Ultrasonic Characterization of Bitumen with Pressure and Temperature: Implications for seismic monitoring of the Grosmont Formation

Abstract

The vastly untapped carbonate Grosmont Formation in north-eastern Alberta may hold in excess of a couple of hundred billion barrels initial volume of bitumen in place. Bitumen, an ultra-heavy hydrocarbon oil, possesses a density comparable to water, but a dynamic shear viscosity million times higher than that of water. Thermal recovery processes, e.g., steam-assisted gravity drainage, are necessary to reduce its extremely high viscosity to make it mobile for the production. During steam injections, the physical properties of the reservoirs change due to the combined effects of increased temperature, pressure, and fluid substitutions. Time-lapse seismic reflection surveys can be used to monitor the alterations, but the proper interpretations require a solid understanding of the host rock’s behaviour and most importantly, the pore fluids properties exposed to such environment. Therefore, we have studied the physical properties of bitumen-saturated carbonates and naturally produced bitumen with temperature and pressure using various ultrasonic methods. The bulk and shear moduli in bitumen hosting carbonates derived from the measured compressional and shear wave speeds see a significant drop with the increase of temperature, but the observations cannot be modelled using standard fluid substitutions theories. Bitumen’s properties are expected to be the most influential factors for such divergences. Next, studies of compressional wave propagation in bitumen reveal a discrepancy between the conventional bulk and longitudinal moduli, particularly at a lower temperature. The complex longitudinal moduli indicate the importance of bulk viscosity in bitumen in addition to the high shear viscosities. The shear properties studies afterward show that bitumen possesses substantial shear moduli and hence, shear wave speed. It also reveals that viscoelastic relaxation and, most importantly, the shear thinning phenomena in a highly viscous material significantly influence the high-frequency viscosity results. The results, in combination with the knowledge of the various phase transitions in bitumen through the compositional and thermophysical characteristics, can be implemented to have a better interpretation of seismic surveys, although further studies at seismic frequency are highly recommended.