Hydrocarbons are a compound constituting hydrogen and carbon. Hydrocarbons find their wide use in the fuel industry. Gasoline, Coal, Natural gas, fuel oil, Kerosene, diesel fuel, jet fuel, and propane are the commonly used hydrocarbon fuels. In this article, we will explore the formation, Migration, and Recovery of Hydrocarbons.
Formation of Hydrocarbons
- Almost all oil and gas come from decayed plants, animals, and bacteria.
- In prehistoric times, conditions for oil formation have been particularly favorable.
- Oil from the North Sea is mainly found in rocks that formed 150 million years ago.
- In this period of time, the seas and swampy areas were rich in microscopic plants, bacteria, and animals. When these died, they slowly sank to the bottom forming a thick sediment layer of organic material, which in turn was covered in layers of mud that trapped the organic material.
- Oil and gas were thus formed by the anaerobic decay of organic material in conditions of increased temperature and pressure. With the layers of mud preventing air from reaching the organic material, the organic material could not rot in the same way as organic material rots away in a compost heap.
- Oil and gas are called ‘hydrocarbons’ because of the high content of molecules of the elements hydrogen and carbon. Crude oil is a complex mixture of hydrocarbons, with small amounts of sulfur, N2, and O2.
Migration of Hydrocarbons
There are two reasons why this happens:
- The oil and gas expand, increasing the pressure in the rock, and then try to escape.
- Being less dense than the surrounding rock and water, it tends to rise upwards. This is a slow process. It will take millions of years for the oil and gas to rise a few kilometers.
What happens to the migrating oil and gas?
Having no seal above the rock source, the oil and gas slowly rise and escapes at the surface. This does happen on occasion but, when it does, no oil and gas deposits can build up.
Porous means that liquids and gas can be held and stored. The pores in rock are the spaces that occur between the individual rock particles. These spaces are created because the rock particles are irregularly shaped and so don’t fit together exactly or closely together. This porous rock is called the ‘reservoir’. Refer to Fig. 1 to understand what a reservoir is.
Type Of Reservoir & Reservoir Fluids
Forces that control the behavior of the Oil and Gas: Viscosity, Gravity, and force of capillary action.
Type Of Reservoir
- Dry Gas Reservoir: no liquid produced surface.
- Wet Gas Reservoir: no liquid is formed in the reservoir, but there will be a production of the liquid surface.
- Gas Condensate Reservoir: Liquid is formed in the reservoir.
- Oil Reservoir:
- Volatile Oil: High GOR.
- Black Oil: Medium GOR.
- Heavy Oil: Law GOR.
Hydrocarbon Recovery Methods
Primary Recovery: Natural Reservoir Energies are used to produce hydrocarbons: Solution gas drive, gas cap drive, natural water drive, and compaction drive.
Artificial Lift: When the Natural flow of the wells declines, Artificial Lift is used to produce more Hydrocarbons like Gas Lift, ESP, and Sucker Rod Pump.
1. Gas Lift (Fig. 3A): This process involves injecting gas down the well through tubing or through the tubing-casing annulus. Injected gas aerates the fluid to make it exert less pressure than the formation does; the resulting higher formation pressure forces the fluid out of the wellbore. Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment.
2. ESP (Fig. 3B): The first electrical submersible pumping unit was developed in Russia in 1917 by Armies, who later migrated to California. Although initially not very successful, the use of ESPs in the oil industry was assured by the help of Frank Phillips of Phillips Petroleum Co. in Bartlesville, Oklahoma. Since that time, the concept has proved to be an effective and economical means of lifting large volumes of fluid from great depths under a variety of well conditions. Today’s ESPs are essentially multistage centrifugal pumps that employ blades, or impellers, attached to a long shaft. The shaft is connected to an electrical motor that is submerged in the well. The pump usually is installed in the tubing just below the fluid level, and electricity is supplied through a special heavy-duty armored cable.
3. Sucker Rod Pump (Fig. 3C): 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.
4. Subsurface Hydraulic Pump: There are two types of hydraulic pumps for artificial lifts. One is a fixed-pump design; the other is a free-pump design. In fixed installations, the downhole pump is attached to the end of the tubing string and runs into the well. Power fluid is directed down an inner tubing string, and the produced fluid and the return power fluid flow to the surface inside the annulus between the two tubing strings. Free-pump installations allow the downhole pump to be circulated into and out of the well inside the power-fluid tubing string, or they can be installed and retrieved by wireline operations. Jet pumps are a special class of hydraulic subsurface pumps and are sometimes used in place of reciprocating pumps. Unlike reciprocating pumps, jet pumps have no moving parts and achieve their pumping action by means of momentum transfer between the power fluid and produced fluid.
5. The future: Artificial-lift technologies of the future will involve software, electronics, sensor technologies, and data transfer and data management. This will require the effort of developers to explore the limits of technology.
EOR:
- 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 EOR refers to any method used to recover more oil from the reservoir than would be produced by a natural mechanism.
- Generally, EOR involves the injection of a fluid, not normally in the reservoir, in the reservoir.
- The major methods of improved oil recovery are water flooding, gas injection, chemical flooding, and thermal recovery. These techniques are used when production from the well starts to decrease.
EOR Method:
Water flooding 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 the water to push oil out of the formation. These chemicals act as surfactants that cause the oil and water to mix and break 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 the 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. The heat from the fire thins out the oil around it, causes the gas to vaporize from it, and changes the water in the reservoir to steam. Steam, hot water, and gas all act to drive oil in front of the fire to production wells.
