Properties of fluids under multiphysical coupling and frequency-dependent rock physics modeling

Abstract

In fluid underground storage and seismic monitoring, accurately characterizing the physical properties of fluid-bearing rocks under multiphysical coupling is crucial. However, in deep geological environments, fluids often exist in a supercritical state with highly nonlinear acoustic properties. Moreover, most existing models lack a unified framework for multiple gas types, often treating fluid properties as constants, which does not adequately reflect their actual state. This study proposes a method for constructing a frequency-dependent rock physics model for fluid-bearing rocks. Three equations of state were used to calculate the density, bulk modulus, and acoustic velocity of carbon dioxide, methane, and hydrogen, which were then compared with reference data from the National Institute of Standards and Technology. The properties of brine under varying temperature and pressure conditions were derived from the Batzle–Wang model, and the properties of mixed fluids were obtained using Wood’s equation, before being integrated into a sandstone matrix. To account for inelastic behavior, the Johnson and White models were applied to analyze the effects of fluid distribution on compressional wave velocity and the attenuation factor under different conditions. The results show that the acoustic properties of fluids are significantly influenced by phase changes. This frequency-dependent rock physics model improves upon most conventional models that treat fluid properties as fixed constants, enabling more accurate calculation of acoustic properties in deep environments. It thus provides a theoretical basis for underground fluid storage and monitoring in practical applications.

Keywords

Multiphysical coupling / Supercritical fluids / Equations of state / Geological storage of carbon dioxide/methane/hydrogen / Johnson model / White model

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