Petroleum foam

Oil foam.

Petroleum foam or oil foam or simply foam in oilfield typically refers to the foam that forms in hydrocarbon production processing separators as a result of forced and rapid oil/gas separation. Foaming risk is related to crude oil properties - density, bulk viscosity, surface tension, asphaltene and resin contents and gas properties play a role as well.

Depending on the nature of the crude oil and the type of separation scheme used, foaming problems can curtail crude oil production and even cause unwanted and unexpected process shutdowns.

Foam formation

In its natural state crude oil contains dissolved gases, held by virtue of high reservoir pressure. When this live crude oil is produced it passes into lower pressure environments, eg a gas oil separator where the dissolved gases are liberated. Gas is taken from the top of the separator, and stabilised crude oil is taken from the bottom. Foam can form during this process, depending on the oil and gas properties.

Foam stability

Interfaces present in an oil foam structures.

Two opposing mechanisms contribute to crude oil foam stability. The destructive mechanism is the drainage of liquid, which proceeds via migration from foam film to Plateau border region as a result of the pressure gradient between these two zones, and from Plateau border regions under the influence of gravity.

The constructive mechanism relates to the ability of the foam film to respond to the effects of film thinning and avoid rupture. Rupture occurs as a consequence of unchecked liquid drainage and/or mechanical agitation. For the constructive mechanism to operate successfully, the rate of surface diffusion and/or bulk to surface diffusion must exceed the rate of film drainage. If bulk to surface diffusion occurs before in-surface diffusion, then the gap between the bubble walls which has thinned will remain thin. In-surface diffusion not only restores the surface film but drags fluid back into the gap and thickens it again conferring a higher stability to the foam.

Liquid drainage is the dominant mechanism in unstable foams.

Production problems associated with foam

Crude oil foam in two- and three-phase separators creates operational problems:

• Poor level control that can lead to platform shutdowns
• Liquid carryover in the gas outlet that can lead to flooding of downstream scrubbers and compressors
• Gas carryunder in the liquid outlet that can lead to increased compression requirements.

Test method

Apparatus for foaming tendency test.

The simplest apparatus to evaluate foams is the gas sparging test. This method can be used to derive two parameters: foaminess index and average foam lifetime. The foaminess index is the ratio of the foam volume generated by a given flow rate to that flow rate\[Foamindex = \frac{Foamvolume}{Flowrate}\] This parameter is independent of flow rate and has the dimension of time. It is a measure of foam forming tendency. The average foam lifetime of a foam is measured by allowing an appropriate volume of foam to be formed before cutting the supply of filler gas. The graduations on the sparging tube (and a stop watch) are then used to measure foam height time data points as the foam collapses. Most studies of the crude oil foaming properties have focused on the average foam lifetime parameter.

The major disadvantage of this simple gas sparging test is that it does not reflect the high pressure field situation in a gas oil separator. In order to replicate the foaming in the separator, a high pressure foaming device needs to be utilized. Crudes having viscosities at or above 150 cP at 37.8 °C produce little or no foam. Thus, the sparge method run at room temperature is not adequate to study crudes of this nature.

Factors impacting foaming tendency

Crude oil viscosity

In view of the mode of foam collapse, any factor that slows the rate of drainage of liquid from a foam will enhance its stability. Therefore, the higher the crude oil viscosity, the higher the foam stability.

Gas composition

The leaner the gas, the higher the diffusion rate and the less stable the foam is.

Asphaltene content

The more asphaltenes, the stabler the foam can be.

Surface tenstion

The higher the surface tension, the stabler the foam is.

Foam control

Chemical

In order to mitigate the foam problems, chemical defoamers or anti-foamers, for example silicone or fluro-siliconed based, are typically injected upstream of separators, typically at a dosage level of 5–10 ppmv.

Mechanical

• Proper separator sizing
• Separator internal structures
  • Transverse Baffles:located at right angles to the direction of flow, they act as skimmers to hold foam back in the upper part of the separator. They are fairly common, and often have perforations in them to allow oil to pass axially along the vessel.
  • Parallel Plates and Vane Packs (Axial):These are very common. The basic principle of operation of the devices is that the separating fluids are forced to flow in thin layers or channels, giving increased separation surface area, and some mechanical enhancement of foam breaking. The orientation of the devices within the vessel varies, but the bulk flow of fluid is normally along the vessel axis.
  • Packing:Random packings, similar to those used in mass transfer columns, can assist in breaking crude oil foams, or in preventing their formation. The packings are normally retained in mesh cages within the separator.
  • Wire Mesh Pads:These are commonly installed at the gas outlet of separators, to act as demisters, removing any entrained droplets of liquid from the gas stream.Special meshes, or lower bulk density, are installed to promote foam collapse. These are situated in the vessel headspace, upstream of the gas demister pads.
• Inlet cyclone
• Gas outlet axial flow demisting cyclones