The Role of Noncondensable Gas in Steam Foams
Authors: Falls Andrew H. , Lawson Jimmie B. , Hirasaki George J. 
Published: December 2007

Field tests suggest that a steam-foam drive is more effective when nitrogen, methane. or the like is added to the formulation. A plausible explanation is that foam lifetime is longest when transport of noncondensable gas limits mass transfer between steam bubbles. On the basis of this hypothesis, a method to estimate the amount of noncondensable gas to be included is presented.

Introduction

The displacement efficiencies of steam-injection processes in heavy-oil reservoirs are high, residual oil saturations in some steam-swept zones are around 10%. Their vertical and/or areal conformance. by contrast, can be poor: because gases are more mobile within pore space than liquids are, steam tends to override or to channel through oil in a formation.

One way to decrease steam mobility is to inject foam-forming components along with the steam. Foam lamellae are generated in situ. Gas relative permeability is reduced and gas apparent viscosity is increased. the degree depending on the average foam bubble size inside the pore space.

In laboratory studies, foams reduce steam mobility up to 40-fold. That they enhance steam drives has also been demonstrated in several field tests. By diminishing steam mobility, foam augments the viscous pressure gradient in the reservoir. Heated oil flows more readily, the steam zone expands more rapidly, and volumetric sweep improves.

A steam-foam formulation must contain steam and at least one surfactant. Polymer, sodium chloride, and noncondensable gases are additives that have been tested in field operations. Polymer has been added only once. Sodium chloride, by contrast. has been used in all Shell Oil Co.'s field tests. Salt was originally included because it enabled sodium dodecylbenzene sulfonates and alpha olefin sulfonates (AOS's) to decrease steam mobility. Since then, its role in the transport of surfactants through reservoirs has also become known.

Noncondensable gas has been another ingredient of steam foams. Foams whose vapor phases consist of steam alone can be generated, but their lifetimes are short. Consequently, even though a foam can control steam mobility, improve injection profiles, and recover additional oil without noncondensable gas, its efficiency is enhanced by inclusion of a material with limited solubility in water and a boiling point much lower than water's. The noncondensable gas appears to lengthen bubble lifetimes (and thus decrease average foam bubble size) by suppressing mass transfer caused by condensation and evaporation of water: this mechanism quickly destroys steam foams outside porous media.

Field data that reveal how noncondensable gas affects the steam foams used to recover heavy, oils are reviewed here. Included are heretofore unpublished data acquired during Shell's steam-foam-drive pilots in the Kern River field, CA, as well as data already recounted in the literature. Collectively, these suggest that a steam foam is more effective when noncondensable gas is present.

To explain these findings, mass transfer between the bubbles of a bulk steam foam is analyzed when it is limited by the transport ofenergy (condensation and evaporation of water alone).surfactant (surface elasticity effects), andnoncondensable gas.

Under conditions representative of steam-foam zones in shallow reservoirs, bubble lifetime is found to be short when the transport of energy controls mass transfer. Moreover, surface elasticity is unlikely to slow the collapse of bubbles composed of pure steam, regardless of whether desorption from the gas/liquid interface or diffusion through lamellae limits the transport of surfactant. Instead, foam lifetime appears to be longest when the transport of noncondensable gas limits mass transfer.

This analysis may need to be modified to describe correctly steam foams in porous media, where bubbles are supported by a solid. Nevertheless, the lower mobilities observed with noncondensable-gas-containing foams probably result because noncondensable gas increases foam stability. From this hypothesis the thermodynamics of steam bubbles is developed to show how the amount of noncondensable gas to be included can be estimated from reservoir temperature and pore size. In hot, permeable reservoirs, only small concentrations are needed.

How Noncondensable Gases Affect Steam Foams

Steam Foam Circulating Fluids. At Shell, steam foams were first designed to clean viscous oil wells. Experiments conducted in heated visual cells showed that steam-foam stability was increased by small amounts of N2. Without noncondensable gas, the foams were coarse-textured and thus ill-suited for lifting particles or sand aggregates. Longer-lived, finer-textured foams could be generated when N2 was incorporated.

Subsequent field testing, with air as the noncondensable gas, verified these laboratory findings. During one test, the supply of aito the foam was cut off. The foam immediately collapsed to a sudsy froth that could not lift solids out of the wellbore.

Shell Tests of Steam Foams for Mobility Control. N2 has been used in steam-foam-drive pilots on the Mecca Lease and Bishop Fee in Kern River field, CA, and the effects of the noncondensable gas were documented.

Mecca Single-Pattern Injection Test, Oct. 1976-April 1977. During a single-pattern injection test on the Mecca Lease, "the importance of including noncondensable gas in the steam foam was ... verified." This statement was based on bottomhole pressure (BHP) data recorded at the beginning of the test. Before the test began, BHP's ranged from 24 to 26 psig [165 to 179 kPa]. Foam components were added gradually to avoid too large a pressure increase. Only surfactant and brine were introduced at first, and their rates did not reach the final levels until about 1 day later. The BHP appeared to level off between 60 and 70 psig [414 and 483 kPa]. Once the operators saw that pressures would stay within acceptable bounds, N2 was added in the same way. After 2 days, it constituted about 0.5 mol % of the vapor phase, and the pressure reached 110 psig [758 kPa].

After the startup, the N2 content was varied between 0.13 and 1 mol% of the vapor phase; little change was observed in BHP.

JPT

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