Foam assisted lift is a deliquification method in the oil and gas industry, which aims to prevent or postpone countercurrent gas-liquid flow in maturing gas wells or to assist in removing downhole accumulated liquids. The creation of foam reduces the density of the liquid that needs to be transported and postpones the transition from annular to churn flow to lower gas velocities, which improves the upward transport of liquid carried by foam. This paper presents a model predicting the foam flow behavior in annular flow conditions down to the transition from annular to churn flow, with which the onset of liquid loading is computed.
Liquid loading is a well-known problem that occurs when ‘wet’ gas wells approach their end-of-life. Due to depletion of the reservoir, its pressure drops and as a consequence the gas flow rate in the well drops as well. When only dry gas is present in the well, the reduced gas flow rates have no further effect. However, when a liquid phase is present (either production fluids or condensed fluids) the reduced gas flow rates may, at some point, not be effective in transporting the liquids to topside and they will accumulate downhole (liquid loading). The critical velocity, Ucrit, determines the onset of liquid loading.
When a well is loaded, it may still produce at a low metastable rate, produce intermittently or may not produce at all. Foam Assisted Lift (FAL) is one of the possible methods to remove the downhole accumulated liquids (deliquification) and/or improve the production. In order to design a suitable FAL-system, the prediction of foam flow is a prerequisite.
An average reduction of Ucrit by the application of a surfactant is about 50%, refs. (1), (2). This value is based on field experience and taken as a rule of thumb in foamer applications. In this paper, a model is presented that aims to give a more detailed description of the effect of foamers on the flow.