Abstract

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.

Introduction

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.

THEORY

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.

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