Time-lapse crosswell seismic monitoring of the Athabasca Tar Sands.

摘要

The Athabasca Tar Sands in northern Alberta contain large amounts of high viscosity petroleum in a shallow, nearly flat lying Cretaceous sand stratum. Cost-effective monitoring methods, such as crosswell seismic, are sought to observe in-situ Steam-Assisted Oil Recoveries (SAOR) and to optimize the oil production processes. This thesis investigates methods of analysing crosswell. seismic data to map structural details of the Athabasca Tar Sand and to observe its modification during oil extraction.;Baseline images of the formations at two different sites (the UTF and Steepbank) in the Athabasca Tar Sands have been derived from crosswell seismic data by traveltime tomography. Particularly, both P- and S-wave velocities of the Steepbank formation have been reconstructed together with a constraint of acceptable values of Poisson's ratio. It has been further demonstrated that full wavefield modeling is capable of quantifying major heterogeneities, intrinsic attenuation, and a stochastic component of velocities. Differences and similarities between the two reservoirs have been noticed and accounted for.;To prepare for interpretation of the time-lapse data, the expected effects of a steam flood on seismic waves have been investigated theoretically by calculating moderately high frequency wave scattering by a cylindrical, spherical, or arbitrarily oriented spheroidal region of altered physical properties. Monochromatic scattering patterns and wavelet responses have been determined to study the possibility of inferring the rock properties changes from diffracted waves.;Seismic images of the Steepbank reservoir after 72 days of continuous steam injection have clearly revealed modification of the reservoir. They indicate about a 10% decrease in P-wave velocity and a 4% increase in S-wave velocity in the affected zones. A likely explanation is that the decrease in P velocity is the combined result of increased temperature and increased gas concentrations whereas the increase in S velocity results from bitumen removal and its replacement by steam. The results also indicate that the steam-affected medium has a complex exterior shape and sharply heterogeneous properties which gives rise to strong P-wave scattering.