The effect of surfactants on air–water annular and churn flow in vertical pipes. Part 2: Liquid holdup and pressure gradient dynamics A.T. van Nimwegen a , L.M. Portela a,⇑ , R.A.W.M. Henkes b,c a Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, Delft, The Netherlands b Department of Process & Energy, Delft University of Technology, Mekelweg 2, Delft, The Netherlands c Shell Projects & Technology, Grasweg 6, Amsterdam, The Netherlands article info Article history: Received 26 July 2013 Received in revised form 28 February 2014 Accepted 27 March 2014 Available online xxxx Keywords: Annular flow Churn flow Foam Liquid loading Gas-well deliquification abstract In this paper, we study the effect of surfactants on both the liquid holdup and the dynamics of the pressure gradient in annular and churn flow in vertical pipes. This effect is linked to the influence of the surfactants on the morphology of the air–water interface, which is studied in a related paper (van Nimwegen et al., 2014). The experimental results, obtained using a vertical flow loop with a 5 cm internal diameter at ambient pressure, show three different effects of the surfactants on the measured quantities, depending on the air and water flow rates. (i) At large air flow rates, in the annular flow regime for air–water flow, the surfactants increase the pressure gradient; this is solely due to the increase of the frictional pressure gradient, caused by the larger interfacial stress between the foamy waves overlaying the foam substrate along the wall and the gas core. (ii) At low air flow rates and low water flow rates, in the churn flow regime for air–water flow, the surfactants decrease significantly both the average pressure gradient and the pressure gradient fluctuations. While the frictional pressure gradient increases, the liquid holdup decreases by more than a factor of two; the foam suppresses the flooding waves, making the flow much more regular, leading to the small pressure gradient fluctuations. Furthermore, there exists an optimum surfactant concentration for decreasing the average pressure gradient and the pressure gradient fluctuations. (iii) At low air and high water flow rates, in the churn flow regime for air–water flow, the average pressure gradient and the pressure gradient fluctuations are both somewhat decreased by the surfactants. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Surfactants are molecules with a hydrophobic tail and a hydro- philic head, causing them to absorb preferentially at an interface between a polar and an apolar medium, such as water and air. This property makes surfactants suitable as emulsifiers, wetting agents or foaming agents (Farn, 2006). In the latter application, they are used to increase the total production from natural gas reservoirs: the surfactants influence the gas–liquid flow in the well tubing, decreasing the minimum gas velocity required to lift the liquid to the surface. Gas production at low reservoir pressure can be severely impeded by the accumulation of liquid at the bottom of the well, which occurs when the gas velocity is insufficient to drag the liquid to the surface. This liquid subsequently exerts a hydrostatic pres- sure on the reservoir, severely limiting or even prohibiting the gas production (Lea et al., 2003). This phenomenon is called liquid loading. It is closely related to the reversal of the flow of the liquid film at the wall of the well tubing, see e.g. Veeken and Belfroid (2011). To explain liquid loading, we consider a gas well as a system of two components placed in series. First, the gas and the liquid flow from the reservoir to the bottom of the well. In Fig. 1, this com- ponent is indicated by the reservoir curve (also known as the Inflow Performance Relation). Increasing the pressure at the bot- tom of the well (the bottom-hole pressure), decreases the flow rate from the reservoir. The second component of the system is the flow of the gas through the well tubing to the surface. The relation between the bottom-hole pressure and the gas flow rate in the well tubing is given by the Tubing Performance Curve (TPC). Since the pressure at the top of the well is fixed, the bot- tom-hole pressure is proportional to the pressure gradient of the multiphase flow along the well tubing. The TPC has a mini- mum: larger gas flow rates cause a large frictional pressure gradi- ent, while smaller gas flow rates lead to liquid loading and a large hydrostatic head. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2014.03.007 0301-9322/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +31 152782842. E-mail addresses: a.t.vannimwegen@tudelft.nl (A.T. van Nimwegen), l.portela@ tudelft.nl (L.M. Portela), r.a.w.m.henkes@tudelft.nl (R.A.W.M. Henkes). International Journal of Multiphase Flow xxx (2014) xxx–xxx Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow Please cite this article in press as: van Nimwegen, A.T., et al. The effect of surfactants on air–water annular and churn flow in vertical pipes. Part 2: Liquid holdup and pressure gradient dynamics. Int. J. Multiphase Flow (2014), http://dx.doi.org/10.1016/j.ijmultiphaseflow.2014.03.007