|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
A wellhead choke controls the surface pressure and production rate from a well. Chokes usually are selected so that fluctuations in the line pressure downstream of the choke have no effect on the production rate. This requires that flow through the choke be at critical flow conditions. Under critical flow conditions, the flow rate is a function of the upstream or tubing pressure only. For this condition to occur, the downstream pressure must be approximately 0.55 or less of the tubing pressure.
Hong, Bingyuan (China University of Petroleum-Beijing) | Song, Shangfei (China University of Petroleum-Beijing) | Wu, Haihao (China University of Petroleum-Beijing) | Li, Xiaoping (China University of Petroleum-Beijing) | Wang, Zhi (Xi'an Changqing science and Technology Engineering Co. Ltd.) | Gao, Jingjing (China University of Petroleum-Beijing) | Gong, Jing (China University of Petroleum-Beijing)
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 actual 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).
Abstract Chokes are used to limit production rates to meet sale contract, comply with regulations, protect surface equipment from wearing out, avoid sand problems due to high drawdown, and control flow rate limited by capacity of the facility. Single gas phase flow through choke is vital to oil industry because not only an accurate estimation of gas flow rate guarantees a reliable supply to the end users, thus the predictable revenue from gas sale for the company, but also protect the equipment from breaking as a result of high gas rate. Nevertheless, importance of gas metering cannot be overemphasized. Gas flow through choke had been studied by numerous investigators, Different choke flow models are available from the literature, and they have to be chosen based on the flow regimes, that is, subsonic or sonic flow. The most common used flow equations developed by Shapiro, Zucrow and Hofmann are used for subsonic and sonic flow, respectively. Sonic flow happens when downstream to upstream pressure ratio is equal to critical pressure ratio. A careful review of these equations indicated that they are not theoretically rigorous and give inaccurate gas flow rate for the real gas. Thus these equations need to be modified in order to be used to calculate gas flow rate under both flow regimes. After a thoroughly analysis and derivation we came up with equations that have solid base. New correlations that reconcile the issue caused by approximation method used to derive the old gas flow equations were based on both engineering judgment and physical phenomenon. The error in the old equation can be corrected with the new equations. New equations provide good approaches to quantify gas flow through choke.
Abstract Gas transient flow in pipeline and gas tank is critical in flow assurance. Not only leak detection requires a delicate model to simulate the complicated yet drastically changed phenomena, but also pipeline and tank design in the metering, gathering, and transportation system demands an accurate analysis of gas transient flow, through which efficient, cost-effective operation can be achieved. Traditionally there are two types of approaches to investigate gas transient flow: one is treating gas as ideal gas so that ideal gas law can be applied; another is considering gas as real gas thus gas compressibility factor comes into play. Needless to say, the former method can result in an analytical solution to gas transient flow yet with a deviation from the real gas performance, which is very crucial in daily operation. The latter approach needs numerical method to solve the governing equation, thus leads to unstable issue but with more accurate result. Our literature review indicated that no study considered the effect of changing gas viscosity on the transient flow is available. Therefore, this effect was included in our study. Our investigation showed that viscosity does have significant influence on gas transient flow in pipe and tank leakage evaluation. In this study, a comprehensive evaluation of all variables was done to find out the most important factors in the gas transient flow. Several case studies were used to illustrate the significant of this study. Engineers can do a more reliable evaluation on gas transient flow by following the method we used in our study.
Wet steam, being a two-phase fluid, has complex critical flow properties. Analytical and experimental data were used to investigate the critical steam flux, and the pressure ratio needed to achieve critical flow for a variety of popular flow-rate controlling devices. Three new parameters -- critical steam flux ratio, dimensionless critical steam flux, and the Napier parameter -- are introduced to correlate the critical flow data. Attention was focused on the effects of stagnation pressure, stagnation quality and design of the flow-rate control device on the critical flux. The study covered stagnation pressures from 200 to 2100 psia and stagnation qualities from 20 to 100%.
Critical flow refers to a situation where any further reduction in the downstream pressure of the flow fails to induce a corresponding increase in the mass flow rate. In other words, critical flow is a phenomenon whereby the flow rate has an upper limit for a given flow-controlling device and the fluid's upstream or stagnation condition.
