Method of locating potential low water cut hydrocarbon reservoirs in

Abstract

A method of identifying potential low water cut hydrocarbon producing zones in sandstone formations lacking significant cementation and having relatively high log-indicated water saturations, implying the presence of significant amounts of unbound water. A formation is logged in a conventional fashion to determine porosity and fractional water saturation along the formation. A linear relation between porosity and water saturation is then derived, preferably by linear regression on pairs of log-indicated porosity and fractional water saturation values, each pair being taken from a different depth along the formation. Irreducible water saturation at any point along the depth of the formation is estimatable from the derived linear function and the log-indicated porosity of a formation at that depth. Potential low water cut pay zones are located by determining irreducible water saturation for at least a length of the formation using the aforesaid linear relation and log-indicated porosity values of the formation along said length, in the manner just described. The irreducible water saturation values thus determined are compared with the log-indicated fractional water saturation values obtained along the same length of the formation, preferably by displaying the two sets of values together in a common log as a function of depth. Depths where the irreducible water saturation approximates or exceeds the log-indicated fractional water saturation are zones in which any hydrocarbons produced will be water free or have a low water cut. The derived linear relationship may also be used with pairs of log-indicated porosity and fractional water saturation values obtained elsewhere within the same formation to identify potential low water cut pay zones at those locations.