Modelling ultrasonic laboratory measurements of the saturation dependence of elastic modulus: New insights and implications for wave propagation mechanisms. (April 2017)

• Record Type: Journal Article
• Authors: Amalokwu, Kelvin; Papageorgiou, Giorgos; Chapman, Mark; Best, Angus I.
• Is Part Of: International journal of greenhouse gas control. Volume 59(2017)
• Page Start: 148
• Page End: 159
• Publication Date: 2017-04
• Subjects: Acoustic properties -- Seismic attenuation -- Elasticity and anelasticity -- Partial saturation -- Wave propagation; Greenhouse gases -- Environmental aspects -- Periodicals; Air -- Purification -- Technological innovations -- Periodicals; Gaz à effet de serre -- Périodiques; Gaz à effet de serre -- Réduction -- Périodiques; Air -- Purification -- Technological innovations; Greenhouse gases -- Environmental aspects; Periodicals; 363.73874605
• DOI: 10.1016/j.ijggc.2017.02.009
• Languages: English
• ISSNs: 1750-5836

Abstract

Highlights: Different mechanisms are combined to model laboratory measurements of saturation dependence of elastic properties of rocks. The results show the interpretation is not unique as different combinations of mechanisms give a good fit to the data. Differences in the low frequency limits of the models could have implications for seismic fluid substitution modelling. Abstract: Seismic time-lapse techniques are a valuable tool used to estimate the mobilization and distribution of stored CO2 in depleted reservoirs. The success of these techniques depends on knowing the seismic properties of partially saturated rocks with accuracy. It is commonplace to use controlled laboratory-scale experiments to determine how the fluid content impacts on their properties. In this work, we measure the ultrasonic P- and S-wave velocities of a set of synthetic sandstones of about 30% porosity. Using an accurate method, we span the entire saturation range of an air-water system. We show that the rocks' elastic behaviour is consistent with patchy saturation and squirt flow models but observe a discontinuity at around 90% gas saturation which can be interpreted in two very different ways. In one interpretation, the responsible mechanism is frequency-dependent squirt-flow that occurs in narrow pores that are preferentially saturated. An equally plausible mechanism is the change of the mobile fluid in the pores once they are wetted. Extrapolated to seismic frequencies, our results imply that the seismic properties of rocks may be affected by the wetting effect with an impact on the interpretation of field data but would potentially be unaffected by the squirt flow effect. This provides strong motivation to conduct laboratory-scale experiments with partially saturated samples at lower frequency or, ideally, a range of frequencies in the seismo-acoustic range.