Field Test of Tapered-Bore Chokes for Steam Flow Control
Authors: Griston Suzanne , Abate Tom 
Published: December 2007

In most heavy oil fields, steam is generated at large centralized plants and distributed through complex piping networks to numerous injection wells. Thornhill-Craver critical flow chokes are commonly used to control the steam rates to each injection well. Unfortunately, pressure loss across the chokes must exceed 40% to maintain critical flow conditions. Consequently, steam must be generated at a pressure greater than twice the required wellhead injection pressure.

In theory, critical flow conditions can be achieved with tapered-bore chokes with only 10% to 20% pressure loss. Although straight-bore and tapered-bore chokes follow the same critical flow principles, the tapered-bore design allows for a more efficient means of controlling flow rate. One obvious advantage of using tapered-bore chokes is the reduced steam generation pressure required to achieve and maintain critical flow. The pressure upstream of the tapered-bore choke need only be about 1.2 times the downstream wellhead pressure.

This paper examines the effectiveness of tapered-bore chokes for controlling steam rates, evaluates the accuracy of the Thornhill-Craver equation for calculating critical flow rates, and presents a set of practical guidelines and procedures for applying tapered-bore chokes to efficiently and cost- effectively control and monitor steam rates to injection wells.

Introduction

Controlling and monitoring steam flow to injection wells is an important element of steamflood reservoir management. Valves, nozzles, orifice plates and critical flow chokes are typically used to regulate steam flow. Of these devices, critical flow chokes have proven to be a very reliable and cost effective means of controlling wellhead steam rates.

A Thornhill-Craver choke with a removable straight-bore bean insert, shown in Figure 1, is the most commonly used choke configuration. The choke assembly consists of a 2-inch cage nipple (or bean housing) threaded into a tee junction that is equipped with a hammer union for safe insertion and removal of the critical flow bean. The drain plug is typically replaced with a blowdown valve prior to installation in the steam line.

Under critical flow conditions, steam flow rate depends only on upstream pressure, steam quality, and bean size. Hence, for a given steam quality, the flow rate can be controlled by setting the upstream pressure or the bean size. The main disadvantage of straight-bore chokes is the large pressure drop required. Hence, chokes are not typically used when the steam supply pressure is considered to be too low, or when the sandface injection pressure is considered to be too high, to establish and maintain critical flow conditions.

In theory, tapered-bore chokes can be used in place of straight-bore chokes to achieve critical flow with considerably less pressure loss. A conventional Thornhill-Craver choke assembly is used, along with a tapered-bore bean insert, shown in Figure 2. Critical flow is achieved in the bean throat, but, the tapered exit section acts as a diffuser that promotes pressure recovery. Steam pressure exiting the choke can be as high as 80% to 85% of the upstream pressure.

Critical Flow Principles

By definition, critical flow of a compressible fluid occurs when the fluid reaches sonic velocity in the bean throat. A shock wave develops and the flow is said to be choked.

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