Abstract

We have used a highly reduced gravity environment to remove the masking influence of normal gravity and examine capillary flow in porous media by combining experiments from high altitude parabolic aircraft flights and from the STS-91 space shuttle flight, with simulations performed using a simple finite-difference numerical model. These studies involved a range of gravitational accelerations, surface tensions, viscosities, wetting preferences, permeabilities, and pore radii on capillary flow. Throughout enhanced to terrestrial to highly reduced gravitational accelerations, bead-pack experiments and numerical simulation results provided quite good agreement when the constraining cell walls were not preferentially wetted by the liquid. When the walls were wetted by the liquid the simulations provided severe underestimates of the experimental capillary flow results, apparently due to pronounced fluid flow along the inner cell walls under highly reduced gravity conditions. For sand-pack 1g experiments the numerical simulations provided reasonable predictions; for nearzero gravity experiments some simulations provided underestimates of the experimental capillary rise results, apparently due to poor conformance on the part of the advancing liquid front through the porous media. This effect was not observable in the shorter time-duration reduced gravity experiments. This work demonstrates the ability of normal gravity to mask remarkable interfacial phenomena and shows the potential value of conducting such experiments in on highly reduced gravity platforms such as high altitude aircraft parabolic flights and the space shuttle.

Introduction

A number of improved oil recovery (IOR) processes are being developed in order to provide economic means of recovering waterflood residual oil(1–4). From an environmental perspective, somewhat analogous problems arise in the filtration of liquid contaminants in soil and their mixing with ground water and transport by convective flows. For both kinds of processes there is a continuing need to improve our understanding of the mechanisms involved, including all relevant pore-scale physics. Among these, the multiphase flow, and trapping, of fluids in porous media is strongly influenced by wettability and capillary forces which need further investigation.

Unfortunately, fluid flows in porous media are strongly but not completely governed by gravity effects. Stated differently, the capillary effects can be shielded by opposing gravitational forces. One way to assess any one such effect is to eliminate the others. Until recently, "effective gravity" effects could only be studied by reducing gravitational acceleration in short-duration freefall experiments(5, 6) or by increasing overall gravitational plus centrifugal acceleration in a centrifuge. Low-g durations of the order of 30 to 40 seconds have become possible via aircraft capable of flying high-altitude, parabolic flight profiles (Figure 1) which provide periods of low gravity (10−2g). Much longer low-g durations of hours to days are now possible via experiments that can be conducted, in orbit, on NASA's space shuttle. We have used both high-altitude aircraft parabolic flights (European Space Agency Caravelle aircraft) and an orbiting space shuttle (NASA Discovery mission STS- 91) to conduct capillary flow experiments in porous media.

These experiments were part of a broader program(7) aimed at improving our understanding of mechanisms involved in processes designed to recover oil and gas from petroleum reservoirs. In the present work we describe capillary flow experiments.

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