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A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of contained in Porosity or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence of high heat and pressure in the Earth's crust.
Reservoirs are broadly classified as conventional and unconventional reservoirs. In conventional reservoirs, the naturally occurring hydrocarbons, such as crude oil (petroleum) or natural gas, are trapped by overlying rock formations with lower permeability, while in unconventional reservoirs the rocks have high porosity and low permeability, which keeps the hydrocarbons trapped in place, therefore not requiring a caprock. Reservoirs are found using hydrocarbon exploration methods.
Oil fields can occur anywhere that the geology of the underlying rock allows, meaning that certain fields can be far away from civilization, including at sea. Creating an operation at an oil field can be a logistically complex undertaking, as it involves the equipment associated with extraction and transportation, as well as infrastructure such as roads and housing for workers. This infrastructure has to be designed with the lifespan of the oil field in mind, as production can last many years. Several companies, such as Hill International, Bechtel, Esso, Weatherford International, Schlumberger, Baker Hughes and Halliburton, have organizations that specialize in the large-scale construction of the infrastructure to support oil field exploitation.
The term "oilfield" can be used as a shorthand to refer to the entire petroleum industry. However, it is more accurate to divide the oil industry into three sectors: upstream (crude oil production from wells and separation of water from oil), midstream (pipeline and Oil tanker transport of crude oil) and downstream (Oil refinery of crude oil to products, marketing of refined products, and transportation to oil stations).
More than 65,000 oil fields are scattered around the globe, on land and offshore. The largest are the Ghawar Field in Saudi Arabia and the Burgan Field in Kuwait, with more than 66 to 104 billion barrels estimated in each. In the modern age, the location of oil fields with proven oil reserves is a key underlying factor in many geopolitical conflicts.
In general, organic sediments buried in depths of 1,000 m to 6,000 m (at temperatures of 60 °Celsius to 150 °C) generate oil, while sediments buried deeper and at higher temperatures generate natural gas. The deeper the source, the "drier" the gas (that is, the smaller the proportion of condensates in the gas). Because both oil and natural gas are lighter than water, they tend to rise from their sources until they either Petroleum seep to the surface or are trapped by a non-permeable stratigraphic trap. They can be extracted from the trap by drilling.
The largest natural gas field is South Pars/Asalouyeh gas field, which is shared between Iran and Qatar. The second largest natural gas field is the Urengoy gas field, and the third largest is the Yamburg gas field, both in Russia.
Like oil, natural gas is often found underwater in offshore gas fields such as the North Sea, Corrib Gas Field off Ireland, and near Sable Island. The technology to extract and transport offshore natural gas is different from land-based fields. It uses a few, very large offshore drilling rigs, due to the cost and logistical difficulties in working over water.
Rising gas prices in the early 21st century encouraged drillers to revisit fields that previously were not considered economically viable. For example, in 2008 Freeport McMoRan passed a drilling depth of over 32,000 feet (9754 m) (the deepest test well in the history of gas production) at the Blackbeard site in the Gulf of Mexico."A Famed Dry Hole Gets a Second Shot", The Wall Street Journal, 21 July 2008, p. B1 ExxonMobil's drill rig there had reached 30,000 feet by 2006, without finding gas, before it abandoned the site.
Roy Nurmi, an interpretation adviser for Schlumberger oil field services company, described the process as follows:
Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70 °C they start to cook. This transformation, this change, changes them into the liquid hydrocarbons that move and migrate, will become our oil and gas reservoir.In addition to the aquatic ecosystem, which is usually a sea but might also be a river, lake, coral reef, or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps:
Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps. The North Sea, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oil fields.
Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land surface to below the surface and are a variety of shapes, sizes, and ages. In recent years, Igneous rock reservoirs have become an important new field of oil exploration, especially in trachyte and basalt formations. These two types of reservoirs differ in oil content and physical properties like fracture connectivity, pore connectivity, and rock porosity.
Petroleum geologists broadly classify traps into three categories that are based on their geological characteristics: the structural trap, the stratigraphic trap, and the far less common hydrodynamic trap.Basin Analysis: Principles and Applications. Allen, P.A. & Allen, J.R. (2005). Second Edition. Publ. Blackwell Publishing The trapping mechanisms for many petroleum reservoirs have characteristics from several categories and can be known as a combination trap. Traps are described as structural traps (in deformed strata such as folds and faults) or stratigraphic traps (in areas where rock types change, such as unconformities, pinch-outs and reefs).
A membrane seal will leak whenever the pressure differential across the seal exceeds the threshold displacement pressure, allowing fluids to migrate through the pore spaces in the seal. It will leak just enough to bring the pressure differential below that of the displacement pressure and will reseal.
