Claude Boyeldieu
Schlumberger Technical Services Singapore
Alan Sibbit
Services Techniques Schlumberger Paris
ABSTRACT
The direct application of the Archie formula, or of more elaborate shaly-sand equations, to the resistivity, Rxo, obtained from the microdevices often fails to satisfy the log analyst. There is a tendency to compute overly high water saturations, Sxo, which when compared with Sw show too much movable oil. The worst examples are in tar sands, where use of a standard relationship and the measured value of Rmf indicate deep invasion, by the filtrate, of an oil which hardly flows.
Recent studies have demonstrated that, in oil bearing zones, mud filtrate displaces some of the oil in place but leaves connate water around the grains of the formation. In some situations, particularly in fine grained shaly-sandstones, large volumes of the original formation water do not even come in contact with the mud filtrate.
This leads us to formulate a model in which three fluids are present within the zone investigated by microdevices; formation water, mud filtrate and residual oil. In this paper, we assume that an irreducible saturation of formation water, Swirr, remains in the flushed zone. New saturation equations are proposed for both clean and shaly formations and the results obtained from this
new approach are presented.