Measurements of Supersaturation and Critical Gas Saturation (includes associated papers 27920 and 28669 )

Abstract

Summary Supersaturation and critical gas saturation have been measured for two coresamples with very different pore structures. In these measurements, a pump witha constant rate of expansion was used to avoid steep pressure changes. Binarymixtures of C1/nC10 at two saturation pressures were used as the fluid system. The surface tension of the binary mixtures varied from 2 to 13 dynes/cm [2 to13 mN/m]. Our measurements show that even at high rates of expansion, thesupersaturation in some porous media could eventually become negligible. Thedata also show that there could be a significant effect of pore structure onthe degree of supersaturation. The supersaturation in a porous media with smallpores may be less than in a porous media with large pores. The critical gas saturation data of this work are less than literaturevalues. Measurements on a chalk sample revealed critical gas saturations ofless than 0.5 percent. Introduction Critical gas saturation (say in a two-phase gas-oil system) is defined asthe minimum gas saturation at which gas phase flow can occur. Its value isoften established from the extrapolation of the gas relative permeability foran external gas drive process. Another approach used less frequently is themeasurement of gas saturation at the point when the gas phase becomes mobilizedunder an internal gas expansion condition. These two methods are fundamentallydifferent, and critical gas saturation is of different nature for external andinternal gas drive processes. The appearance of the gas phase in an internalexpansion process is a heterogeneous bubble nucleation phenomenon, where theformation of a new phase requires an expenditure of work to be supplied by theenvironment surrounding the locality at which the nucleus will appear. (Thework of nucleus formation is 1/3 S where is the surface tension, and S is thesurface area of the nucleus). Also, thermodynamic considerations requiresupersaturation for the initiation of the nucleation. The growth of the initialnucleus (critical nucleus) due to diffusion and expansion, agglomeration ofvarious nuclei and the mobilization of the connected bubbles in an internal gasexpansion process is entirely different from the mobilization of gas phase inexternal gas-drive systems. The subject of this study is the critical gassaturation for an internal gas expansion process.