Effect of pressure and fluid on pore geometry and anelasticity of dolomites
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ORIGINAL CONTRIBUTION
Effect of pressure and fluid on pore geometry and anelasticity of dolomites Wei Cheng 1 & José M. Carcione 1,2 & Stefano Picotti 2 & Jing Ba 1 Received: 17 February 2020 / Revised: 23 July 2020 / Accepted: 3 August 2020 / Published online: 11 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The acoustic properties of rocks depend on porosity, pressure, and pore fluid and also on pore geometry. Anelasticity (attenuation and velocity dispersion) is more affected by crack aspect ratio and fraction (soft pores) than by equant (stiff) pores. To study this fact, we have performed ultrasonic measurements on two dolomite samples under variable pressure and fluid content, and used the EIAS (equivalent inclusion-average stress) model to obtain the crack aspect ratio and fraction from the bulk and shear moduli of the rock. The theory has an excellent agreement with the experimental data, and the results show that the crack attributes decrease with increasing differential pressure and are higher for a stiffer fluid. In fact, the interpretation of the experiments with the model shows that crack fraction and aspect ratio increase with the bulk modulus of the fluid (water and oil). Then, by extending the theory to all frequencies, using the Zener mechanical model, we obtain the phase velocities and quality factors as a function of frequency. Our findings reveal the importance of considering differential pressure and fluid type to analyze pore geometry and rock anelasticity. Keywords Crack aspect ratio . Crack fraction . Anelasticity . Differential pressure . Fluid type . EIAS model . Zener model . Dolomites
Introduction The anelastic properties of rocks—wave velocity dispersion and attenuation—have gained much attention in recent years. The applications cover a variety of fields, including geophysical prospecting, soil mechanics, and underwater acoustics. In particular, in the exploration of hydrocarbon reservoirs, it is important to predict porosity, permeability and the presence of fluids (type and saturation) (Carcione 2014). Moreover, anelasticity is closely related to the microstructural properties, fluid content, and in situ conditions, basically, pore geometry and density, and pore pressure (Jones 1986; Müller et al. 2010; Cheng et al. 2020). Gassmann (1951) equation can be used to predict the wave velocities (Murphy 1984). However, it is only valid at the low frequency limit, due to the assumption of complete fluid pressure equilibration between cracks and stiff pores (Cleary * Jing Ba [email protected] 1
School of Earth Sciences and Engineering, Hohai University, Nanjing 211100, China
2
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42c, 34010 Sgonico, Trieste, Italy
1978; Mavko and Nolen-Hoeksema 1994; King and Marsden 2002; Ba et al. 2016, 2017). Moreover, this equation incorporates pore geometry information through the use of empirically determined stiffnesses and therefore cannot be used to predict the dependence of t
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