Acoustic velocities as a function of effective pressure in low to moderate porosity shaly sandstones, Part 1 - experimental results

Abstract

Compressional and shear wave velocities were measured as a function of effective pressure on a set of cores from gas producing reservoirs in the Cooper Basin in South Australia. The aim of this study was to investigate the relationship between the stress sensitivity of acoustic velocities and geological and petrophysical characters of low to medium porosity shaly sandstones. The suite of samples consists of 22 core plugs from depths between 1917 m and 2564 m with helium porosity ranging from 2.6% to 16.5%. The samples are mainly fine to medium-grained sublitharenite. The echo-pulse technique is used to measure ultrasonic velocity and attenuation in dry and fully water-saturated cores under effective pressures of 5 MPa to 60 MPa. Experimental results show that compressional and shear wave velocities increase non-linearly with effective pressure. The influence of pressure was more pronounced below 40 MPa for both compressional and shear wave velocities. The rapid increase in wave velocities with effective pressure between 5 MPa and 40 MPa is attributed mainly to the closure of low aspect ratio pores such as micro-cracks and loose grain contacts within the rock framework. Compressional wave velocity in dry samples shows stronger pressure sensitivity, whereas there is no significant difference between the stress dependency of shear wave velocity in dry and water saturated samples. There is a positive correlation between change in velocity with pressure and core porosity and permeability. However, this association is weak and diminishes with increasing effective pressure. The pressure dependence of acoustic velocity in the Cooper Basin samples is not related to total clay content, but samples with a greater amount of pore filling and grain-coating clay particles appear to be less pressure sensitive at dry condition. Cooper Basin sandstones show higher pressure sensitivity compared to data from other studies.