PETROLEUM PRUDUCTION

Production

The production stage is the most important stage of a well's life, when the oil and gas are produced. By this time, the oil rigs and work over rigs used to drill and complete the well have moved off the wellbore, and the top is usually outfitted with a collection of valves called a Christmas tree or Production trees.

These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work is needed. From the outlet valve of the production tree, the flow can be connected to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations, or oil export terminals.

As long as the pressure in the reservoir remains high enough, the production tree is all that is required to produce the well. If the pressure depletes and it is considered economically viable, an artificial lift method mentioned in the completions section can be employed.

Oil extraction and recovery

Primary recovery 
Hydrocarbons come in varying densities and viscosities; reservoir traps also have variations in porosity and permeability, pressures, and temperatures. All of these factors exert an influence on how easily hydrocarbons can be removed from a trap.

The forces include:

1. Water located below the oil layer may be pressing upward; when this occurs, it is referred to as a water drive system  
2. If the gas cap located above the oil is causing a downward pressure or expansion of the natural gas at the top of the reservoir it is referred to as a gas cap drive system.
3. If the primary driving pressure is the result of pressure release from dissolved gas in solution with the oil, then it is referred to as a solution gas drive system. Buoyancy resulting from the movement of oil within the reservoir from the upper to the lower parts

Secondary recovery

Over the lifetime of the well the pressure will fall, and at some point there will be insufficient underground pressure to force the oil to the surface. After natural reservoir drive diminishes, secondary recovery methods are applied. They rely on the supply of external energy into the reservoir in the form of injecting fluids to increase reservoir pressure, hence replacing or increasing the natural reservoir drive with an artificial drive. Sometimes pumps, such as beam pumps and electrical submersible pumps (ESPs), are used to bring the oil to the surface. Other secondary recovery techniques increase the reservoir's pressure by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the bottom of an active well, reducing the overall density of fluid in the wellbore.

Tertiary recovery

Steam is injected into many oil fields where the oil is thicker and heavier than normal crude oil. Thermally enhanced oil recovery methods (TEOR) are tertiary recovery techniques that heat the oil, thus reducing its viscosity and making it easier to extract. 

Steam injection is the most common form of TEOR. This form of recovery is used extensively to increase oil extraction in the San Joaquin Valley, which has very heavy oil, yet accounts for 10% of the United States' oil extraction. 

In-situ burning is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil. Occasionally, surfactants (detergents) are injected to alter the surface tension between the water and oil in the reservoir, mobilizing oil which would otherwise remain in the reservoir as residual oil. Another method to reduce viscosity is carbon dioxide flooding. Tertiary recovery allows another 5% to 15% of the reservoir's oil to be recovered 

Microbial treatments is another tertiary recovery method. Special blends of the microbes are used to treat and break down the hydrocarbon chain in oil thus making the oil easy to recover as well as being more economic versus other conventional methods. In some states, such as Texas, there are tax incentives for using these microbes in what is called a secondary tertiary recovery.

Pressure Maintenance and Artificial Lift

When an oil well first starts producing oil, it can usually flow to surface naturally because of the high pressure in the reservoir formation. As oil is produced over the months or years, however, the reservoir pressure gradually decreases. This phenomena is normally counteracted by establishing a pressure maintenance program which involves injecting water or natural gas into the reservoir to balance the oil removed. Without pressure maintenance, some form of artificial lift may be required to help raise the crude oil to the surface and obtain the maximum production from the field.

Common artificial lift methods are : 

1. Surface pump: Sucker rod pump.
2. Down hole pumps: 
3. Electrical Submerged Pump (ESP)   
4. Injecting water, natural gas reinjection or Gas lift: Air, Carbon dioxide or some other natural gas of an active well. 
5. Sucker rod pump (Surface Artificial Lift)

A common form of artificial lift is to install a pump to pump the oil up to the surface. The figure below shows an example of a widely used type of sucker rod pump.

Downhole Pumps

Downhole pump insert the whole pumping mechanism into the well. In modern installations, an Electrical Submerged Pump (ESP) is inserted into the well. Here the whole assembly consisting of a long narrow motor and a multiphase pump, such as a PCP (progressive cavity pump) or centrifugal pump, hangs by an electrical cable with tension members down the tubing.