Some Aspects of 4-D Seismics for Reservoir Monitoring

摘要

This contribution demonstrates how the seismic technique canbe a valuable tool to improve the accuracy of simulationpredictions and optimize the efficiency of hydrocarbonproduction. We illustrate how time-lapse 3D (or 4D) seismiccan be used to monitor the movement of oil-water contacts andmap changes in saturation throughout a reservoir over discreteintervals in time.Quantitative data can be extracted from the monitoringquantity, i.e. either seismic amplitude data or acousticimpedance data, depending on whether the flow regime beingconsidered is segregated or diffusive. For segregated flowconditions, 4D seismic can be used to monitor the movementof the oil-water contact (OWC) in time and space. It can alsoclearly identify undrained compartments of a producingreservoir and be a valuable guide for planning new wells.Potential problems such as coning can be ‘seen’ prior to waterbreakthrough and be prevented by controlling production andinjection rates. For diffusive flow, where there is no explicitOWC, acoustic impedances can be used to map changes insaturation and evaluate areal sweep efficiency.Two examples are discussed to illustrate 4D seismicmonitoring of segregated and diffusive flow respectively. Thefirst example is from a sandstone reservoir at 1600m depth inthe North Sea, with porosity of 27% and permeability of 4Darcy. The second example is of an inverted 5-spot waterflood pilot project, where 4D seismic was used over a timespan of 6 months for a 90m thick carbonate reservoir at 2000mdepth, with porosity of 21% and permeability of 31mD. Wewill also show results of two physical fluid-flow models whereair and kerosene were injected and monitored with 4Dacoustics under controlled conditions. These results serve asan impressive demonstration of the ultimate resolving powerof the seismic technique in measuring actual flow ina reservoir.