The next stage in completion includes placing various pieces of hardware-referred to as jewelry-in the well; the jewelry is attached to production tubing. Tubing, the conduit between the producing formation and the surface, is the infrastructure upon which almost all completions are built. Its strength, material and size-weight/unit length and internal diameter-are chosen according to expected production rates, production types, pressures, depths, temperatures and corrosive potential of produced fluids.
Jewelry almost always includes packers, which seal against the inside of the casing. Packers isolate producing zones within the casing-tubing annulus in the same way cement does outside the casing. If the zone being produced is the deepest in the well, fluids flow from the formation below the packer and through the end of the tubing to the surface. In wells with multiple zones, a more common scenario, flow enters the well between an upper and lower packer and into the tubing through perforations or sliding sleeves (at bottom right). A sliding sleeve is a valve that is opened or closed mechanically; a specially designed tool on slickline or coiled tubing moves the valve's internal perforated sleeve up or down.
Nearly all completions also include safety valves. These come in a variety of forms but all are placed in the tubing within a few hundred feet of the surface. They are designed to automatically shut in the well when the surface control system is breached. They can also be closed manually to add an extra barrier between the well and the atmosphere when, for example, the well is being worked on or a platform is being evacuated in preparation for a storm.
With the basic jewelry deployed, many refinements are possible, depending on the specific needs of the field or well. For example, intelligent completions (ICs) are often used in situations or locations where entering the well to change downhole settings is costly or otherwise problematic. ICs include permanent, real-time remote pressure and temperature sensors and a remotely operable flow control valve deployed at each formation.
In other wells, the formation pressure is, or eventually becomes, insufficient to lift the formation fluids out of the well. These wells must be equipped with pumps or gas lift systems. Electric submersible pumps (ESPs) pump fluids to the surface using a rotor and stator. Pump rotor drives can be located on the surface. Reciprocating pumps, called pump jacks, may be used to lift the fluid to the surface through a reciprocating vertical motion.
Gas lift systems pump gas down the annulus between two casing strings. The gas enters the tubing at a depth below the top of the fluid column. This decreases the fluid density enough for buoyancy to lift the fluid out of the well. The amount of gas entering the well may be regulated through a sequence of valves located along the length of tubing, or it may be streamed in at one or more locations.
Also in low-pressure formations, water or gas may be injected down one well to push oil through the formation to producing wells. The producers may be fitted with injection control devices(ICDs) that regulate how much and where fluid enters the wellbore.
Before designing a completion, engineers take into consideration-for every well-”the types and volumes of fluids to be produced, downhole and surface temperatures, production zone depths, production rates, well loca-tion and surrounding environment. Engineers must then choose from the most basic openhole completion that may not have even a production casing string, to highly complex multilateral wells that consist of numerous horizontal or high-angle wellbores drilled from a single main wellbore, each of which includes a discrete completion.
The indispensable underpinnings of the optimal completion are solid FE, data from nearby offset wells and flexibility. Armed with reliable knowledge of target zones, how nearby wells accessing those formations were completed and how they produced, engineers are often able to plan the basic completion before the well is drilled. But completion engineers know that not every well will behave as expected, so they include contingencies in their completion plans and are prepared to implement them. In the end, how a well is completed—the culmination of all the decisions about jewelry and processes—directly impacts the rate at which and how long hydrocarbons will be produced from that well.
Oilfield Review 27, no. 2 (September 2015).
Copyright © 2015 Schlumberger.
For help in preparation of the article, thanks to Kyle Hodenfield, Houston, Texas, USA.