In steam-enhanced oil recovery (EOR) operations, the flow rate of steam is usually controlled by a flow controlling device, such as an orifice, nozzle, venturi, choke, adjustable valve, etc. A constant flow rate of such devices can be maintained for a varying downstream pressure when the flow through the devices is under critical flow conditions.
Most steam used in EOR operations is a wet steam whose flow properties are more complex than single-phase fluids. In this study, attention was given to the effect of stagnation pressure, quality and the design of the flow controlling device on the critical flow properties. For representative field applications, this study covered steam pressures ranging from 200 to 2100 psia [1378 to 14500 kPa] and steam qualities from 20 to 100%. Flow-controlling devices discussed in the experiments include orifices, nozzles, venturi, static chokes and adjustable chokes. New parameters are proposed to correlate critical flux data. Results from this study offer a better understanding of the critical flow properties of steam so as to improve steam management for steam flooding or stimulation operations, and to explore the properties that can be used as a basis for developing new and innovative controlling and measuring devices for steam quality and steam flow rate.
ANALYTICAL RESULTS OF THE CRITICAL FLOW PROPERTIES OF WET STEAM
Various models have been proposed to describe the critical flow behavior of steam. The homogeneous equilibrium model (HEM) is a simple model that gives a good representation of the critical flow properties of steam. It is used as the basis of this study.
Understanding the thermodynamic properties of steam at critical flow conditions is important to the design, operation and management of steam distribution network and hence the cost of production in steam-enhanced recovery projects. Using the homogeneous equilibrium model, the properties of steam at critical flow properties of steam at critical flow conditions are presented. These properties include critical velocity, critical pressure, critical temperature, critical pressure, critical temperature, critical steam quality, critical specific volume, critical steam flux and Napier's parameter. The critical velocity, Napier's parameter and the dimensionless form of other properties are shown to be functions of the properties are shown to be functions of the stagnation steam quality. The steam pressure is shown to have a slight effect on pressure is shown to have a slight effect on some of the properties. Empirical correlations for these properties are established for selected operating ranges and experimental results are presented to verify the model's predictions.
In steam-enhanced oil recovery operations, the flow rate of steam is usually controlled by a flow-rate controlling device. Flow restrictions, such as nozzles, venturi, chokes and valves can be used to maintain a constant flow rate when the flow through such restrictions is under critical flow conditions. Critical flow refers to a situation where any change in the downstream pressure of the flow fails to induce a pressure of the flow fails to induce a corresponding change to accelerate the mass flow rate. In other words, critical flow is a phenomenon whereby the flow rate has an upper limit for a given set of upstream or stagnation conditions.
Where steam exists as a two-phase fluid, it consists of a saturated liquid and a saturated vapor. In this state, it behaves differently from a single-phase fluid. various models have been proposed to describe the critical flow behaviors of steam. The homogeneous equilibrium model is a simple model that gives a good representation of the critical flow of steam. Results of this model are used in this study. These properties include critical velocity, critical pressure, critical temperature, critical specific volume, critical steam quality and critical steam mass flux.
The critical pressure, critical temperature, critical specific volume, critical steam quality are made dimensionless by dividing each with the corresponding property at the stagnation condition. The critical steam flux is made dimensionless using dimensionless critical steam flux and a new parameter, Napier's parameter, is introduced parameter, Napier's parameter, is introduced as a correlation parameter for the critical flow of steam.
These properties are plotted as a function of the stagnation steam quality using the steam pressure as a parameter. Their results are discussed and empirical correlations of these properties are established for selected operating ranges. Experimental results are presented to compare the critical pressure ratio, the dimensionless critical steam flux and Napier's parameter with the predictions of the homogenous equilibrium model.
Two-phase flow through wellhead chokes, including both critical and subcritical flow and the boundary between them, was studied. Data were gathered for air-water and air-kerosene flows through five choke diameters from 1/4 in. (6.35 mm) to 1/2 in. (12.7 mm), and results were compared to published correlations. A new theoretical model for predicting flow rates and the critical-subcritical flow boundary was tested against these data, as well as data from two published studies. The new model substantially improves the existing methods for predicting choke behavior in two-phase flow.
Chokes are widely used in the petroleum industry to protect surface processing equipment from slugging, to protect surface processing equipment from slugging, to control flow rates from wells, to provide the necessary backpressure to a reservoir to avoid formation damage from excessive drawdown, to maintain stable pressure downstream from the choke and dampen large pressure fluctuations.
