In the past several years, laboratory studies of the temperature dependence of seismic velocity and in-situ monitoring tests at active thermal projects have proven the concept of seismic monitoring to track the horizontal extent of thermal fronts in the subsurface. In addition, work in progress at the AOSTRA-AMOCO-PETRO CANADA GLISP site as reported by Puliin et al (3), has shown that an exceptionally high level of resolution can be obtained through careful field planning and acquisition. It now appears that not only is it possible to track the horizontal extent of the thermal zone, but by careful analysis of the changes in seismic character, the vertical characteristics of the flood can be determined as well.
Based on published laboratory data and field resolution limitations, this study explores the limits of thermal EOR characterization using synthetic examples based on occurrences in the various Alberta deposits. The results show that for an initial oil saturation greater than 60% at API 10 or less, the horizontal extent can be determined to the 50 degree Centigrade isotherm or better. In addition, the various modes of flood movement such as uniform radial displacement, gravity override, aquifer leakage and thief zone losses have differing seismic responses which are diagnostic of their occurrence. These patterns can be recognized and interpreted in processed seismic sections or in inverted pseudo-velocity sections.
In the face of declining conventional oil production, the successful exploitation of our vast bitumen and heavy oil reserves is becoming increasingly important. Continued development of these reserves in the present economic situation forces an even greater level of optimization in order to remain competitive.
Recent advances in seismic technology have provided new techniques which if properly applied, can help optimize the various thermal extraction methods. The basis of these techniques is the temperature dependence of acoustic velocity in porous, fluidl saturated materials. The study reported by Wang and Nur (4) using several differing matrix materials and pore saturants measured at reservoir conditions, has shown a decrease in compressional velocity due to heating. In samples saturated with tar and heavy oil, this decrease ranges from 1.0 to 30 percent as the temperature is raised from 25 to 15U degrees Centigrade. This effect appears to be due to both melting and viscosity changes in the pore saturant. In addition, there is a slight velocity-temperature dependence of 4 to 0 percent in water saturated cores. These temperature effects can be exploited using the seismic method. If a baseline seismic survey is conducted prior to the inlt1acioIl of the thermal EOR process, then subsequent surveys will reflect the induced slowness of the heat front. By compar1ng these surveys with the baseline study, a thermal flood progress map can be generated.
Over the past several years this concept has been successfully tested at several sites. Most of this work has been done using standard surface seismic techniques yielding relatively low resolution.