About Chokes in Oil and Gas wells

A well is normally producing the fluid with a choke at the surface to control the flow rate. Most flowing wells have surface chokes for the following reasons:
· to reduce the pressure and improve safety
· to maintain a fixed allowable production limit
· to adjust the desired rate to the optimum as it is match with the well’s productivity and the predicted production plan
· to match the surface pressure of a well into a multi-well gathering line and to prevent back flow
There are several different types of chokes currently in use. They may be divided into two broad categories: variable or adjustable chokes and positive or fixed orifice.
Positive chokes have a fixed orifice dimension which may be replaceable and is usually of the bean type. The flow path is normally symmetric and circular. Fixed orifice chokes are commonly used when the flow rate is expected to remain steady over an extended time.

Normal beans are 6 inches long and are drilled in fractional increments of th-inch up to -inch. Smaller bean inserts, known as X-type, are used to provide closer control. Ceramic, tungsten carbide, and stainless steel beans are used where sand or corrosive fluids are produced. Changing the size of a fixed orifice choke normally requires shutting off flow, removing and replacing the bean. Some continuously variable or adjustable chokes operate similarly to a needle valve and allow the orifice size to be varied through a range from no flow to flow through a full opening.

Controlling the flow is obtained by turning the hand wheel which opens or closes the valve. Graduated stem markings indicate the equivalent diameter of the valve opening. Another type uses two circular discs, each of which has a pair of orifices. One disc is fixed while the other can be rotated so as to expose the desired flow area or block the flow altogether.

It is important in the design of the surface control system to understand the pressure versus flow rate performance of the choke at critical flow rates.
We can use some software like WellFlo, Reo, and Gap to modelling the choke performace.
In my experience it is important to consider the compositional properties of fluid when designing the choke type. I have ever done to do choke replacement due to leaking that caused by the abrasive fluid.