Seismic tomography at a fire‐flood site

Abstract

A crosshole seismic experiment was conducted to locate and characterize a firefront at an enhanced oil recovery (EOR) pilot project. The reservoir engineers involved in the project were interested in finding out why the burnfront apparently had stalled between two wells 51 m apart. In a noisy producing environment, good quality seismic data were recorded at depths ranging from 710 to 770 m. The frequency range of the data, 500 to 1500 Hz, allows resolution of the velocity structure on a scale of several meters. The moveout of first arrivals indicates that there are large velocity variations in the study region; a high‐amplitude, late arriving channel wave points to the existence of a low‐velocity channel connecting the boreholes. Using an iterative, nonlinear scheme which incorporates curved ray tracing and least‐squares inversion in each iteration, the first‐arrival times were inverted to obtain a two‐dimensional model of the compressional seismic velocity between the boreholes. The velocities range from 1.5 km/s to 3.2 km/s, with a low‐velocity channel at the depth of the producing oil sand. Sonic, core, and temperature logs lead us to conclude that the extremely low velocities in the model are probably due to gases produced by the burn. Increased velocities in an adjacent shale may be a secondary effect of the burn. The velocity model also indicates an irregularity in the topography at the bottom of the reservoir, an irregularity which may be responsible for blocking the progress of the burnfront.