Heavy oils: Their shear story

Heavy oils are important unconventional hydrocarbon resources with huge reserves and are usually exploited through thermal recovery processes. These thermal recovery processes can be monitored using seismic techniques. Shear-wave properties, in particular, are expected to be most sensitive to the changes in the heavy-oil reservoir because heavy oils change from being solid-like at low temperatures to fluid-like at higher temperatures. To understand their behavior, we measure the complex shear modulus (and thus also the attenuation) of a heavy-oil-saturated rock and the oil extracted from it within the seismic frequency band in the laboratory. The modulus and quality factor (Q) of the heavy-oil-saturated rock show a moderate dependence on frequency, but are strongly influenced by temperature. The shear-wave velocity dispersion in these rocks is significant at steam-flooding temperatures as the oil inside the reservoir losesviscosity. At room temperatures, the extracted heavy oil supports a shear wave, but with increasing temperature, its shear modulus decreases rapidly, which translates to a rapid drop in the shear modulus of the heavy-oil-saturated rock as well. At these low to intermediate temperatures (30°C–100°C), an attenuation peak corresponding to the viscous relaxation of the heavy oil is encountered (also resulting in significant shear-wave velocity dispersion, well described by the Cole-Cole model). Thus, shear-wave attenuation in heavy-oil rocks can be significantly large and is caused by both the melting and viscous relaxation of the heavy oil. At yet higher temperatures, the lighter components of the heavy oil are lost, making the oil stiffer and less attenuative. The dramatic changes in shear velocities and attenuation in heavy oils should be clearly visible in multicomponent seismic data, and suggest that these measurements can be qualitatively and quantitatively used in seismic monitoring of thermal recovery processes.