Estimation of flow coefficient for subsonic natural gas flow through orifice-type chokes using a simple method Alireza Bahadori * Southern Cross University, School of Environment, Science and Engineering, PO Box 157, Lismore, NSW 2480, Australia article info Article history: Received 21 March 2012 Received in revised form 3 May 2012 Accepted 16 May 2012 Available online 26 June 2012 Keywords: Orifice-type choke Vandermonde matrix Choke flow coefficient Natural gas Predictive tool abstract Choke valves are critical elements in gas production facilities. Malfunction or failure of a choke valve can seriously affect safety, the environment and production rates. Choke is a device installed at wellhead or down hole to control the flow rate for maintaining sufficient back pressure to prevent formation damage, to prevent water/gas coning, to stabilize the flow rate and to produce the reservoir at the most efficient possible rate. In this work a simple-to-use method is developed to estimate the choke flow coefficient for natural gas subsonic flow through orifice-type chokes as a function of Reynolds number and the ratio of choke diameter to pipe diameter. The results can be used in follow-up calculations for rapid estimating gas passage through a choke under subsonic flow conditions. Estimations are found to be in excellent agreement with reported data in the literature with average absolute deviation being less than 0.4%. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Chokes are one of the most important flow controllers in oil and gas producing wells (Bahadori, 2012a). Choke valves are severe service valves which are designed specifically for oil and gas wellhead applications, both in a surface and subsurface context (Morris, 1990). They are used for controlling the flow on production, reinjection and subsurface wellheads. Choke valves are subjected to typical wellhead extreme conditions which can cause erosion, corrosion and other damage (Morris, 1990). Typically this can include high fluid velocity, slugging, sand production and multiphase of oil, gases and water. Also a choke valve has to have a very high turndown capability as it has to cover a wide range of flow rates. Thus the design of Choke valves is required to be very robust with careful selection of valve configu- ration, flow path profiles, materials and ease of maintenance (Keith and Crowl, 2005). Accurate modeling of choke performance and selection of optimum choke size is vitally important for an engineer in production from oil and gas reservoirs due to high sensitivity of oil and gas production to choke size (Bahadori, 2012a). The choked flow of a flowing gas is a limiting point which occurs under specific conditions when a gas at a certain pressure and temperature flows through a restriction into a lower pressure environment for meeting limitations of rate or pressure imposed by production facilities and satisfying production limits set by regu- latory authorities (Chong et al., 2009; Al-Attar, 2008). Flow through a surface choke can be described as either critical or sub-critical. Critical flow occurs when the velocity through the choke is greater than the sonic velocity of the fluid (Bahadori, 2012a). These result in a Mach number of the fluid that is greater than or equal to one. For fluids with a velocity greater than sonic velocity, any downstream perturbation is unable to propagate upstream and the mass flow rate through the choke is solely a function of the upstream parameters, in other words, in a critical flow region, the mass flow rate reaches a maximum value that is independent of a pressure drop applied across the choke (Grace and Frawley, 2011). Therefore, once critical flow is reached, any disturbance introduced downstream of the choke will have no effect on upstream conditions (Nøkleberg and Søntvedt, 1998). Conversely, in sub-critical flow, the flow rate depends on pressure difference across the choke and changes in the downstream pressure affect the upstream pressure. Thus pressure drop across well chokes is usually very significant (Schüller et al., 2006). There is no universal equation for predicting pressure drop across the chokes for all types of production fluids. Different choke flow models are available from literature, and they have to be chosen based on the gas fraction in the fluid and flow regimes, that is, subsonic or sonic flow (Nøkleberg and Søntvedt, 1995). It is important to note that although the gas velocity * Tel.: þ61 2 6626 9412. E-mail address: Alireza.bahadori@scu.edu.au. Contents lists available at SciVerse ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse 1875-5100/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jngse.2012.05.005 Journal of Natural Gas Science and Engineering 9 (2012) 39e44