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Production is the operation that brings hydrocarbons to the surface and prepares them for processing. Production begins after the well is drilled. The mixture of oil, gas and water from the well is separated on the surface. The water is disposed of and the oil and gas are treated, measured, and tested. Production operations include bringing the oil and gas to the surface, maintaining production, and purifying, measuring, and testing. Completing the Well After a well has been drilled, it must be completed before oil and gas production can begin. The first step in this process is installing casing pipe in the well. Oil and gas wells usually require four concentric strings of pipe: conductor pipe, surface casing, intermediate casing, and production casing. The production casing or oil string is the final casing for most wells. The production casing completely seals off the producing formation from water aquifers. The production casing runs to the bottom of the hole or stops just above the production zone. Usually, the casing runs to the bottom of the hole. In this situation the casing and cement seal off the reservoir and prevent fluids from leaving. In this case the casing must be perforated to allow liquids to flow into the well. This is a perforated completion. Most wells are completed by using a perforated completion. Perforating is the process of piercing the casing wall and the cement behind it to provide openings through which formation fluids may enter the wellbore. Tubing and Packers After cementing the production casing, the completion crew runs a final string of pipe called the tubing. The well fluids flow from the reservoir to the surface through the tubing. Tubing is smaller in diameter than casing-the outside diameter ranges from about 1 to 4-1/2 inches. A packer is a ring made of metal and rubber that fits around the tubing. It provides a secure seal between everything above and below where it is set. It keeps well fluids and pressure away from the casing above it. Since the packer seals off the space between the tubing and the casing, it forces the formation fluids into and up the tubing. Subsurface Safety Valve A subsurface safety valve is installed in the tubing string near the surface. The valve remains open as long as fluid flow is normal. When the valve senses something amiss with the surface equipment of the well, it closes, preventing the flow of fluids. Multiple Completions The operator uses a multiple completion when one wellbore passes through two or more zones with oil and gas in them. Usually, a separate tubing string width packers is run in for each producing zone. Wellhead The wellhead includes all equipment on the surface that supports the various pipe strings, seals off the well, and controls the paths and flow rates of reservoir fluids. All wellheads have at least one casinghead and casing hanger, usually, a tubing head and tubing hanger, and a Christmas tree. Each string of casing usually hangs from a casinghead, a heavy steel fitting at the surface. Metal and rubber seals in the casinghead prevent fluids from moving within the wellhead or escaping to the atmosphere. Each casing head also has a place for a pressure gauge to warn of leaks. The tubing head supports the tubing string, seals off pressure between the casing and the inside of tubing and provides connections at the surface to control the flowing liquid or gas. The tubing head often stacks above the uppermost casinghead. Like the casingheads, it has outlets to allow access to the annulus for gauging pressure or connecting valves and fittings to control the flow of fluids.
Gauges measure pressure in the casing and in the tubing. Valves control the flow of hydrocarbons from the well. The choke controls the rate of production from the well. Starting the Flow Before oil production can begin the drilling mud must be removed from inside the casing. Salt water is pumped into the tubing to remove this mud. In some cases the well has too much salt water in the tubing and some must be pumped out. Production flow can also be started by forcing high-pressure gas into the tubing. Sometimes after starting the flow the well does produce at a fast enough rate. In this situation, flow from the reservior may be increased by stimulation. Stimulation is one of several processes that enlarge or create channels in the reservoir rock so that the oil and gas can move through it and into the well. Gas wells are generally completed in the same way as oil wells except that natural gas usually flows without help.
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