This contribution demonstrates how the seismic technique can be a valuable tool to improve the accuracy of simulation predictions and optimize the efficiency of hydrocarbon production. We illustrate how time-lapse 3D (or 4D) seismic can be used to monitor the movement of oil-water contacts and map changes in saturation throughout a reservoir over discrete intervals in time.
Quantitative data can be extracted from the monitoring quantity, i.e. either seismic amplitude data or acoustic impedance data, depending on whether the flow regime being considered is segregated or diffusive. For segregated flow conditions, 4D seismic can be used to monitor the movement of the oil-water contact (OWC) in time and space. It can also clearly identify undrained compartments of a producing reservoir and be a valuable guide for planning new wells. Potential problems such as coning can be ‘seen’ prior to water breakthrough and be prevented by controlling production and injection rates. For diffusive flow, where there is no explicit OWC, acoustic impedances can be used to map changes in saturation and evaluate areal sweep efficiency.
Two examples are discussed to illustrate 4D seismic monitoring of segregated and diffusive flow respectively. The first example is from a sandstone reservoir at 1600m depth in the North Sea, with porosity of 27% and permeability of 4 Darcy. The second example is of an inverted 5-spot water flood pilot project, where 4D seismic was used over a time span of 6 months for a 90m thick carbonate reservoir at 2000m depth, with porosity of 21% and permeability of 31mD. We will also show results of two physical fluid-flow models where air and kerosene were injected and monitored with 4D acoustics under controlled conditions. These results serve as an impressive demonstration of the ultimate resolving power of the seismic technique in measuring actual flow in a reservoir.