Abstract

Experiments in homogeneous and heterogeneous sand-packed columns showed that the foam mobility in the two cases could differ by two orders of magnitude. The difference is due to the generation of foam lamellae by snap-off for flow across an abrupt increase in permeability. This mechanism is shown to be dependent on the degree of permeability contrast and the gas fractional flow. It has important implications for the degree of gravity segregation of gas and liquid in field scale recovery processes.

Introduction

Foam has been used to improve the sweep efficiency of enhanced oil recovery processes 1–4 and aquifer remediation. 5–8 Gas as a bulk fluid has a very low viscosity compared to oil and water. However, when gas is a dispersed phase as in foam, its apparent viscosity is greatly increased, i.e., its mobility is greatly reduced. This reduced mobility is responsible for the improvement in sweep efficiency. Yet, foam still has a lower average density than resident liquids in a reservoir or aquifer. This density contrast tends to promote gravity segregation. Stone 9 and Jenkins 10 developed models to predict the extent of gravity segregation in water-alternating-gas (WAG) injection. A key parameter in this model is the permeability anisotropy or the kv/kh ratio, where kv and kh are vertical and horizontal permeabilities, respectively. This anisotropy is based on single-phase flow. We infer here from one-dimensional experiments and calculations that the effective anisotropy for foam flow in stratified systems may be much greater than for single-phase flow. The implication is that gravity segregation for foam flow in stratified systems may be much less than predicted by the Stone-Jenkins model.

Background
Field test

Anisotropy in foam mobility was seen in an aquifer remediation field test that we conducted in a heterogeneous alluvial formation beneath Hill Air Force Base, Utah. 6,8 Foam was used to divert injected surfactant solution to zones of lower permeability and to the base of the aquifer where the contaminant was located. The field test showed that foam could be generated in situ and propagated even in a shallow unconfined aquifer. Air was injected alternating with aqueous surfactant solution at low injection pressure (2(8 psi above hydrostatic), and foam was produced at the multi-level monitoring well located 15 ft [4.6 m] away from the injector, as shown in Fig. 1. The photograph clearly shows foam being produced from the two upper intervals (B and C, see Fig. 2). Although the lowest monitoring interval A is producing emulsified contaminant in the photograph, foam was intermittently produced there as well even though it is below the injection interval. The observations indicated that foam was propagated horizontally despite relatively high foam mobility.

In simulating the field test using a modified UTCHEM reservoir simulator,8 an anisotropy in foam mobility must be used in order to history-match field observations. With an effective foam viscosity of 1 cp, matching required use of a ratio of vertical to horizontal gas mobilities (lv/lh)g, of 0.1 in addition to explicit modeling of permeability layering. The additional resistance of foam to vertical flow is attributed to both capillary entry effect and foam generation by snap-off at permeability discontinuities. Fig. 2 shows the simulated foam propagation profiles at three different times. As can be seen, considerable amount of gas was propagated horizontally from the injector to the extractor. Simulation details are reported in a separate publication. 8

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