Foams for conformance improvement

Currently, the three major applications of conformance improvement oilfield foams are as a mobility control agent during steamflooding, a mobility-control agent during CO₂ flooding, and gas blocking/plugging agents placed around production wells, often applied in conjunction with a gas flooding project.

History

Although the use of foams for oil-recovery applications has been actively considered and studied for more than forty years, widespread application of foams for improving oil recovery has not occurred to date. In the pioneering work of the late 1950s and through the early 1970s, foam was identified to be a promising candidate for improving mobility control and sweep efficiency of oil-recovery drive fluids, especially gas-drive fluids. Early R&D personnel observed the following foam characteristics:

• Foams can be quite effective at reducing gas mobility
• On the microscopic scale, the gas and liquid phases of foam flow separately through porous media with the liquid usually flowing as thin films or lamellae that are separated by gas bubbles
• The pressure gradient during foam flow is proportional to the liquid flow rate, but quite independent of the gas flow rate
• The macroscopic effective viscosity of foam during its flow in porous media is a function of the number and strength of the lamellae (alludes to the importance of foam texture and bubble size)
• Foams, at times, tend to promote larger mobility reductions in high-permeability porous media, as compared with lower-permeability porous media (an attractive property for improving conformance and reducing channeling)
• Foams might be good candidates for use as gas-flow blocking agents

These early workers also noted that oil in porous media often tends to destabilize most aqueous foams and tends to harm oilfield foam performance. A number of the earliest oil industry proponents of the use of foam hoped that foams would eventually lead to routine “air flooding” of reservoirs. This has not come to fruition.

The earliest study of foams for use during oil-recovery flooding operations attempted to capitalize on the ability of numerous aqueous-based foams to significantly reduce the mobility of gas flow during gas flooding and to be able to substantially improve oil-recovery flood sweep efficiency when flooding with a gas. In concept, foam flooding offers an alternative to polymer flooding. That is, foams can also provide mobility control during oil-recovery flooding operations. Early study and development focused on foam use for mobility-control purposes during oil-recovery flooding projects, especially during gas-flooding operations.

The focus of foam development and application has changed in more recent years. Two major factors have been largely responsible for promoting this change.

• It is unclear if foams (especially steam and natural gas foams) can be propagated distances of more than 100 ft in an oil reservoir because of the substantial minimum pressure gradient required for foam propagation and in view of the small pressure gradients that exist in most of the volume of matrix rock reservoirs.
• Economics now tend to favor small-volume chemical treatments (e.g., gel conformance improvement treatments) over chemical-based improved oil recovery flooding operations.

Thus, the focus of oilfield foam development and application has shifted somewhat toward the use of foams as blocking/plugging agents that are part of relatively small volume treatments applied through production wells, especially for use as blocking agents to gas flow. The fluid-flow-blocking and permeability-reducing propensity of foams is one of the major factors hampering effective application of mobility-control foams (especially steam and natural gas foams) in the far-wellbore environment. The significant negative impact that crude oil often exerts on the desired performance of foams during mobility-control flooding also helped to shift the focus of oilfield foam use from mobility-control applications to fluid-flow-blocking treatments.

When, where, and why to use foams

Conventional foams (i.e., not polymer-enhanced foams and foamed gels) are considered effective only when placed in matrix reservoir rock and are not applicable when placed in reservoir fracture channels with aperture widths on the order of greater than 0.5 mm. The application of foams for sweep-improvement and gas/water-blocking purposes is considered to be an advanced and nonroutine form of an oilfield conformance improvement operation. It is recommended that the average petroleum engineer not undertake a foam conformance improvement operation without in-house or commercially available technical support and/or without support from an organization that has expertise in conformance improvement foam technologies. In addition, before implementing a foam conformance improvement operation, it is usually necessary to perform a laboratory evaluation of the proposed foam formulation and the actual foam process to be used in the field.

The use of foams is most advantageously applied during gas flooding or for reducing gas coning and cusping in one of two manners:

• Foams can be used to improve sweep efficiency and improve oil recovery during gas flooding (e.g., steam, CO₂, and hydrocarbon-miscible flooding). Such mobility-control foam is usually injected from the injection well side.
• Foams can be used as gas-blocking agents to reduce excessive, deleterious, and competing gas production. Such gas-blocking foam is most often placed from the production well side.

Foams for use as both mobility-control and gas-blocking agents are attractive because they are relatively inexpensive on a unit-volume basis. The low unit-volume cost results from the combination of the bulk of the foam volume usually being a relatively low-cost gas, and the surfactant chemicals for the foaming solution are relatively inexpensive and used at relatively low concentrations.

Advantages and disadvantages of foams

There are a number of somewhat contrasting advantages and disadvantages for the use of foams for improving conformance during oil recovery operations.

Advantages

The following is a list of the advantages of the use of foams.

• Foams are exceptionally effective in reducing gas mobility during gas flooding
• Foams can be an effective gas-blocking agent
• Foams are a conformance improvement material that has the tendency, in numerous instances, to reduce permeability and mobility to a greater degree in higher permeability matrix reservoir rock
• Foams are shear-thinning fluids resulting in relatively good injectivity and in more effective mobility control in the far-wellbore region where such mobility control is most needed
• Foams possess low effective density that can often be exploited to help selectively place foams high in a reservoir thereby impeding problematic gas flow where it is most likely to occur
• Foams are considered, in general, to be an environmentally friendly material for use in conformance-improvement operations

Disadvantages

The following is a list of the disadvantages of the use of foams.

• Foams are a relatively complex technology, both chemically and operationally, to apply successfully
• Oil tends to destabilize and deactivate many conformance improvement foams
• Many mobility-control foams (e.g., steam and natural gas foams) are difficult or impossible to propagate in the intermediate- to far-wellbore environment under the differential-pressure conditions encountered in most reservoirs
• Surfactant adsorption/retention has a substantial negative impact on the performance and economics of mobility-control foams
• Fluid-blocking (e.g., gas-blocking) foams used in production-well treatments have limited strength under high differential pressure conditions
• Fluid-blocking (e.g., gas-blocking) foams are also limited by the inherent lack of long-term stability and the associated lack of long-term treatment effectiveness
• The high viscosity and poor injectivity of preformed foams limit the application of this otherwise often favored foam-injection mode

The limited and sometimes poor ability to effectively form foam in situ in matrix reservoir rock during the co-injection or sequential injection of the foam’s gas and liquid phases limit the effectiveness and the efficiency of the co-injection and sequential-injection modes for foam formation and placement in a reservoir.

Design strategies for field application

Rossen suggests the following design strategies for the field application of conformance-improvement foams. The initial steps are to:

• Characterize the field and its conformance problem
• Determine that there is sufficient recoverable oil to render the foam process economic
• Determine process goals, for example:
  • Increase oil recovery or recovery rate
  • Reduce operating costs
• Perform a preliminary economic evaluation of the foam project

Next, the surfactant to be used should be chosen by conducting wet-chemistry testing, conduct foam-property testing in porous media, and determining surfactant retention with reservoir core material, if possible. Finally, determine the injection strategy to be used.

Noteworthy papers in OnePetro

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External links

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See also

Foam properties

Foam behavior in porous media

Foams as mobility control agents

Foams as blocking agents

Field applications of conformance improvement foams

Conformance improvement

PEH:Polymers,_Gels,_Foams,_and_Resins