Passive microseismic monitoring of cyclically steamed production wells has been gaining popularity as a potential tool for optimizing drilling by delineating lateral and vertical steam migration within the reservoir. The initial challenge with passive monitoring was to create robust systems capable of permanently recording data with semi-automated triggering thresholds and remote data transfer capabilities for further data processing.

While these technical challenges have been overcome in the past few years, the present challenge with long-term monitoring is in accounting for the dynamic processes taking place within the reservoir as steam is introduced. The introduction of fluid and varying temperatures during cyclical steaming cycles have direct influence on the pre-existing physical properties of the rock and as a result, the velocity model in use needs to be calibrated accordingly. Since passive source waveforms are influenced by the dynamic nature of the reservoir properties it is sufficient to say that their associated event locations need to account for these changes. To this end, we employ a Particle Swarm Optimization (PSO) approach to invert for both the velocity model and the spatial locations of the events. Based on this approach, we observed P-wave velocity reductions up to 16% at the reservoir level for any given cycle. The re-located microseismic events appear ‘well behaved’ in the context of reservoir processes, and the location uncertainty of the re-located events dropped by approximately 21%. The approach further opens opportunities to examine the anisotropic conditions in the reservoir and assist in re-defining velocity derived lithologic boundaries.

Sary Zantout12, Ted Urbancic1, and Peter McGillivray3