Onshore Production Wellhead

Crude Oil extraction methods

The exploitation of the well # is caused by the reservoir pressure being greater than the weight of the hydrocarbon column from the reservoir to the wellhead. This pressure, which allows the exploitation of the well, can be of three types, that is:

• DISSOLVED GAS DRIVE
• GAS CAP DRIVE
• WATER DRIVE

These natural push mechanisms often coexist.

Primary extraction methods, i.e. methods based on the exploitation of the natural expulsion forces that push the oil from the reservoir rocks to the extraction wells, are generally not very efficient. Often a large part of the original oil remains in place, in fact as the delivery proceeds the volume and pressure of the gas produced decrease until reaching a reservoir pressure such as to be insufficient for the fluid to reach the surface. Primary extraction methods allow for the recovery of a very low percentage of oil which actually is present, sometimes this percentage is just 10% and rarely exceeds 50%. Therefore the quantity of oil extracted is 1/3 of the total. This vast reserve (remaining 2/3) of oil stimulates the search for increasingly efficient recovery of the remaining oil. There are many methods:

1. Beam-Balanced Pumping (mechanical method)
2. Pneumatic pumping (GAS-LIFT)
3. Hydraulic pumping (KOBE submersible pump)
4. Electric pumping (REDA submersible pump)
   
   

Beam-Balanced Pumping

Delivery is made by a pump placed in the tubing at a variable depth which depends on the dynamic level of the oil in the well. The piston of the pump is connected by rods to a balance wheel installed at the wellhead which is driven by an electric or petrol engine. Thanks to a reducer placed between the engine and the balance wheel it is possible to regulate the speed, therefore the number of piston strokes per minute, corresponds to the flow rate of the well. It is clear that the downward stroke of the piston and rods occurs by the free fall of the piston itself, therefore the pumping speed is conditioned by the fall speed of the piston-rod system. This must therefore always be greater than the pumping one. Fig.3 represents the internal parts in a pumping stand.

Figure 3 - Beam-Balanced Pumping Drawing

GAS-LIFT

This method consists in allowing gas to enter the tubing from the casing, at a certain depth, which will push the oil to the surface. The insufflation of gas takes place through valves mounted along the tubing at regular intervals, in a variable number from 2 to 7. The opening mechanism is regulated by the injection of gas at established pressure.

The most used methods for the extraction of oil by using the "GAS-LIFT" are two:

1. Intermittent lifting:  Consists of injecting a certain quantity of gas from the casing to the tubing at a pre-established pressure and for a certain time interval.
2. Continuous lifting: Consists of continuously injecting gas which, mixing with the liquid, decreases its density. In this case, the valve must be sensitive to the hydrostatic column: if this becomes too light, the valve must intervene by limiting the quantity of gas to be injected, on the contrary, means becomes too heavy, the valve must intervene by sending more gas in the casing. Fig. 4 represents the simplified diagram of a well in production with GAS-LIFT.

Figure 4 - Gas Lift Pumping

Hydraulic pumping (KOBE submersible pump)

It is an artificial lifting system in which a submersible pump is operated by pressurized hydraulic oil. As you can see in Fig.5, it consists of a surface pump, a storage tank for the driving fluid, a control station and a submersible pump located in the tubing. This submersible pump (Fig.5A) consists of a "motor" and a "production pump" combined in the same assembly. The driving fluid is pumped back to the surface together with the fluid and sent to the tank. Production can take place behind the tubing or via a second tubing. Installation and removal are done with WIRE-LINE operation.

Figure 5 - KOBE Hydraulic pumping

Figure 5A - Diagram of the Kobe Hydraulic Pump

Submersible electric pumps (REDA)

They are multistage centrifugal pumps, whose shaft is directly connected to the submersible electric motor. The entire complex can be directly immersed at the desired depth (directly in the well fluid). The pump and the motor are built to reach a depth of up to 5000 meters.

Generally as shown in Fig.6, the system consists of:

• Motor:  Submersible induction electric motor, filled with oil for lubrication. Variable power (0.3 +/- 780 HP). RPM: 2880 RPM.
• 3600 RPM Protector:  Located between the pump and the motor, it is used to balance the internal pressure of the motor with the external one of the well.
• Pump:  Multi-stage centrifugal pump which is built in a wide range of flow rates and pressures.
• Gas-separator:  If the well also produces gas, the gas-separator avoids the cavitation phenomenon by sending the gas to the casing and the oil to the suction.

Figure 6 - Submersible electric pumps

English Terminology of Pumping Units

• pumping unit
• walking-beam
• saddle bearing
• Tail bearing
• pìtman bearing
• pitman
• crank
• crank arm
• counterweight
• crankshaft
• unit sheave
• engine sheave
• gear reducer
• Samson post
• horse head
• bridle
• polished rod clamp
• sucker rod
  
  
  

  Wellhead manifolds

  
  Wellhead manifold mean the set of equipment installed at the wellhead. The composition of the manifold varies with the variation of the delivery system of the well and with the characteristics of the crude oil.
  For wells with low pour point crude oil, the manifold can be equipped with a heater or light product flush system to ensure the transportation of the crude from the well to the oil centre.
  For wells in spontaneous flow, the manifold is equipped with a flow Choke Valve regulator.
  For pumping wells with rocker arm and/or submersible pump, the wellhead will have the necessary equipment installed for the operation of the submersible pump, hydraulic oil circuit in the case of KOBE pumps, cables and electrical panels in the case of REDA pumps.
  Regarding gas-lift wells, will be a system of automatic valves on the manifold to regulate the pressure and quantity of gas to be injected into the well.