A New Technique to Measure the Breakthrough Capillary Pressure

Abstract

Abstract The breakthrough capillary pressure is an important macroscopic property of porous media that is used to predict permeability and to correlate capillary pressure curves. It is normally determined using the porous plate method, where the non-wetting phase is introduced into the medium in increasing steps of pressure until it establishes a continuous pathway through the sample. In mercury porosimetry, breakthrough capillary pressure is identified as the pressure corresponding to about 10 - 20% non-wetting phase saturation or the point of inflection in the drainage capillary pressure curve. A new method for measuring the breakthrough capillary pressure has been developed, involving a constant rate injection process as opposed to constant injection pressure. The scope of this paper is to report the accuracy and reliability of the new technique in measuring the breakthrough capillary pressure by reporting test results on transparent micromodels and Berea sandstone core samples. Air or mercury was used as the non-wetting phase while water (or brine, in the case of core samples) was the wetting phase. Constant rate injection was provided by means of a syringe pump, using water to displace a slug of either air or mercury in a tube connected to the porous medium at rates of about 1.8 × 10−2 cm3/min. Pressure was measured at the sample inlet using a pressure transducer and data were recorded using a data acquisition system. The injection pressure vs. time plot reveals the highest pressure established in such a test, which is identified as the breakthrough capillary pressure. It was concluded that the new test enables the determination of the breakthrough capillary pressure in micromodels with excellent accuracy. The new test was also validated using a sandstone core sample. Introduction In the literature of immiscible displacement in porous media, the terms "breakthrough capillary pressure," "threshold pressure," "entry pressure," and "bubbling pressure" are frequently encountered. It has been observed, for example, that in order to make a non-wetting phase, such as air, flow through a porous medium saturated with brine, the applied capillary pressure on the air side has to exceed a critical limiting value, Pc °, at which air can flow through the medium (i.e., can break through and emerge as bubbles from the outlet face of the sample). The pore channels in the medium used by the air to break through the sample are made up of pore sizes greater than or equal to the pore size De °, defined by Equation (Available In Full Paper) Where γ is the interfacial tension, θ is the contact angle, and De ° is the (equivalent) diameter of a capillary tube of circular cross-section which requires the same capillary pressure, Pc °, for penetration as that measured at breakthrough capillary pressure in the porous medium. The value of De ° calculated using Equation (1) is the minimum capillary size that is necessary for the non-wetting phase to invade and break through the medium.