A hydraulic seal occurs in rocks that have a significantly higher displacement pressure such that the pressure required for tension fracturing is actually lower than the pressure required for fluid displacement—for example, in or very tight shales. The rock will fracture when the pore pressure is greater than both its minimum stress and its tensile strength then reseal when the pressure reduces and the fractures close.
Unconventional reservoirs and their associated unconventional oil encompass a broad spectrum of petroleum extraction and refinement techniques, as well as many different sources. Since the oil is contained within the source rock, unconventional reservoirs require that the extracting entity function as a Mining rather than Drilling rig and Pumpjack like a conventional reservoir. This has tradeoffs, with higher post-production costs associated with complete and clean extraction of oil being a factor of consideration for a company interested in pursuing a reservoir. Tailings are also left behind, increasing cleanup costs. Despite these tradeoffs, unconventional oil is being pursued at a higher rate because of the scarcity of conventional reservoirs around the world.
The next step is to use information from appraisal wells to estimate the porosity of the rock. The porosity of an oil field, or the percentage of the total volume that contains fluids rather than solid rock, is 20–35% or less. It can give information on the actual capacity. Laboratory testing can determine the characteristics of the reservoir fluids, particularly the expansion factor of the oil, or how much the oil expands when brought from the high pressure and high temperature of the reservoir to a "stock tank" at the surface.
With such information, it is possible to estimate how many "stock tank" barrels of oil are located in the reservoir. Such oil is called the stock tank oil initially in place. As a result of studying factors such as the permeability of the rock (how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to estimate the recovery factor, or what proportion of oil in place can be reasonably expected to be produced. The recovery factor is commonly 30–35%, giving a value for the recoverable resources.
The difficulty is that reservoirs are not uniform. They have variable porosities and permeabilities and may be compartmentalized, with fractures and faults breaking them up and complicating fluid flow. For this reason, computer modeling of economically viable reservoirs is often carried out. Geologists, geophysicists, and reservoir engineers work together to build a model that allows simulation of the flow of fluids in the reservoir, leading to an improved estimate of the recoverable resources.
Reserves are only the part of those recoverable resources that will be developed through identified and approved development projects. Because the evaluation of reserves has a direct impact on the company or the asset value, it usually follows a strict set of rules or guidelines.
Modern production includes thermal, gas injection, and chemical methods of extraction to enhance oil recovery.
This occurs when the natural gas is in a cap below the oil. When the well is drilled the lowered pressure above means that the oil expands. As the pressure is reduced it reaches bubble point, and subsequently the gas bubbles drive the oil to the surface. The bubbles then reach critical saturation and flow together as a single gas phase. Beyond this point and below this pressure, the gas phase flows out more rapidly than the oil because of its lowered viscosity. More free gas is produced, and eventually the energy source is depleted. In some cases depending on the geology the gas may migrate to the top of the oil and form a secondary gas cap. Some energy may be supplied by water, gas in water, or compressed rock. These are usually minor contributions with respect to hydrocarbon expansion.
By properly managing the production rates, greater benefits can be had from solution-gas drives. Secondary recovery involves the injection of gas or water to maintain reservoir pressure. The gas/oil ratio and the oil production rate are stable until the reservoir pressure drops below the bubble point when critical gas saturation is reached. When the gas is exhausted, the gas/oil ratio and the oil rate drops, the reservoir pressure has been reduced, and the reservoir energy is exhausted.
It is best to manage the gas cap effectively, that is, placing the oil wells such that the gas cap will not reach them until the maximum amount of oil is produced. Also a high production rate may cause the gas to migrate downward into the production interval. In this case, over time the reservoir pressure depletion is not as steep as in the case of solution-based gas drive. In this case, the oil rate will not decline as steeply but will depend also on the placement of the well with respect to the gas cap. As with other drive mechanisms, water or gas injection can be used to maintain reservoir pressure. When a gas cap is coupled with water influx, the recovery mechanism can be highly efficient.
With a water-drive reservoir, the decline in reservoir pressure is very slight; in some cases, the reservoir pressure may remain unchanged. The gas/oil ratio also remains stable. The oil rate will remain fairly stable until the water reaches the well. In time, the water cut will increase, and the well will be watered out.
The water may be present in an aquifer (but rarely one replenished with surface water). This water gradually replaces the volume of oil and gas that is produced out of the well, given that the production rate is equivalent to the aquifer activity. That is, the aquifer is being replenished from some natural water influx. If the water begins to be produced along with the oil, the recovery rate may become uneconomical owing to the higher lifting and water disposal costs.
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