Okay, so some of you have looked at my picture and said “What on earth is that thing in your hand?”
It is a choke used in oil and gas wells. Chokes are usually used in relatively high-rate wells. They have reduced diameters compared to the tubing and/or flowlines and cause a pressure drop. As we remember from our first petroleum engineering class, flow rates will decrease if we introduce larger pressure drops in the flowlines.
Chokes can be introduced at the surface or downhole. This particular choke is huge by typical oilfield standards. It has a 2″ internal diameter. Choke sizes (in the US) are usually expressed in 64″ of an inch while they are generally referred to in millimeters elsewhere. So a 14/64 choke has an ID of 14/64 of an inch.
This particular choke was one of two chokes on the well. The well was completed with 4″ tubing; production was split in half at the wellhead and each flowline had a 2″ choke. The internal area of the two chokes added up to only about half of the tubing area. This flow restriction was required in order to control the very high rates of gas flow from the well so as not to overcome the production facilities. if you inspect the internal portion of the choke you can see significant erosional grooves in the choke. These were caused by the high gas velocities. The first chokes installed on the well only lasted a few hours. As the reservoir pressure dropped, the chokes lasted longer and eventually were unnecessary. It was easy to tell when erosion was occurring as the pressure drop across the chokes were decreasing and the flow rates were increasing. Chokes can also be eroded or plugged by sand or other solids production. The interior surfaces of a choke are often coated with erosion resistant materials to minimize erosion.
Production from a gas well is generally sold via pipeline; however, it is often processed prior to transportation to remove condensate, water, H2S, etc. High-pressure gas wells use chokes to gradually lower gas stream pressures to facility or pipeline pressures. Sometimes chokes are used in wells to maintain a certain level of pressure drop, resulting in more stable production or to prevent freezing at the production facilities. In many gas wells, Joule-Thomson cooling can be very substantial and cause a well to “freeze up.” You can imagine Joule-Thomson cooling easily with a simple experiment. If you hold your palm in front of your mouth with your mouth as wide open as possible and exhale, you will feel that your breath is warm, nearly the same temperature as it was in your lungs. With your mouth wide open there is very little pressure drop. Now purse your lips and blow onto your palm and the air is cooler. The same thing can happen with a choke in a flowline. Because produced natural gas contains water vapor, the flowline can actually clog with ice. Producers either run the gas through a heater (that burns natural gas and heats the flowline and its contents) or inject methanol to suppress freezing.
The Joule-Thomson affect does not always cause temperature drops with pressure drops. At very high pressures and temperatures, typical natural gases may actually warm with pressure drops. At room conditions hydrogen, helium and neon warm with such pressure drops. However, the majority of oilfield situations result in cooling with pressure drops. Because this cooling can be very substantial, operators can place chokes downhole in the tubing string. Taking some of the pressure drop downhole can result in improved safety and lower operational risks by decreasing the likelihood of surface freezing. In many cases downhole chokes can be recovered easily from the surface when the pressure drop is no longer desirable.
Baker Hughes makes a packer-type bridge plug design makes the tool capable of setting anywhere in the tubing string without requiring nipples for location. It can be set on slickline, electric line, or coiled tubing and retrieved on slickline. The design accepts chokes and other devices, such as a blanking plug, check valve, instrument hanger, and tubing-conveyed perforating gun hanger.
Chokes are relatively simple technology. Baker Hughes provides some advanced choking capabilities that can be adjusted in support of intelligent wells. More on intelligent wells later.