How are Crude Oil and Natural Gas Produced?

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.

Wells are equipped with a group of valves and fittings called a Christmas tree. The valves and fittings are used to regulate, measure, and direct the flow of hydrocarbons from the well. 

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 reservoir 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.
 
Drive Mechanisms 

Natural Drive 
 

After the well has been completed, the hydrocarbons flow from the reservoir to the surface. In the first stage of a reservoir's producing life, pressure from the reservoir forces the hydrocarbons from the pores in the formation, moves them to the well, and up to the surface. This stage of production is known as primary recovery. The three principal primary recovery drive mechanisms are water drive, gas drives, and gravity drainage.

Water drive uses the pressure exerted by water below the oil and gas in the formation to force hydrocarbons out of the reservoir. The greater the depth of the reservoir the higher the pressure.  Water drive is the most efficient natural drive and can be used to produce 50% or more of the oil in the reservoir.

The two types of gas drives are dissolved-gas drives and gas-cap drives. These drives use the pressure of gas in the reservoir to force oil out of the reservoir and into the well. In a dissolved-gas drive the hydrocarbons in the oil are light enough that they become gaseous when the well releases pressure from the reservoir. This is similar to dissolved carbon dioxide in a soft drink. If the can or bottle is shaken, the soda gushes out when the can is opened. When the well is opened the lighter hydrocarbons turn into gases and the oil and gas flow up to the surface. The amount of oil recovered from dissolved-gas drives varies from 5 to 30%.

In some reservoirs, gas may be present in a space on top of the oil. This gas cap provides pressure to push the oil into the well. As the level of oil in the reservoir drops, the gas cap expands and continues to push the oil into the well and up to the surface. The more space the oil leaves empty in the porous reservoir rock, the more the gas expands to take its place. The pressure of a gas-cap drive depletes more slowly than a dissolved gas drive. From 20 to 40% of the oil in the reservoir is recovered with gas-cap drives.

Artificial lift 
 

Artificial lifting methods are used when the pressure from natural reservoir drive decreases to the point where the well stops producing.  Artificial lift uses pumps and gas injection.

The most common method of pumping oil in land-based wells is beam pumping.  The beam pumping unit sits on the surface and creates an up-and-down motion to a string of rods called sucker rods.  The top of the sucker rod string is attached to the front of the pumping unit  and hangs down inside the tubing.  A sucker rod pump is located near the bottom of the well.  The walking beam's reciprocating action moves the rod string up and down to operate the pump.

Gas lift describes methods in which a gas is used to increase oil well production.   Gas  lift-dissolved-gas drive or gas-cap drive may provide natural drive to the reservoir.  Natural gas can also be injected into the well to lift the oil artificially on the same principle. 

The natural gas makes oil in the wellbore column much lighter in weight. Because the liquid column is lighter, it exerts less pressure on the bottom of the well. With the pressure lower at the bottom, the pressure remaining in the reservoir becomes sufficient to push reservoir fluids to the surface through the tubing. 

Gas lift is common when a supply of gas is economical and available and when the amount of petroleum it will lift justifies the expense.

Enhance Recovery Techniques

After a well has used up the reservoir's natural drives and gas lift or pumps have recovered all the hydrocarbons possible, statistics show that 25 to 95% of the original oil in the reservoir may still be there. This amount of oil can be worth recovering if prices are high enough.  The major methods of improved oil recovery are waterflooding, gas injection, chemical flooding, and thermal recovery.  These techniques are used when production from the well starts to decrease.

Waterflooding is a technique where water is injected into the formation using wells that have ceased production.  The injected water enters the reservoir and displaces some of the remaining oil toward producing wells in the same reservoir. The producing wells then pump up the oil and water. Several injection wells surround each producing well. Water flooding is the least expensive and most widely used secondary recovery method.

Production can also be increased by injecting gas, such as natural gas or nitrogen, into the reservoir.  The injected gas expands to force additional volumes of oil to the surface. 

Chemical flooding uses special chemicals in water to push oil out of the formation. These chemicals act as surfactants that cause the oil and water to mix and breaks the oil into tiny droplets that can be more easily moved through the reservoir to the well.

Thermal recovery is used when the oil is so viscous, or thick, that it cannot flow through the reservoir and into a well. When the oil is heated, its viscosity is decreased and the flow increases. Recovery techniques that use heat are called thermal processes or thermal recovery. 

Steam Drive or steam injection involves generating steam on the surface and forcing this steam down injection wells and into the reservoir. When the steam enters the reservoir, it heats up the oil and reduces its viscosity.  The heat from the steam also causes hydrocarbons to form gases which also increases flow.  The gases and steam provide additional gas drive and the hot water also moves the thinned oil to production wells.

Another way to use heat in a reservoir is fire flooding, or in situ (in-place) combustion.  In fire flooding, the crew ignites a fire in place in the reservoir. They inject compressed air down an injection well and into the reservoir.  A special heater in the well starts a fire. As the fire burns, it begins moving through the reservoir toward production wells. Heat from the fire thins out the oil around it, causes gas to vaporize from it, and changes water in the reservoir to steam. Steam, hot water, and gas all act to drive oil in front of the fire to production wells. 

Offshore Operations

Offshore operations are fundamentally the same as onshore operations with the major difference being in the complexity of the production sites and hence their costs. Offshore production facilities are self-contained production sites.  The platforms are semi-permanent structures from which many wells are drilled and completed.