determining the degree of saturation in situ from seismic logs, with particular applicability to evaluation of steam content in geothermal reservoirs and of gas content in oil reservoirs. Introduction In recent years seismic attenuation has become of increasing interest to seismologists, including the study of bright spots in hydrocarbon exploration, reservoir evaluation, and well logging.Seismic methods are particularly important in exploration for and evaluation of geothermal reservoirs, including physical property determinationssuch as Poisson's ratio and seismic wave attenuationfor delineating the boundaries and state of reservoirs. The latter measurements are particularly interesting for the possible distinction between steam-bearing and hot-water domains. To make full use of seismic data, it is necessary to interpret attenuation in terms of the physical properties of rocks, and the mechanisms responsible for loss of energy in seismic waves. In this paper we show some of the important effects of fluids on wave velocities and attenuation in porous and cracked rocks. We further show that a fundamental physical link may exist between seismic wave attenuation in rocks and the hydraulic permeability in these rocks.The effects that pore fluids have on seismic velocities are well-documented. It is natural to suppose that pore fluids will also influence seismic attenuation, but very little experimental work has been done in this area. There is, however, no shortage of theoretical models of fluid loss mechanism. Flow models based on partial saturation of individual cracks have been developed by White and by Mavko and Nur, and a thermoelastic partial saturation mechanism has been presented by Kjartansson and Nur. The models show invariably that attenuation is related to the local hydraulic diffusivity. Thus, if the local and global hydraulic properties can be related, we may eventually be able to infer permeability from seismic measurements. SPEJ P. 450＾

The effective pressure law for permeability of clay-rich sandstones

“…In addition, the results obtained in this paper confirm that n k is constant and higher than 1.0 in sandstones with a substantial amount of clays (Zoback and Byerlee, 1975;Nur et al, 1980;Berryman, 1992;Al-Wardy and Zimmerman, 2004). From X-ray analysis data, the clay content in the sandstone samples was more than 10%.…”

“…Three groups of experimental results can be summarized: 1) n k equals 1.0 (Brace et al, 1968;Coyner, 1984;Bernabé, 1987). 2) n k ranges from 0 to 1.0 (Nur et al, 1980;Bernabé, 1986;Warpinski and Teufel, 1992;Z heng et al, 2008;Li and Xiao, 2008;Li et al, 2009a;2009b). Nur et al (1980) studied two artificial rocks and observed that n k were constant (n k values of 0.43 and 0.86) while the others found that n k varied with confi ning pressure and pore pressure.…”

“…2) n k ranges from 0 to 1.0 (Nur et al, 1980;Bernabé, 1986;Warpinski and Teufel, 1992;Z heng et al, 2008;Li and Xiao, 2008;Li et al, 2009a;2009b). Nur et al (1980) studied two artificial rocks and observed that n k were constant (n k values of 0.43 and 0.86) while the others found that n k varied with confi ning pressure and pore pressure. 3) n k values are greater than 1.0 (Zoback and Byerlee, 1975;Nur et al , 1980;Al-Wardy and Zimmerman, 2004;Ghabezloo et al, 2009), indicating that por e pressure has more effect on permeability than confi ning pressure.…”

“…Nur et al (1980) studied two artificial rocks and observed that n k were constant (n k values of 0.43 and 0.86) while the others found that n k varied with confi ning pressure and pore pressure. 3) n k values are greater than 1.0 (Zoback and Byerlee, 1975;Nur et al , 1980;Al-Wardy and Zimmerman, 2004;Ghabezloo et al, 2009), indicating that por e pressure has more effect on permeability than confi ning pressure. Ghabezloo et al (2009) found that n k was variable while the others observed constant n k .…”

Effective stress law for the permeability of a limestone

“…Experimental studies on both sedimentary and crystalline rocks can be found in the literature. Most of the studies on sedimentary rocks have been conducted on clay-rich sandstones [7][8][9][10][11][12][13], and resulted in effective stress coefficients which are greater than one and which increase with the clay content. The evaluated effective stress coefficient by Walls [11] varies from 1.2 for clean sandstone to 7.1 for sandstone containing 20% clay.…”