Abstract
The attenuation characteristics of compressional (P) and shear (S) waves in dry water, and varyingly saturated brine (10, 20 and 30% NaCl by weight) saturated Hawkesbury sandstone samples were measured in the laboratory within the ultrasonic frequency range of 0.1-1.5MHz. The results were analyzed using the pulse transmission technique and spectral ratios were used to calculate the attenuation coefficient and quality factor (Q). The values were calculated relative to a reference sample of aluminium with negligible attenuation, and the effect of salinity on the attenuation characteristics of sandstone was evaluated. Velocity dispersion was observed for both P and S waves for all the tested conditions. It is observed that the attenuation coefficient is frequency-dependent, and the attenuation coefficient of the tested sandstone is linearly proportional to frequency for both P and S waves. Interestingly, with respect to dry value, the attenuation coefficient increases with the fluid saturation (both water and brine). Moreover, the calculated Q values reveal that the values are highly dependent on saturation condition, and reduce with saturation. Varyingly saturated brine (10, 20 and 30% of NaCl) was then used to simulate the brine saturation effect. According to the results, although the sample saturated with 10% NaCl had similar attenuation characteristics to the water-saturated sample, 20 and 30% NaCl saturated samples displayed considerable variations in attenuation coefficient and quality factor, where the attenuation coefficient decreases with increasing salinity level of the pore fluid and consequently, the quality factor of the rock formation is also increased.
1. INTRODUCTION
Carbon capture and storage (CCS) in geological reservoirs is one of the best ways to reduce anthropogenic CO2 emission into the atmosphere. It is now well accepted that CO2 geo-sequestration in deep saline aquifers can accommodate large amounts of captured anthropogenic CO2 compared to other geological reservoirs [1, 2] such as depleted oil/gas and coal formations. Generally, the most preferable saline aquifers are of sandstone and most are highly saline [3]. During long-term injection in CO2 sequestration, the reservoir undergoes different mineralogical reactions, including the dissolution and precipitation of rock minerals, which in turn alter the hydro-mechanical properties of the formation. Therefore, it is essential to evaluate the possible changes in the hydro-mechanical properties of undisturbed formations before initiating the injection process.
