A Dual Gas Tracer Technique for Determining Trapped Gas Saturation During Steady Foam Flow in Porous Media

Abstract

ABSTRACT Foam is a possible mobility control agent for effective oil displacement from reservoirs. Thus, it is important to understand the mechanisms by which foam flows in porous media. Micromodel studies and prior gas-phase tracer experiments show that a significant fraction of the gas in a foam exists as trapped bubbles which, therefore, have a major impact on the flow resistance. Unfortunately, in the tracer experiments performed to date, partitioning of the tracer into the trapped gas has not been accounted for. Currently, only qualitative information is available on the actual amounts of trapped gas. To overcome these limitations and obtain quantitative measurements of trapped gas saturations, we have developed a unique experimental apparatus employing dual gas tracers. During steady foam flow in a porous medium, dilute sulfur hexafluoride (SF6) and methane (CH4) tracers in a nitrogen carrier are injected, and the effluent concentration is monitored by gas chromatography. The measured tracer histories are fit to a simple mass transfer model which describes any partitioning between the mobil and trapped foam phases. Tracer effluent concentrations predicted by the model are strongly influenced by the solubility of each tracer in the liquid phase. This behavior is observed in the experimental histories as well. Hence, multiple gas tracers provide a discriminating assessment of trapped gas saturation during foam flow through porous media. New trapped gas saturations are reported for an aqueous C14-16α-olefin sulfonate foamer solution and nitrogen flowing through a 2.3-μm2 fired Berea sandstone at 105 Pa (1 atm) back pressure and at room temperature. Total superficial velocities range from 0.4 to 4 m/day while inlet gas fractional flows are varied from 0.8 to 1.0. We find large fractions of trapped gas between 80 and almost 100% depending on the particular flow conditions. The importance of trapped gas to understanding foam-flow behavior is again confirmed.