Petroleum Extraction
Information from this article includes data from Wikipedia. Please see primary reference.
The extraction of petroleum is the process by which usable petroleum is extracted and removed from the earth.
Geologists use seismic surveys to search for geological structures that may form oil reservoirs. The “classic” method includes making an underground explosion nearby and observing the seismic response that provides information about the geological structures under the ground [1]. However, “passive” methods that extract information from naturally-occurring seismic waves are also known.[1]
Locating the oil field
Other instruments such as gravimeters and magnetometers are also sometimes used in the search for petroleum. Extracting crude oil normally starts with drilling wells into the underground reservoir. When an oil well has been tapped, a geologist (known on the rig as the “mudlogger”) will note its presence. Such a “mudlogger” is known to be sitting on the rig. Historically, in the USA, some oil fields existed where the oil rose naturally to the surface, but most of these fields have long since been used up, except in certain places in Alaska. Often many wells (called multilateral wells) are drilled into the same reservoir, to ensure that the extraction rate will be economically viable. Also, some wells (secondary wells) may be used to pump water, steam, acids or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and so maintain an economic extraction rate.
Drilling
The oil well is created by drilling a hole into the earth with an oil rig. A steel pipe (casing) is placed in the hole, to provide structural integrity to the newly drilled wellbore. Holes are then made in the base of the well to enable oil to pass into the bore. Finally a collection of valves called a “Christmas Tree” is fitted to the top, the valves regulating pressures and controlling flows.
Oil extraction and recovery
Primary recovery
During the primary recovery stage, reservoir drive comes from a number of natural mechanisms. These include: natural water displacing oil downward into the well, expansion of the natural gas at the top of the reservoir, expansion of gas initially dissolved in the crude oil, and gravity drainage resulting from the movement of oil within the reservoir from the upper to the lower parts where the wells are located. Recovery factor during the primary recovery stage is typically 5-15%.[2]
While the underground pressure in the oil reservoir is sufficient to force the oil to the surface, all that is necessary is to place a complex arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing.
There are three elements essential for fluid extraction within a resource bearing reservoir. Porosity, Permeability, and Pressure are the necessary factors for removal of these resources, and must be present together in order for a well to be commercially viable.
- Porosity: The percentage of pore volume or void space;
- Permeability: The measurement of the ability to transmit fluids;
- Pressure: The pressure of fluids within the pores of a reservoir;
Porosity is a rocks ability to store fluids within it. Effective porosity is the volume containable through the interconnected pores, and is usually less than total porosity. Thus, porosity alone is not enough to ensure that fluids can move through a rock structure. These pores must be interconnected in order to have any chance of a predicable reservoir.
Permeability, as defined above, is a rocks ability to move fluids. Permeability is directly related to a formations Effective Porosity, as these interconnections within a rocks structure are the necessary transmission lines for a fluid to flow. The largest reservoir, with little to no porosity and permeability, is useless.
Pressure, in this instance, is the amount of force a fluid exerts within the pores of a reservoir. Pressure within a reservoir changes over time, therefore it is necessary to define a reference, which in this case would be the Initial Pressure. Good reservoir pressure is the third essential element for resource recovery.
To understand how these three necessities work together, we need only to look to the common sponge. A sponge, to be useful, must have good Porosity and Permeability. This is what gives a sponge the natural ability to store fluids. But how do we get the fluid out of the pores. In the instance of a sponge, this is easy, as any kind of pressure applied will cause the sponge to give up the fluids. Nature, aside from the sponge, is rarely so obliging when it comes to the recovery of natural resources.
The oil and gas industry has come up with many ways to increase porosity and elevate reservoir pressure. The most common way to increase porosity is to fracture (frac) the targeted zone. This entails setting off a focused charge within the pipe, at a predetermined depth, in order to reach the zone targeted for production. This charge puts holes in the pipe, as well as breaks up the rock in the immediate area of the blast. These holes are used as an access point for a fracturing fluid to be injected into the zone. Different zones require different types of fluids and mixtures to be effective, most common are the acid salt mixtures. Injected with these mixtures is generally a large amount of water, at high pressures. This mixture and pressure combined is a highly effective means of increasing porosity.
Increased porosity is only half the story. Without the necessary reservoir pressures, porosity becomes moot. The industry has devised many different ways to combat low pressures, the most common being the pumpjack. The pumpjack however, only creates suction, and does nothing to increase pressures. One remedy for lower pressures are injection wells, that replace fluids as they are taken, or even moves fluids towards the producing well.
The best solution for managing pressure within a well is to maintain the initial pressures as long as possible. For example, a can of spray paint can last for an extended period of time by stirring the paint and maintaining a proper spraying angle. The manufacturer sets the choke at the top of the can to moderate the rate of flow. Cut the top of the can off however, and what you have left is no pressure and a lot of paint that not easily obtained for its intended use.
A well has similar properties. An operator can open the bore completely, and produce at extremely high numbers for a short period of time. But to get the highest rate of return possible, managing Porosity, Permeability, and Pressure is key.
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 andelectrical 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. Typical recovery factor from water-flood operations is about 30%, depending on the properties of oil and the characteristics of the reservoir rock. On average, the recovery factor after primary and secondary oil recovery operations is between 35 and 45%.[2]
Tertiary recovery
Steam is injected into many oil fields where the oil is thicker and heavier than normal crude oil
Tertiary, or enhanced oil recovery methods increase the mobility of the oil in order to increase extraction.
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, and is often done with a cogeneration plant. In this type of cogeneration plant, a gas turbine is used to generate electricity and the waste heat is used to produce steam, which is then injected into the reservoir. 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.[3]
Another method to reduce viscosity is carbon dioxide flooding.
Tertiary recovery allows another 5% to 15% of the reservoir’s oil to be recovered.[2]
Tertiary recovery begins when secondary oil recovery isn’t enough to continue adequate extraction, but only when the oil can still be extracted profitably. This depends on the cost of the extraction method and the current price of crude oil. When prices are high, previously unprofitable wells are brought back into use and when they are low, extraction is curtailed.
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. Very few companies supply these, however companies like Bio Tech, Inc. have proven very successful in waterfloods across Texas.
Recovery rates
The amount of oil that is recoverable is determined by a number of factors including the permeability of the rocks, the strength of natural drives (the gas present, pressure from adjacent water or gravity), and the viscosity of the oil. When the reservoir rocks are “tight” such as shale, oil generally cannot flow through but when they are permeable such as in sandstone, oil flows freely. The flow of oil is often helped by natural pressures surrounding the reservoir rocks including natural gas that may be dissolved in the oil (see Gas oil ratio), natural gas present above the oil, water below the oil and the strength of gravity. Oils tend to span a large range of viscosity from liquids as light as gasoline to heavy as tar. The lightest forms tend to result in higher extraction rates.
Petroleum engineering is the discipline responsible for evaluating which well locations and recovery mechanisms are appropriate for a reservoir and for estimating recovery rates and oil reserves prior to actual extraction.
References
1.^ A technology web site of a passive – seismic based company
- ^ a b c E. Tzimas, (2005) (PDF). Enhanced Oil Recovery using Carbon Dioxide in the European Energy System. European Commission Joint Research Center. Retrieved 2008-08-23.
- ^ “New Billions In Oil” Popular Mechanics, March 1933 — ie article on invention of water injection and detergents for oil recovery
