Effective porosity for Gassmann fluid substitution

摘要

In this study, we have analyzed the complexity of pore structure and its interaction with pore fluid. The specific surface area is an important parameter related to water retention in reservoir rocks. The surface tension between mineral surface and pore fluids might be so strong that some portion of pore fluids does not respond as fluid under small pressure disturbance. Integrated study of ultrasonic lab measurement data, petrographic data, mercury injection capillary pressure (MICP) data and NMR T_2 data shows the rationality of using an effective porosity as input for Gassmann equation. The effective porosity for Gassmann equation should be frequency-dependent. Knowing the pore geometry, if an empirical correlation between frequency and threshold pore throat size or NMR T_2 is set up, Gassmann equation can be applicable to data of any frequency measurement. Without information of pore geometry, the irreducible water saturation can be used to estimate effective porosity; the "modified" Gassmann equation should give more reliable prediction of saturation effect.