Drainage Capillary-Pressure Functions and the Influence of Connate Water
Authors: Dumore J.M. , Schols R.S. 
Published: December 2007

Drainage capillary-pressure functions of reservoir rock-fluid-fluid systems are usually obtained by determining mercury-air capillary-pressure functions on the rock material and subsequently transforming the functions into those of the relevant systems. This procedure is followed because mercury-air capillary pressures can be measured rapidly and easily. The question arises whether this indirect procedure gives the same results as direct procedure gives the same results as direct measurements on the system in question. To investigate ibis, drainage capillary-pressure functions have been determined on the same rock material with various fluid-fluid combinations, including mercury-air. The measurements have been carried out by the semipermeable-membrane method under carefully controlled conditions. It appeared that with the aid of the measured interfacial tension, permeabilities, antiporosities, the capillary-pressure permeabilities, antiporosities, the capillary-pressure measurements could be transformed into one dimensionless capillary-pressure function without large discrepancies. Thus, the indirect method can indeed be applied in obtaining reasonably accurate results for the systems considered. The mercury-air interfacial tension of the mercury used in the experiments bas been found to be time-dependent as measured by the pendent-drop method. After a certain aging time of the drop, the interracial tension reaches an almost constant value and it is this value that must be used in the transformations. Additional measurements of drainage capillary-pressure functions for the rock-gas-oil system in the presence of connate water show that at sufficiently high gas-oil capillary-pressures, very low residual-oil saturations are obtained, irrespective of whether or not the oil phase spreads on water in the presence of gas. Low oil saturations are also obtained in gravity-drainage experiments after long drainage times when the capillary pressure is sufficiently high, possibly as a result pressure is sufficiently high, possibly as a result of oil "film flow." The contribution to oil production from film flow in secondary gas caps is production from film flow in secondary gas caps is thought to be generally negligible during the lifetime of a reservoir. In primary gas caps, however, where film flow may have occurred during geologic time spans, very low oil saturations may occur.

Introduction

The efficiency of oil recovery in the gas- and water-invaded zones of an oil reservoir is, apart from other properties, affected by the capillary properties of the rock-gas-oil and rock-water-oil properties of the rock-gas-oil and rock-water-oil systems present. Particularly in highly fractured oil reservoirs, being composed of a multitude of separated matrix blocks, the ultimate recoveries in gas- and water-invaded regions depend strongly on the capillary entrapments.

Capillary properties are generally characterized by capillary-pressure functions. Those functions relate the capillary pressure - i.e., the pressure difference between two fluids present in the reservoir rock and the saturation of the rock by those two fluids. The functions depend on the way the saturations have been reached, a phenomenon known as capillary hysteresis. When the wetting-fluid saturation decreases monotonically from initially complete saturation, the function is called the "drainage capillary-pressure function"; when it increases monotonically from its irreducible saturation, the function is called "the imbibition capillary-pressure function." Between these two extremes many other capillary-pressure functions exist, depending on the initial value and change (not necessarily monotonic) of the wetting-fluid saturation. Capillary-pressure functions can be determined directly on the system in question, for instance by the time-consuming semipermeable-membrane method' or a centrifuge method, or indirectly with another fluid-fluid combination on the same rock material (for instance, by Purcell's rapid mercury-injection method), The functions obtained by the latter method must be transformed to those of the relevant system.