Effects of Saturating Fluids On Seismic Velocities In Shale

to slippage along grain contacts. The weakening effects are especially strong in shales containing smectite. We studied the effects of saturating fluids on seismic velocities in low porosity shales collected from several formations. Because of the very low permeability of the shales, we show that the Biot critical frequency than the ultrasonic frequency used in l (fc) is too much higher aboratory for the Biot fluid flow effect to be considered. For the same reason, the transition frequency between the low and high frequency responses related to the local flow effect is much lower than the borehole seismic frequency. Therefore, assuming that Biot flow and local flow are the only two dominant mechanisms, no correction is needed when applying laboratory measurement results to borehole sonic or seismic interpretation as far as frequency-related velocity dispersion is concerned. However, other effects of saturating fluids in shales, including the mechanical stiffening by fluid, the chemical weakening of clay minerals, and the physical-chemical weakening of grain contacts, were found to be very strong. The total changes in elastic constants caused by fluid saturation were decomposed into the above effects. We find that the chemical and physical-chemical weakening effects are most pronounced in shales containing even small amounts of smectite. EXPERIMENTAL PRINCIPLE Shale samples for this study represent the worldwide collection of more than 50 cores of shales that we have generated. The most significant feature of these shales is the organic richness and the very fine lamination of kerogen-rich laminae and clay-rich laminae along with preferred orientation of clay particles. The study by Vernik and Nur (1992) of the Bakken shale has experimentally verified the elastic symmetry as being transverse isotropic. Therefore, five velocity measurements were made on the three plugs drilled for each sample along the bedding, normal to bedding, and 45 degrees to bedding to fully characterize its elastic properties. The measurement technique was the same as that used in Lo (1985) and Vernik and Nur (1992). In order to distinguish the chemical from the physicalchemical weakening effect, we used two different saturating media, brine (5% NaCl) and oil (dimethylpoly-siloxane, viscosity 5 centi-stokes at Sigma Chemical Company). INTRODUCTION LABORATORY VERSUS FIELD MEASUREMENTS The effects of saturating fluids on seismic velocities in rocks have been predominantly studied in two aspects: (1) the static mechancial stiffening of the rock frame by fluids, as predicted by the Gassmann (1951) and Brown and Korringa (1975) relations, and (2) the dynamic velocity dispersion due to wave-induced fluid flow. Two mechanisms for velocity dispersion have been proposed. One of them, known as Biot flow (Biot, 1956), considers fluid-solid mass coupling due to inertia and viscous effects in fluids. A critical frequency, which is related to the permeability and porosity of rock and the viscosity and density of fluid, separates the high and low frequency velocity responses. The other mechanism, called Below we evaluate the effect of velocity dispersion by considering the critical frequency defined in Biot theory and the transition frequency operating in the local flow mechanism. The Biot critical frequency is given by: local flow, deals with the local stiffening of pore structure by unrelaxed fluids (Mavko and Jizba, 1991). Similarly, a transition frequency (Dvorkin and Nur, 1993), which is related to the frame modulus, porosity, permeability and the squirt-flow length of rock and the viscosity of fluid, is used to separate the high frequency, unrelaxed behavior from the low frequency, relaxed behavior. According to both mechanisms, velocity dispersion correction is needed when going from the high frequency range (such as that used in laboratory ultrasonic measurements) to the low frequency range (such as that used in borehole sonic/seismic studies). In this work, we studied the effect of saturating fluids on the elastic properties of shale in order to understand the relationships between (1) the laboratory and field measurements and (2) the dry and wet shale properties. The study showed that because of the very low permeability of shale, the laboratory ultrasonic measurements and borehole surveys are in the same frequency range as far as velocity dispersion is concerned. So there is no need for dispersion correction when using laboratory measurements for borehole sonic/seismic interpretations. When relating dry to wet shale properties, the mechanical effects are shown to be overshadowed by the chemical weakening due to water absorption into clay platelets and the physical-chemical weakening due where and are porosity and permeability of rock and and are viscosity and density of fluid, respectively. The permeability measured in selected high porosity shales is on the order of md. The majority of shales have much lower porosities (1 to 6%) and so are the permeabilities. On average, md . The fluid viscosity = Pa l s and density we choose and accordingly = Therefore, the Biot critical frequency 0.04 l l 3.14 l l 6 x Hz or 6 x MHz. Compared with the ultrasonic frequency (1 MHz) that we use in our velocity measurements, the Biot critical frequency is much too high for any high frequency effect to be considered. We notice that this very high critical frequency is a direct result of the very low permeability of shale. The transition frequency is more ambiguous because of the difficulty with estimating the squirt-flow length R which enters the equation: