This paper aims to study the application of choke models as a part of virtual metering system (VMS) to predict the flow rate of natural gas wells. In this paper, three semi-empirical choke models, including Perkins, Sachdeva and Al-Safran, are investigated. Combining with the 12424 groups azctual production data obtained by flowmeters of A1 and A2 wells in the underwater gas field in the South China Sea, the flow coefficients are defined to modify those models. The results show great agreement with field production data and the results of wellbore model (a calculation module of the VMS). In addition, Perkins model of which the average error is 8.23% and the standard deviation is 5.48% is more accurate relative to Sachdeva and Al-Safran model. Considering the error of flowmeter and the fluctuation of production data, those choke models can be developed as a part of VMS to predict natural gas flow rate.
With the further development of the oil and gas industry moving to the seas continuously, underwater oil and gas production process have been born. Achieving continuous measurement of oil and gas wells is one of the most basic technical requirements during its producing processes. The harsh natural environment in the deep sea makes it difficult for the flowmeter to be installed and it costs a fortune to maintain (Varyan et al., 2015). As the alternative, a relatively new single well metering management software, virtual metering system (VMS), has been gradually adopted in the domestic and foreign offshore oil and gas field production systems (Bello et al., 2014). The flow rate of oil and gas in the single well can be calculated out through several different kinds of modules with VMS technology utilizing the field basic process parameters and the real-time instrumental data obtained from the Data Communication System (DCS). VMS technology is highly integrated, which provides great convenience for the operators. In contrast to traditional techniques, VMS is more convenient in operation as its highly integration, cheaper for installation and maintenance (Paz et al., 2010), (Petukhov et al., 2011). Another advantage of VMS is that it can be employed in combination with real-time flow management system. Hence, VMS can be used as a replacement of traditional multiphase metering equipment or as a supplementary or backup scheme of an entity flowmeter in an offshore gas field (Wu et al., 2015a). Some technology companies in this area have developed a variety of corresponding systems which were successfully used and achieved good results in some deep-water oil and gas fields in North Sea, Mexico Bay and West Africa, such as the FAS system of FMC company, OLGA online system of SPT company, ISIS system of BP, WPM system of TOTAL (Wang et al., 2015). The research in this area has just begun in China. Our group is researching and developing a flow monitoring and management system relying on multiphase flow simulation technology. In January 2014, this system based on wellbore model was successfully put into use in a certain gas field in the South China Sea (Wu et al., 2015b) (Wang et al., 2014). After nearly three years of operation, it turned out that the VMS system has been running smoothly and the hardware malfunctions never happened. The individual flow and total flow calculated by VMS are in good agreement with the measurement results of the flowmeter on the platform. Our VMS meets the accuracy requirements of engineering practical production, thus creating considerable economic benefits and value (Wang et al., 2013).