Oil Reservoir Monitoring in Middle East Carbonates – Passive Seismic as an Alternative Approach to 4D Seismic

Many of the supergiant oil fields in the Middle East produce from prolific carbonate reservoirs. Collectively these carbonate rock reservoirs hold over 50% of the proven world oil reserves (Nurmi, 1997). The ultimate oil recovery from these reservoirs could be optimized by monitoring the water flood areal sweep efficiency and by monitoring the changes in the remaining oil saturation between the wells, as the reservoir is produced. The sparse well spacing and large drainage areas for producing wells in these fields along with the rock physics properties of the reservoir matrix and fluids, however, make such monitoring a technical challenge. <br><br>The limestone-dolomite reservoir rock matrix has high rigidity modulus and the reservoir pore fluids show only a small elasticity contrast, as the oil is replaced by aquifer water. This is primarily due to relatively low gas-oil-ratio (GOR) in these undersaturated oil reservoirs, absence of a gas cap and mixed salinity water due to the injection of lower salinity brine in the reservoir. The combination of these reservoir properties produces a weak 4D seismic or time-lapse seismic effect from oil production in these reservoirs. For fluid monitoring in such reservoirs an alternative to time-lapse or 4D seismic technique is needed. <br>