Either critical or subcritical flow may exist. Since different methods apply for predicting choke behavior in these regimes, the prediction of the critical-subcritical flow boundary is also important. The majority of correlations available apply to critical flow only. Pressure drops through chokes can be substantial. For example, in critical flow the pressure downstream from the choke may be as low as pressure downstream from the choke may be as low as 50% or even 5% of the upstream pressure. Modern techniques, like Nodal* Analysis, of analyzing the entire production system require two-phase models of production system require two-phase models of comparable accuracy for each system component. Thus, to optimize the performance of the entire production system, an improved two-phase choke model is required.
For the purpose of modeling, a wellhead choke can be treated as a restriction in a pipe. Two types of two-phase flow can exist in a choke: critical and subcritical flow. During critical flow, the flow rate through the choke reaches a maximum value with respect to the prevailing upstream conditions. The velocity of the fluids flowing through the restriction reaches the sonic or pressure wave propagation velocity for the two-phase fluid. This implies that the flow "choked" because downstream disturbances cannot propgate upstream. Therefore, decreasing the downstream propgate upstream. Therefore, decreasing the downstream pressure does not increase the flow rate. If the pressure does not increase the flow rate. If the downstream pressure is gradually increased, there Will be no change in either the flow rate or the upstream pressure until the critical-subcritical flow boundary pressure until the critical-subcritical flow boundary is reached. If the downstream pressure is increased slightly beyond the boundary conditions, both flow rate and upstream pressure are affected. The velocities of fluids passing through the choke drop below the sonic velocity of the upstream fluids. Here, the flow rate depends on the pressure differential and changes in the downstream pressure affect the upstream pressure. This behavior characterizes subcritical pressure. This behavior characterizes subcritical flow.
Although it is often desirable to operate wells under critical flow conditions with uniform flow rate and downstream pressure, Fortunate' reports that a majority of wells in the field operate under subcritical conditions. However, most of the correlations available to petroleum industry are for critical flow.
Existing Methods A complete model for two-phase flow through chokes should define the boundary between the critical and subcritical flow regimes and predict the functional relationships of flow rate through the choke and the pressure differential across the choke for a given set of fluid properties and flow conditions. Most existing methods model critical flow only and a few even attempt to define the criticalsubcritical flow boundary. These models are surveyed.
A multiphase flow equation describing the behavior of orifice flow may be used directly to evaluate well performance as a function of choke size; upstream choke pressure; choke temperature; producing and solution GOR; gas, oil, and water gravities; and a discharge coefficient. The coefficient compensates for nonideal factors excluded in the development of the equation and relates theoretical oil production rates through chokes to field-measured rates. Discussion Various developments have been published that present theory and correlations for describing simultaneous liquid and gas flow though a restrictive orifice. The correlations of Poettmann and Beck were intended to aid in the prediction of gas-liquid flow through chokes. Their development followed the original presentation by Ros and was derived for an average orifice discharge coefficient. Poettmann and Beck considered the polytropic expansion of the gaseous phase of the fluid expanding through the choke. The polytropic expansion theory was used successfully by Ros and is perhaps the most rigorous development in the application of orifice flow theory to oilfield conditions. Basically, the derivation of any orifice relationship is dependent on two main criteria. First, an expression must be written relating the flowing fluid specific volume and velocity to the mass flow rate. Second, an independent equation must be written incorporating the behavior of the gaseous phase of the fluid with pressure. The above stipulations are met by the following relationships. The energy balance around a fluid flowing through an orifice may be written as The polytropic expansion equation relating the specific volume of the gas, (Vf - Vl), to the confining pressure, p, the polytropic expansion constant, b, and the ratio p, the polytropic expansion constant, b, and the ratio of specific heat at constant pressure to specific heat at constant volume, n, is When an expression for the orifice velocity, v2, is achieved with Eqs. 1 and 2, a relationship for the mass rate of flow through the choke is achieved by using the resulting relationship: where C is the orifice discharge coefficient, and Subscript 2 denotes downstream orifice throat conditions. The solution of Eqs. 1 through 3 is given in Appendix A. The results are summarized below. For critical orifice flow, the critical pressure ratio, Ec, defined as the ratio of the upstream pressure to the downstream choke pressure, occurs when The condition following from the solution of Eqs. 1 and 3 for flow is JPT P. 843