The effects of stress and fluid on the anisotropy of reservoir rock: case study of a sandstone from the harvey 3 CCS site, Western Australia

Abstract

In rock physics one of the important purposes of the determination of P- and S-wave velocities is to obtain the elastic constants and anisotropy parameters. This method is standardised in American Society for Testing and Materials (ASTM) by a standard D2845-95. The most challenging yet important part of these experiments is picking the first arrivals correctly and consistently. A sharp, clean first arrival of a wave is not always possible, especially when testing dry, porous sandstones at low pressures. The signal transmitted through such samples is weak due to a high damping effect, and picking the first break introduces uncertainties. Harvey sandstone is a sample type with high porosity and, therefore, some degree of anisotropy is expected. In order to study the anisotropy parameters of the Harvey 3 sandstone which belongs to Lesuer- Wonnerup Member of the Yalgorup Member, three core plugs in directions of horizontal, vertical and diagonal, were experimentally investigated. The laboratory measurements on three different samples which are cut at three different directions with regards to the base deposition system enabled us to calculate the anisotropy parameters using a standard acoustic equipment while applying stress. We concluded that Harvey 3 poses a more complex symmetry axis of anisotropy, and no sign of low-grade anisotropy was observed through the experimental data. To study the anisotropy degree of the Harvey 3 sandstone, a pressure cell, pore fluid injection pump, and ultrasonic system (consisting of P- and S-wave transducers, oscilloscope and pulser/receiver) were used to record the elastic waves passing through the samples. The specimens were fully saturated inside the pressure cell using vacuum injection for the best possible comparison of dry and saturated status. The results confirmed that the sample is far from weak anisotropy and possesses more axes of symmetries than simple layering anisotropy due to the pore orientation and distribution in the matrix of the background rock. The study also confirms that the anisotropy degree decreases under full compaction and full saturation, and the Harvey 3 anisotropy becomes elliptical when it’s fully saturated. Large, stress-dependent changes in the ultrasonic velocities for porous media were observed in this experimental study, which confirms the usefulness of such studies in examining the inner structure change process.