Laboratory Experiments and Numerical Simulation on Bitumen Saturated Carbonates: A Rock Physics Study for 4D Seismology

Abstract

The change in seismic reflectivity from a reservoir — during in situ bitumen recovery processes such as SAGD — can be substantial due to the combined effects of increased temperature, pore pressure and effective stress changes, and the substitution of bitumen with water and steam. These physical property changes can be observed with time lapse seismic monitoring e.g. 4D seismology. The proper interpretation of geophysical observations, however, requires a solid understanding of the saturated reservoir rock’s behavior and pore fluid’s properties under changing conditions. Our first suite of ultrasonic measurements with bitumen saturated carbonate show the P- and S-wave velocities decrease by ~11.5% and ~8.5% for a temperature increase from 10°C to 102°C, respectively at a constant effective pressure of 5 MPa. In the next effort, direct measurements with bitumen show ~29% decrease of P-wave velocity for 10°C to 130°C change in temperature. Different slopes in velocity versus temperature plot may also indicate the possible states of quasi solid and liquid in bitumen. The change in fluid bulk modulus with temperature drives the drop in the P-wave velocities. We may also infer that the decline in S-velocity in core sample is due to greatly lowered viscosity of the fluid with temperature.

We also attempt to simulate numerically the ultrasonic pulse-transmission through rock saturated with viscous fluid, which exhibits similar trends of P-wave velocity drop with temperature. However, the decrease in velocity is not quite large as experimental studies with bitumen saturated carbonates; may be due to the difference in pore structure and fluid properties. In addition, the dynamic moduli of saturated rocks at seismic frequencies on core scale using the strain-stress method indicate its strong dependency on viscosity of pore fluid and (or) the frequency.