Reconciliation of contradictory ros data from a Venezuelan light-oil sandstone reservoir

Abstract

In the context of the Joint Maraven/Intevep/Shell Enhanced Oil Recovery Research Agreement an evaluation was made of all data available on waterflood-residual oil saturations ( ros) in the Eocene C5 sandstone reservoirs in the Horst of Block I, Lake Maracaibo. Special attention has been paid to contradictory data obtained in the same well by log evaluation, pressure coring and a single-well tracer test. It was found that the absolute values of the log-derived ros's are unreliable. The pressure core, on the other hand, has yielded the most complete and dependable information on the vertical distribution of the residual oil, while the single-well tracer test was inferred to have investigated an uncored sand unit. Widely varying ros values were also obtained from core-plug measurements, which further revealed a significant dependence on the degree of flooding. On the basis of this dependence it could be shown that the apparently conflicting in-situ ros data reflect, in all likelihood, different degrees of flooding of the relevant interval. The residual oil saturation in the N2.3 sands at the test well was concluded to vary from 23% in the well developed sands at the bottom of the unit to 35% in the poorer sands in the top half. Similar variations can be expected throughout the reservoir.