Modern logging methods provide good values of porosity and water saturation in most reservoir formations. These parameters are essential in the computation of in-place hydrocarbons. However, these values alone do not define either the type or rate of fluid production to be expected from the formation. Additional reservoir and fluid properties must be considered. To determine whether a zone should produce clean hydrocarbons, or with a water cut, the computed water saturation is compared with the irreducible water saturation of the interval. The latter value can frequently be determined from a plot of porosities versus water saturations throughout the formation of interest. This method of defining irreducible water saturation has been known and used for many years. Today, with the increased accuracy and efficiency of modern logging methods, these plots are more effective than in the past. For intervals indicated to be above the transition zone, the computed values of porosity and water saturation can be used in an empirical method to determine the permeability of the interval. Then, augmented by knowledge of formation pressure, and fluid viscosities and volume factors, these data provide a reasonable estimate of initial rate of production from the untreated intervals. Additional empirical methods are used for the estimations of relative permeabilities to oil and water for intervals in the transition zone. They provide reasonably accurate estimations of the water fraction of production. These methods are useful in carbonates and sand formations; both clean and shaly. They extend the utility of log data, they signal unusual formation properties that might otherwise cause an incorrect prediction of production, and they provide a check on the efficiency of the completion.

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