Utilizing the onset of time-lapse changes: a robust basis for reservoir monitoring and characterization

Time-lapse monitoring is useful for imaging changes in geophysical attributes due to fluid flow. In trying to use time-lapse data for reservoir characterization, difficulties often arise when relating changes in geophysical observations to changes in fluid saturation and pressure. As an alternative approach for reservoir monitoring and characterization, we introduce the idea of an onset time, the time at which a measured quantity, such as a seismic traveltime or a reflection amplitude, begins to deviate from its background value. We illustrate the idea and demonstrate its utility through the consideration of traveltimes recorded by the continuous active source seismic monitoring system at the Frio pilot site near Houston, Texas. The system, which transmits an elastic wave every 15 min, is used to monitor the movement of carbon dioxide injected into a permeable sand formation. From these data we can estimate the onset of changes in seismic traveltimes to six receivers in an adjacent borehole. Numerical simulation and synthetic tests indicate that the onset times are not very sensitive to the method used to compute the effective fluid bulk modulus and, correspondingly, the seismic compressional velocity. Rather, the onset times are strongly influenced by saturation changes within the formation, specifically by the break-through time of the injected fluid phase. By means of an iterative inversion algorithm we use the onset times to estimate permeability variations between the boreholes at the Frio pilot site.