Effect of Mobile Oil on Residual Oil Saturation Measurement by Interwell Tracing Method

摘要

Interwell tracing has been recognized as a reliablernmethod for determining residual oil saturation betweenrnwells. Sponsored by the United Nations InternationalrnAtomic Energy Agency, China Institute of AtomicrnEnergy is currently conducting several tests in variousrnChina oil fields to determine residual oil saturation andrnto delineate reservoir problems. Interwell methodrnassumes tracers contacting immobile oil in the watered-outrnzone and the oil saturation is calculated from thernpartitioning tracer propagation rate. In the presence ofrnmobile oil, the partitioning tracer velocity wouldrnincrease as it can contact, partition into and move withrnthe flowing oil. As a result, assuming zero oil rate wouldrnlead to underestimate of oil saturation.rnIn an unsteady state displacement where tracersrnare injected with water into a virgin reservoir, thernpartitioning tracer will move with oil at a constantrncharacteristic velocity governed by the partitionrncoefficient. Since tracers are normally injected after thernstart of waterflood, the injected tracers will lag behindrntheir respective characteristic fronts. The time requiredrnfor the partitioning tracer to catch up with itsrncharacteristic front and the breakthrough time of therntracer in a pseudo 1-D system were theoreticallyrnanalyzed in this paper. The produced oil cut, which isrndominated by streamline distribution, sweep andrnlayering, is not a definite indicator of mobile oil in thernwater zone. The presence of moving oil and its effect onrnoil saturation measurement can be inferred from thernresponses of two tracers with different partitionrncoefficients.