After a well was successfully drilled, the hydrocarbons must be produced. For this, a variety of methods are used, depending on the characteristics of the reservoir and the produced fluid.
Production methods
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.
Natural lift
All reservoirs are under a certain pressure. The temperature increases with the depth, by about 3° C per 100 m.
In many reservoirs there is gas (gas cap) above and water (aquifer) below the oil.
Due to the pressure and heat in the reservoir, dissolved natural gas is always present in the oil. The reservoir pressure drops when a well is drilled into it (the reservoir) and production begins. The gas can now be released from the oil, and its volume can expand. The oil is forced to the surface in a similar manner to soda when the bottle is shaken.
The expanding gas in the gas cap can also help keep the pressure in the oil column high enough for the oil to flow to the surface without artificial lift. The water in the aquifer can also help maintain the pressure in the oil column.
When oil flows to the surface without artificial assistance this is known as natural flow. Natural flow can also be kept up by artificially maintaining the pressure in the reservoir high (by means of water or gas injection in the reservoir).
Natural flow is the cheapest form of production, which is why every effort is made to sustain it for as long as possible.
Gas production methods
Gas production is considerably simpler than oil production. Normally the tubing is run down until just above the producing formation, and then the casing is perforated with a perforating gun in the same way as with oil production.
Thereafter the well fluid (usually water) is removed to the point that the pressure of the gas in the reservoir exceeds the pressure of the cushion (well fluid) so allowing the gas to flow to the surface.
The well is capped with a Christmas tree, which is used to shut in the well, measure the pressure and regulate the volume of gas produced.
The gas is directed to a separator which separates it from the produced liquids.
Artificial Lift methodsAs mentioned before, 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 (e.g. offshore).
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.
Treatment
Before oil and gas can be transported for further processing, they are treated at the production site. Treatment takes place at the gathering station.
Here, sand and water are removed from the effluent, and if oil and gas are produced together, they are separated. Both processes take place in a separator. The oil, gas and water mixture is broken down into its original components under pressure.
The gas rises and is drawn off. The water that collects in the bottom of the separator (due to its higher density) is filtered several times until it is completely free of oil contaminants. It is then injected into the reservoir to maintain pressure or cleaned and disposed.
The pretreated oil is conveyed to a tank where residual water and natural gas are separated. The crude oil has now been in large part purified, and can be transferred to a collection tank before transportation to the refinery for further processing.
Following pretreatment in the separator, all condensation must be removed from the gas. This is performed in a dehydration unit. If necessary, the gas is desulfurized in the plant as well. The sulfur is gathered for later use elsewhere (e.g. in the chemicals industry).
The desulfurized and dried gas is ready for use, and is sent on its way to consumers.
