Viscoelasticity of Ells River bitumen sand and 4D monitoring of thermal EOR processes

摘要

Samples of Ells River bitumen sand (Broughton, 2012) from Alberta, Canada were measured at low frequencies (0.2-205 Hz) to determine the frequency dependence of velocities and attenuations. With residual air in the pores, at 5°C, there is strong dispersion in the P-wave modulus and a peak in attenuation at seismic frequencies. The attenuation peak shifts by three orders of magnitude in frequency as temperature is increased from 5° to 48°C, consistent with the bitumen's viscosity. When the empty pore space is filled with water, the bulk modulus increases from the empty frame value (0.89 GPa) to 8.33 GPa as predicted by Gassmann's equation. Increasing the temperature from 5° to 49°C causes a 34% reduction in the saturated bulk modulus and only a 6% reduction in the shear modulus. This can only be explained by the temperature-dependent bulk modulus of bitumen. There will be a progressive reduction in velocities, i.e., -11% at 25°C and -17% at 49°C, in areas of formation heating, but without steam or gas in the pores. While smaller than the effect of steam, the effect of formation heating alone is large enough to be easily detected by today's 4D surveys. With local rock physics calibration, it should be possible to map the areal extent of formation heating using 4D seismic data.