The effect of confining pressure on elastic wave velocities and dynamic to static Young’s modulus ratio

Abstract

We carried out laboratory experiments under dry conditions on limestone core specimens of Sarvak formation obtained from an oil well in the southwest of Iran. Our objective was to study the effect of confining pressure on the compressional and shear wave velocities ([Formula: see text], [Formula: see text]), and on the dynamic to static Young’s modulus ratio ([Formula: see text]). Furthermore, we made attempts to predict [Formula: see text] and [Formula: see text] at atmospheric pressure based on the same velocities at various confining pressures. These analyses revealed that, below a critical pressure with an increase in confinement [Formula: see text] and [Formula: see text] increased exponentially, representing a poroelastic regime. Above a critical pressure, however, the trend was linear. Likewise, we observed that with an increase in confinement, [Formula: see text] initially decreased exponentially, followed by a linear decreasing trend above the critical pressure. This indicated that [Formula: see text] is more responsive than [Formula: see text]. Accordingly, these observations infer that it is possible to predict [Formula: see text] based on [Formula: see text] at different confining stresses. This is an important improvement for geomechanical modeling of hydrocarbon and geothermal reservoirs because static parameters are more realistic input parameters. Besides, we derived the coefficients of the velocity-pressure equation for Sarvak limestone using least square regression analysis. More interestingly, we predicted [Formula: see text] and [Formula: see text] at atmospheric pressure based on these coefficients. Good agreement was observed between measured and predicted velocities at atmospheric pressure. Analysis of similar published experiments on oceanic basalts strongly confirmed these observations.

Characterisation and multifaceted anisotropy assessment of Corvio sandstone for geological CO2 storage studies

“…We observe that, when increasing confining pressure, both velocities increase from 2.92 to 3.69 km s −1 and 1.51 to 2.09 km s −1 , respectively. Below 10 MPa, the loading effect follows an exponential curve consistent with the previously described crack closure stress stage, i.e., an effect that has been reported or inferred from experimental data in a number of works (Asef and Najibi 2013;King 1983;Mikhaltsevitch et al 2014;Xu et al 2006). The unloading path is slightly different from the loading one, with higher average velocities.…”

“…a and b). Furthermore, comparing dynamic to static moduli, we observe that E dyn is higher than E st , whereas ν dyn is lower than ν st , which has been significantly identified in the past (Asef and Najibi ; Eissa and Kazi ; King ; Fjær ). The reasons for such differences include the properties of the pore fluid, frequency of cracks, the stress path, and the inherent difference of strain amplitudes between static and dynamic (at ultrasonic frequencies) methods (Blake and Faulkner ; Fjær, Stroisz and Holt ).…”

“…This effect becomes less pronounced above 10 MPa loading or at higher confining pressures. This means that closing of compliant cracks and pore compaction must be kept in mind when considering the ultrasonic characterisation of plugs in the laboratory (Asef and Najibi ; King ).…”

“…This effect becomes less pronounced above 10 MPa loading or at higher confining pressures. This means that closing of compliant cracks and pore compaction must be kept in mind when considering the ultrasonic characterisation of plugs in the laboratory (Asef and Najibi 2013;King 1983). Based on the previous elastic wave velocities and the dry density of the samples, we calculated the dynamic moduli of the rock: Young's modulus and Poisson's ratio (E dyn and ν dyn ; Fig.…”

Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge

“…Deformation may lower the acoustic wave velocities of our four samples due to the presence of cracks/fractures in the matrix as observed in the thin sections, even though the cracks are expected to close during the early stages of loading (Asef and Najibi, 2013). For example, Brereton (1992) reported very little variation of the elastic wave velocities of oceanic basaltic rocks above 25 MPa confining pressure.…”