Relationships Between Rock Acoustic Dispersion and Petrophysical Properties for Enhanced Formation Evaluation

Abstract

Abstract Understanding the dissipation of acoustic energy in a fluid-saturated rock formation can divulge some important clues about their petrophysical properties. Rock properties such as bulk density, porosity, permeability, fluid type and viscosity affect the acoustic propagation velocity. The objective of the present work is to study the effects of the afore mentioned petrophysical properties on the acoustic dispersion phenomenon at a variety of practical application frequency scales. Experimentation was performed on sandstone rocks to measure petrophysical properties such as bulk density, porosity, permeability and acoustic velocities of compressional waves sweeping frequency, ranging from downhole logging (~20 KHz) to laboratory ultrasonic measurements (1 MHz). Sandstone samples, with different range of porosity and permeability were selected in order to study their effect on acoustic dispersion. Dispersion analysis for each rock type was performed and related to the petrophysical properties. Acoustic velocity measurement method at variable frequencies is described in this study. Laboratory measurements of p-wave velocity at variable frequencies ranging from 60 KHz to 1 MHz were performed on sandstone rocks of different porosity and permeability. Biot-Gassmann and squirt-flow models were applied to describe the p-wave velocity dispersion behavior. The dry measurements showed insignificant change in p-wave velocity with respect to frequency which reaffirms the claims of earlier studies of frequency independence of acoustic velocity at dry condition. The squirt-flow model predicted a transition from low to high-frequency regime at or above logging frequencies. These theoretical models can be used to derive acoustic velocities at any required frequency ranging from seismic to logging and ultrasonic frequencies. Squirt flow model can be used to derive rock permeability given the characteristic squirt-flow length is known and model parameter Z derived from laboratory measurements. Samples with higher porosity exhibit more dispersion compared to lower porosity samples can give a qualitative measure of the porosity. Further research and development in this area may lead to improved methods for characterizing sandstone reservoirs and optimizing hydrocarbon exploration and production. The novelty of the present workflow lies in utilizing and extracting additional potential of acoustic data to estimate petrophysical properties from multi frequency modern acoustic logs.