Foam diverters

Foam diverter stage Foam diverter pill (70% quality) ... 

The foam, having a viscosity greater than the displacing medium, will preferentially accumulate in the well-swept and/or higher permeability zones of the formation. The displacing medium is thus forced to move into the unswept or underswept areas of the formation. It is from these latter areas that the additional oil is recovered. However, when a foam is used to fill a low oil content area of the reservoir, the oil contained therein is, for all practical purposes, lost. This is because the foam functions to divert the displacement fluid from such areas [1574-1576]. 

Laboratory studies of foam flow in porous media suggest that the relative foam mobility is approximately inversely proportional to the permeability. This means that foam has potential as a flow-diverting agent, in principle sweeping low-permeability regions as effectively as high-permeability regions [716]. 

In recent years there has been considerable interest in the use of foams in chemical steam flood, CO2, and low tension processes. To date, principal applications have been as diverting agents where the foam has been used to block high permeability, low oil saturation zones and hence force drive fluids through lower permeability, higher oil saturation zones. The utility of foams in more general mobility control roles has not been extensively... 

Desirable foams Foam drilling fluid Foam fracturing fluid Foam acidizing fluid Blocking and diverting foams Gas-mobility control foams... 

The use of surfactant-stabilized foams to counteract these kinds of problems was suggested several decades ago (7, 2) and has recently become actively pursued in laboratory and field tests (3—8). The use of foam is advantageous compared with the use of a simple fluid of the same nominal mobility because the foam, which has an apparent viscosity greater than the displacing medium, lowers the gas mobility in the swept or higher permeability parts of the formation. This lowered gas mobility diverts at least some of the displacing medium into other parts of the formation that were previously unswept or underswept. From these underswept areas, the additional oil is recovered. Because foam mobility is reduced disproportionately more in higher permeability zones, improvement in both vertical and horizontal sweep efficiency can be achieved. 

Steam-based processes in heavy oil reservoirs that are not stabilized by gravity have poor vertical and areal conformance, because gases are more mobile within the pore space than liquids, and steam tends to override or channel through oil in a formation. The steam-foam process, which consists of adding surfactant with or without noncondensible gas to the injected steam, was developed to improve the sweep efficiency of steam drive and cyclic steam processes. The foam-forming components that are injected with the steam stabilize the liquid lamellae and cause some of the steam to exist as a discontinuous phase. The steam mobility (gas relative permeability) is thereby reduced, and the result is in an increased pressure gradient in the steam-swept region, to divert steam to the unheated interval and displace the heated oil better. This chapter discusses the laboratory and field considerations that affect the efficient application of foam. 

As a result of an increased flow resistance, foam injection is often accompanied by an increase in the injection-well bottomhole pressure (BHP). Temperature surveys at injection, production, and observation wells indicate whether foam is successfully diverting steam. By minimizing gravity override, a successful foam application increases the temperature... 

The types of diverters vary extensively, from mechanical systems to chemical diverting techniques. The latest generation of diversion techniques involves pumping an immiscible mixture of two fluids such as emulsions or foams. Emulsions have not been used as extensively as foams as a diversion technique, because they are more difficult to clean up in low-pressured reservoirs. Foams, however, with similar properties to emulsions, tend to clean up very easily. 

Foam properties vary with changes in pressure and temperature thus foam qualities increase as they flow away from the wellbore. The property is advantageous when treating an interval with more than one zone having different pressures. In the lower pressure zone, the foam will have a higher quality and potentially more diverting effects. 

Investigations (34, 35) to study the effects and limitations of brine and acid foams as diverting agents for nonfoamed acids developed several interesting conclusions. In several case studies of actual stimulated wells... 

Brine foams in certain circumstances are more acceptable as diversion fluids than add foams, as shown in Tables III and IV. The data illustrate that either high-porosity or high-permeability limestones reacted with the foamed acid to create poor diversion charaderistics. Foam brines that will not react with the carbonate formation produced better diverting fluids. 

---