SEISMIC TOMOGRAPHY IN OIL SANDS FOR MONITORING THERMAL RECOVERY PROCESSES

Crosshole seismic measurements before and after steam injection were obtained for the purpose of delineating the heated zone within a tar sand reservoir.; Analysis of an initial well-to-well experiment of limited temporal and spatial sampling showed significant changes on seismic character after steam injection. It was possible to amplitude normalize the 'before' and 'after' seismic records by making use of the tube waves. Extensive computer simulation of realistic and carefully designed experimental geometries was carried out assuming a dense tomographic coverage of the area of interest, and a variety of shapes of the heated zone. Before and after steam injection versions of the field experiment are suggested in order to reduce the dimension of the reconstruction problem. Inversion was by solution of an overdetermined system of linear equations. A direct least squares solution provided acceptable results but superior reconstructions were obtained by using a fully constrained algebraic reconstruction approach employing the method of projections. Further improvements were achieved when the above technique was combined with data interpolation and radial smoothing.; In the field tomography experiment, data were acquired with a DFS-V system recording at 2,000 samples/second from a fixed downhole string of 24 hydrophones. The source was a downhole air gun at a pressure of 2300 PSI. Inversion of the data revealed dispersion effects from which the intrinsic attenuation of the Clearwater was estimated. Independently, attenuation was determined by using the concept of average frequency, and by the spectral ratio method. Q was found to be close to 30. Attenuation was much greater within the heated zone where Q was close to 10. This has been explained in terms of the dramatic increase of permeability in the heated zone as a result of bitumen mobilization. There is some evidence that attenuation increases at frequencies greater than 150 Hz. The Poisson's ratio within the zone was found to be as high as 0.40. These findings indicate the presence of large amount of fluids inside the region of the reservoir affected by the steam stimulation. Optimization of recording techniques will certainly allow one to obtain the spatial variation of the Poisson's ratio and perhaps the attenuation.