A laboratory study of the dependence of elastic wave velocities on pore scale fluid distribution

Abstract

Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone. Compressional wave velocity (Vp) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with the level of water saturation (Sw) during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically. When Sw was decreased, pronounced saturation‐induced hysteresis was observed in the region 03 < Sw < 0.9, with Vp measured during drainage greater than Vp measured during imbibition. Similar results were obtained for Vs versus Sw, with Vs during drainage greater that Vs during imbibition in the saturation range Sw > 0.4. As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i.e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air‐filled or water‐filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, “crack‐like” pores, these variations in fluid distribution cause related variations in velocities.

Estimating rock porosity and fluid saturation using only seismic velocities

“…(Murphy, 1982;1984;Knight and Nolen-Hoeksema, 1990) of Figure 1 and for three limestones (Cadoret, 1993;Cadoret et al, 1995;Cadoret et al, 1998). Figure 3: Examples of the correlation of slopes with porosity: (a) three sandstones (Murphy, 1982;1984;Knight and Nolen-Hoeksema, 1990), (b) five limestones (Cadoret, 1993;Cadoret et al, 1995;Cadoret et al, 1998), (c) 11 fused glass-bead samples (Berge et al, 1995), (d) Westerly granite (Nur and Simmons, 1969) at four pressures. The observed trend is that high porosity samples generally have lower slopes than lower porosities on these plots, although there are a few exceptions as discussed in the text.…”

“…In that case, for the plot of vs. , instead of data following a horizontal line with a jump up at the high saturation end (e.g., Figure 1b), the ideal patchy saturation model (for completely segregated liquid and gas pockets) would predict that the data should lie on another straight line connecting to the two end points (dry and saturated) on this plot. These straight lines have been superimposed on the plots [obtained using data from Murphy (1982;1984) and from Knight and Nolen-Hoeksema (1990) Plots of velocity versus saturation and of versus for two sandstones that apparently do not behave according to Gassmann's model are shown in Figures 1c-1f. These apparent deviations from the range of expected behaviors (from purely homogeneous mixed fluids to purely segregated patchy saturation) are resolved by including another display for these three sandstones (Murphy, 1982;1984;Knight and Nolen-Hoeksema, 1990) in Figures 2a,c,e and corresponding plots for three limestones (Cadoret, 1993;Cadoret et al, 1995;Cadoret et al, 1998) in Figures 2b,d,f.…”

“…These straight lines have been superimposed on the plots [obtained using data from Murphy (1982;1984) and from Knight and Nolen-Hoeksema (1990) Plots of velocity versus saturation and of versus for two sandstones that apparently do not behave according to Gassmann's model are shown in Figures 1c-1f. These apparent deviations from the range of expected behaviors (from purely homogeneous mixed fluids to purely segregated patchy saturation) are resolved by including another display for these three sandstones (Murphy, 1982;1984;Knight and Nolen-Hoeksema, 1990) in Figures 2a,c,e and corresponding plots for three limestones (Cadoret, 1993;Cadoret et al, 1995;Cadoret et al, 1998) in Figures 2b,d,f. Now the ratio is plotted versus saturation measured in the laboratory, and we observe in all these cases that the basic plot structures we had anticipated for Figures 1b, 1d, and 1f are in fact confirmed.…”

In situ seismic measurements in borehole LB‐08A in the Bosumtwi impact structure, Ghana: Preliminary interpretation

“…This last statement unfortunately oversimplifies the relationship between porosity and velocity, and many other elements, particularly the type of porosity encountered, can have a large effect (e.g., Knight and Nolen-Hoeksema 1990). In a first simplification, one may consider the pore space of a rock to consist of either equant "spherical" pores or low aspect-ratio, crack-like planar pores.…”

Effects of pore fluids in the subsurface on ultrasonic wave propagation

“…Seismic methods have been applied in the petroleum industry to detect oil-bearing formations (Clark, 1992). A variety of laboratory experiments have been performed on sedimentary rocks to study the effect of pore fluid distribution on attenuation and velocity for different fluids under high pressure (Endres and Knight, 1989;Knight and Nolen-Hoeksema, 1990). However, the same mechanisms are not directly applicable to NAPL in groundwater aquifers, since the in-situ stresses at depths where oil and contaminating NAPL can be found are orders of magnitude 9 lower.…”