FIELD AND LABORATORY OBSERVATIONS OF REMAINING OIL SATURATIONS IN A LIGHT OIL RESERVOIR FLOODED BY A LOW SALINITY AQUIFER

Increase in microscopic oil recovery with decrease in brine salinity has been first put in evidence by Tang & Morrow in 1998. Conflicting laboratory results have been published since and there is little field evidence yet of the benefits of waterflooding with dilute brine. Field observations of remaining oil saturations in a watered-out sandstone, light oil bearing reservoir, produced by depletion and low salinity aquifer encroachment are presented in this paper. Finally, these results are compared with early core water flood results where the injected brine had the same salinity (17,000 ppm) as the resident brine in the oil leg. A sound estimate of the amount and vertical location of remaining oil was deemed to be critical. The evaluation process consists of: • Well production tests to confirm the watercut in the zone of interest, as well as the salinity and the composition of flowing brine, • A single-well tracer test (SWTT) which was conducted on a watered-out producer, • Direct measurements of oil saturation from cores cut with water-base muds in a new well drilled in the watered-out zone • A suite of logs, including resistivity, carbon/oxygen and NMR tools This paper focuses on the comparison between ROS data from direct cores measurements in the waterflooded zone, SWTT results and laboratory core flood results. The well production tests showed that the zone of interest was watered-out. The produced brine salinity was about 2.5g/l. Thus, the salinity of the brine originating from the aquifer had significantly lower salinity than the original salinity of the resident brine. The interpretation of resistivity data in clean aquifer zones on downflank wells confirms that this reservoir has been flooded by a low salinity aquifer. Special attention was devoted to the estimation of possible oil flushing from the cores during coring and expulsion due to gas expansion as the core is lifted to the surface. This paper also describes the dedicated coring equipment used to ensure that all mobile oil will be recovered. It is concluded that the residual oil saturation obtained by the single well tracer test is in very good agreement with those determined by direct core measurements in the waterflooded area, but is significantly lower than the core flood results obtained with the injected brine having the same salinity as the resident brine (17,000 ppm).