When oil or gas is encountered, the first step is to feed a pipe into the well. The oil or gas rises to the surface through this tubing. Equipment to shut in the well and regulate the flow (Christmas tree) is mounted at the surface.
There are several well completion systems. The purpose of a well completion is to connect the hydrocarbons producing reservoir to the surface so that fluids can be produced. One of the most common completion system is the cemented and perforated casing completion which has a casing (piece of pipe) across the production zone, which is cemented and perforated to allow production. The perforation gun can make up to 40 perforations per meter with diameters of up to 2 centimeters in the casing. The oil or gas enters the well through the perforations and rises to the surface through the tubing.
Reservoirs are often having high pressure, meaning that oil and gas can naturally flow to the surface. There is a point at which the energy of the well that makes the hydrocarbons to be produced is reduced after some time of production. To continue producing the fluids out of the well it is then necessary to provide extra energy to it. One of the ways to do it is by using artificial lift methods, which basically help to produce the fluids by reducing the pressure acting downhole over the producing reservoir.
Artificial Lift methods
As mentioned above, artificial lift becomes necessary when the pressure in the reservoir becomes too low for the oil to flow naturally to the surface, following the extraction of oil and associated gas.
A brief description of the most used methods is described below:
Gas lift
With gas lift production, gas is injected into the well (most time via the annulus - the space between the tubing and the casing) and forced into the tubing via valves (gas injection). The gas bubbles streaming upwards through the oil reduce the density of the oil column, and provide additional energy that helps lifting the fluids.
The advantage of gas lift is that the production equipment has a very long lifetime. The intervals between repairs or replacements can be as long as 15 years. As a result, this method tends to be preferred in locations where maintenance is very expensive.
Pumping
Pumping is the most commonly used artificial lift method. There are different types of pumps.
Beam pumping - typically, beam pumps consist of a number of components. The drive unit on the surface is known as the pump jack or "horse-head" pump. The power is transmitted downwards by the sucker rods (steel or fiber glass) to a piston with valves (plunger) located in a cylinder (barrel) that can lift oil columns of up to 2,000 meters in length.
Electrical submersible pumping (ESP) – this method uses a system that transfers electrical energy from surface to a downhole motor which converts it into a mechanical force (torque – rotational movement). This turns the inside components of the pump and lifts the well fluids to the surface.
Progressing cavity pumps – these pumps are based on rotary fluid displacement. Mechanical energy is transferred via sucker rods to a downhole spiral (rotor) that spins inside a housing (stator) and provides additional energy to the well to lift the fluids. Hydraulic pumps – these pumps operate similar to a gas lift system, but instead of using high pressure gas as an energy source, it uses a fluid usually called power fluid.
