Analysis of temperature dependency of elastic moduli in heavy oil deposits

Abstract

Seismic monitoring of oil sands during hot or cold production requires a valid rock physics model. The effective elastic properties of heavy oil deposits depend on temperature variations, which consequently alter the P- and S-wave velocities in thermal recovery processes. The authors discuss the relationship between temperature, apparent shear modulus, frequency, and velocity dispersion. We use log data from wells drilled in heated and cold zones of a bitumen reservoir to calculate bulk and shear moduli along the wellbore. We apply various filters to control the unwanted effects of other variables, such as porosity and water saturation. We demonstrate that sonic velocities of steam-saturated sands at reservoir conditions can be lower than the compressional velocity of seismic waves in water. We use modulus-temperature crossplots to verify the existence of the liquidation temperature and apparent shear modulus of the heavy oil. In our study, we observe two rapid-decline events in bulk modulus at around 20°C and 200°C. The first event exhibits the ideas of the viscoelastic model of Maxwell. We attribute the second event to effective replacement of liquid phase with steam. For temperatures between 20°C and 200°C, we use a linear relationship to model bulk modulus decline with temperature. The calculated shear modulus shows a wide range of variations at cold temperatures due to a slight change in the bitumen's API gravity with depth. We attribute the weakening of the shear moduli at temperatures greater than 100°C to thermal expansion of the rock.