Foam Flow Modelling for UBD Applications
Authors: Gregory G.A. 
Published: March 2010

Introduction
While naturally occurring foaming crude oils can present unwanted operating problems in production facilities, specially created foams have been used for a variety of applications in the petroleum industry for many years. These include, but are not limited to, foam drilling fluids, foam stimulation fluids, and foams used for controlling the mobility of various other fluids and solids. More recently, interest in the properties and behaviour of foams has increased significantly due to the use of foam drilling fluids in underbalanced drilling (UBD). In UBD, the bottomhole pressure is intentionally maintained lower than the reservoir pressure during the drilling operations. This underbalanced state is usually achieved by using a gas-liquid mixture in place of traditional (and much heavier) drilling mud. The combination of low density (i.e., low pressure loss due to hydrostatic head) and high viscosity (i.e., good capacity for cuttings transport) makes foam a strong candidate as a UBD drilling fluid.
Because of the technical complexity of UBD, the success of such operations can depend on the accuracy of a detailed engineering analysis that is typically carried out both before and during the actual drilling. Accurate computer modelling of the hydraulics of flow in the drill string, the bottomhole assembly (BHA), and the return annulus is a critical component of this design and performance analysis process. When foam is used as the drilling fluid, this generally means that complex questions of foam rheology and flow behaviour must be answered using technology which is simple and can be applied with relatively limited measured data.
A very important component of the modelling process, from a cuttings transport perspective, is the determination of the local foam velocity in the entire return annulus, due to changes in the cross-sectional area (e.g., between the BHA and the open hole, the open hole-drill tubing, presence of casings). Since this parameter can (and does) change during drilling because of reservoir fluid influx, the modelling must be an ongoing part of the monitoring.
This paper examines the extent to which relatively simple methods, based on limited data, can be applied to give the accurate performance predictions required for the planning of complex underbalanced drilling operations. Some of the basic concepts of foam rheology are introduced, and some recent work based on detailed measurements gathered during a UBD pilot study is reviewed, along with several applicable technologies.

Foam Rheology
Foam is created by mixing gas, liquid, and a foaming agent (i.e., surfactant). The latter stabilizes the foam by reducing the gas-liquid surface tension. The liquid phase is frequently an aqueous solution of one or more polymer materials that are used to increase the effective viscosity of the resulting foam over that obtained with pure water.
For modelling purposes, foam is generally treated as a single phase fluid using a homogeneous mixture density and an effective viscosity. The latter is computed using a yield-power law model (frequently referred to as a Herschel-Bulkley model), over some range of conditions.