Macro‐flow and velocity dispersion

Abstract

Deformation resulting from a compressional wave can cause pore fluid motion on the order of the wavelength. If the fluid mobility is high, pressure can be equilibrated between regions of gas versus brine saturation. This can result in a relaxed, drained velocity even lower than dry or gas saturated velocities. This diffusion of fluid pressure can cause a gas-water contact that looks sharp at high (logging) frequencies to be gradational at seismic frequencies. This fluid motion can also result in high attenuation. One of the most straight-forward descriptions was developed by Cole and Cole (1941) and applied to attenuation measurements by Spencer (1981). The result is coupled attenuation and velocity as functions of frequency as shown in Figure 2. Introduction We have seen in several of our low frequency experiments, how fluid flow can influence the seismic velocities (Figure 1). Pore pressure changes slightly as a seismic wave passes, causing the rock to be slightly stiffer and increasing velocity. This pressure change depends on the compressibility of the fluid and the compliance of the pore space. This process is described by Gassmann's (1951) Equations. However, these equations were derived under the assumption that that pore pressure has equilibrated throughout the rock. This assumption then depends on fluid mobility. At low mobility, pressure can not equilibrate and Gassmann's assumptions are violated. With increasing mobility, 'squirt' flow mechanisms can operate over a small distance, and adjacent pores can reach equilibrium. With high mobility, a more global or 'Macro' flow can occur on a scale approaching to the seismic wavelength. 0 1 2 3 4