Foam Generator
A foam generator is a device that combines a surfactant solution with a gas (most commonly nitrogen) to produce a stable foam for injection into a well or pipeline. The generator works by forcing the liquid and gas through a chamber or porous element that breaks the gas into very fine bubbles dispersed uniformly in the surfactant liquid. The resulting foam behaves like a viscous fluid with a very low density, which is what makes it useful for well operations: it can lift water out of a gas well, carry proppant into a fracture, or fill the annular space during cementing without the hydrostatic pressure that a conventional liquid column would impose.
Key Takeaways
- Foam quality is the volume fraction of gas in the foam, expressed as a percentage. A 70-quality foam is 70 percent gas and 30 percent liquid by volume. Drilling and cementing foams typically run at 50 to 80 quality. Higher quality foams are lighter but less stable.
- Foam generators produce consistent foam quality by mixing the gas and liquid at a controlled ratio and shearing them through a foam head, a series of restriction plates, or a packed element. Consistent quality is important because variable foam quality changes density and viscosity throughout the pumping operation.
- In gas well deliquification, liquid surfactant (foam sticks or foam concentrate) is sometimes injected into the wellbore without a foam generator; the foam forms naturally as the gas in the well agitates the liquid. In more controlled applications (foam fracturing, foam cement), a surface foam generator ensures consistent properties.
- Foam cement is used in low-pressure or depleted reservoirs where conventional cement slurry density would fracture the formation. The foam generator mixes nitrogen with a slurry that contains a foaming surfactant, reducing the final cement density to as low as 800 kg/m³ compared to 1,900 kg/m³ for conventional class G slurry.
- Foam fracturing uses nitrogen foam as the fracturing fluid in low-pressure, water-sensitive formations where a conventional water-based fracturing fluid would either fail to create the target fracture geometry or damage the relative permeability to gas. The foam generator is the critical quality-control point for the entire foam frac operation.
What a Foam Generator Does
Mix dish soap and water in a bowl. Whisk vigorously and foam forms: millions of tiny air bubbles wrapped in a thin film of soapy water. The whisking shears the air into small bubbles and the soap stabilizes each bubble by reducing the surface tension at the air-water interface. A foam generator does the same thing mechanically and continuously, at controlled pressures and flow rates.
Most oilfield foam generators have a manifold where nitrogen (or air, in some surface pipeline applications) and liquid foamer are combined at a preset ratio. The mixed stream then passes through a foam head, which is a short pipe section packed with a wire mesh, ceramic beads, or a series of orifice plates. The restriction forces the gas-liquid mixture through small openings at high velocity, shearing the gas into fine bubbles. The foam exits the downstream side of the generator at a consistent quality, ready for injection.
For foam cementing operations, the generator is positioned on the cementing truck or skid, receiving a nitrogen supply from a liquid nitrogen pumper and a cement slurry from the cement pumping unit. The three streams (cement, foamer additive, and nitrogen) combine at the generator at flow rates set by the design engineer, and the foam exits to the pump that drives it down the casing.
Fast Facts
Foam cement was developed in the 1970s as a solution to low-pressure and depleted zone cementing in Appalachian Basin gas wells in the eastern United States. Conventional cement slurry at 1,900 kg/m³ fractured the formation before it could set, creating channels for gas migration behind the casing. Foam cement at 1,100 to 1,300 kg/m³ matched the weak formation more closely, set without fracturing the zone, and provided better annular isolation. Today foam cement is used worldwide in similar low-pressure applications, including coalbed methane wells in Alberta and Queensland, shallow gas wells in the Bowland Shale in the UK, and geothermal wells.
Foam Fracturing Applications
Foam fracturing became an important technique in the Rocky Mountain region and the Canadian Prairies during the natural gas development of the 1990s and early 2000s. Shallow, low-pressure, tight gas formations in the Foothills and the Shallow Gas Play in Alberta were being completed with water-based hydraulic fracturing, and the water frequently damaged the gas relative permeability of the formation (water blocking). Switching to nitrogen foam as the fracturing fluid addressed the water-blocking problem because the foam left very little liquid behind in the formation after the treatment.
A foam fracturing treatment in a Cardium or Rock Creek tight gas well in west-central Alberta uses nitrogen at 60 to 80 quality, mixed with a small volume of gelled water carrying proppant. The foam generator on the blending equipment mixes the nitrogen and liquid at the target quality before the combined stream enters the high-pressure pump. Because nitrogen is compressible, foam quality changes with pressure throughout the treatment, and the foam generator's ratio control adjusts continuously to compensate.
In Queensland, Australia, foam fracturing has been used in coalbed methane (coal seam gas) wells where the coal matrix is sensitive to water and where near-zero reservoir pressure makes water cleanup from the fracture impractical. Santos-operated Spring Gully and Fairview fields in the Bowen Basin have used foam fracs in wells where conventional water-based stimulation would have created long cleanup periods before gas production could be established.
Synonyms and Related Terminology
A foam generator is sometimes called a foam head, a foam mixing skid, or a foaming unit in field language. Related terms include foam quality (the volume fraction of gas in a foam, expressed as a percentage; a fundamental design parameter that controls foam density and viscosity), foam cement (a lightweight cement slurry in which nitrogen gas is dispersed as bubbles by a foaming surfactant; used to cement low-pressure or depleted formations without fracturing the rock during the job), foam fracturing (a hydraulic fracturing technique that uses nitrogen foam as the primary fracturing fluid; suited to water-sensitive, low-pressure, or gas-producing formations where water-based fluids would damage relative permeability), deliquification (the set of methods used to remove accumulated liquid from a gas well that has loaded up; foam treatment is one of the simplest deliquification methods, requiring no downhole equipment in some applications), and surfactant (a surface-active agent that reduces the surface tension at the interface between two immiscible phases; in foam, the surfactant stabilizes the gas-liquid interface and prevents the bubbles from coalescing).
When the Foam Generator Failed Mid-Job on a Shallow Gas Cementing Operation
A service company was running a foam cement job on a shallow gas casing string in the Horseshoe Canyon coalbed methane play near Drumheller, Alberta. The well had been drilled into a pressure-depleted coal seam at 450 metres. Conventional cement at 1,860 kg/m³ would have fractured the seam and channeled to surface. The design called for 1,050 kg/m³ foam cement at 75-quality nitrogen.
Sixty percent of the way through the displacement, the foam generator's nitrogen inlet valve seized. Nitrogen flow stopped. The liquid cement slurry continued to pump without foaming. The job was completed with conventional-density cement occupying the upper portion of the annulus. A post-job cement bond log showed good bond in the foam cement zone but poor bond in the unfoamed zone, as the slurry had fractured a thin, low-pressure gas sand at 310 metres, creating a micro-annulus channel.
The well required a squeeze cementing job to seal the channel, adding CAD 35,000 to the completion cost. Review found that the nitrogen inlet valve had accumulated ice from moisture in the nitrogen supply line. Adding an inline nitrogen heater and dehydrator upstream of the foam generator was added to the standard equipment list for winter cementing operations in Alberta. A CAD 4,200 heater unit prevented a repeat event on the next 12 foam cement jobs run that winter.