Petroleum reservoir

A petroleum reservoir, hydrocarbon reservoir, oil reservoir or gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability. Reservoirs are found using hydrocarbon exploration methods.

Reservoirs are often made of sandstone, which consists of grains of the mineral quartz (pure silica, Si02).The hydrocarbons are situated in the spaces, or pores, between the grains. The porosity of the reservoir is the fraction of the total reservoir volume that consists of pores. Hydrocarbons cannot flow into or out of the reservoir unless the pores are connected together. The permeability of the reservoir is a measure of the connectivity of the pores.

Sources of hydrocarbons

Hydrocarbons are normally not generated in the reservoir - they migrate into the reservoir from a source rock. Source rocks must contain large amounts of organic material, from which oil and gas are created. Most source rocks are shales, as these rocks contain 90% of all the organic material which is found in sediments. Crude oil found in oil reservoirs formed in the Earth's crust from the remains of living things. Source rocks themselves rarely form economic reservoirs because they have extremely low permeability. Source rocks need to be heated to over 10O˚C for a significant period of geological time in order to mature the remains of microscopic plant and animal into oil and natural gas.

In ocean environment, 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 changes them into the liquid hydrocarbons that move and migrate, will become oil and gas reservoir.

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 oilfields.

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 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and ages.

Traps

The reservoir must also have a three-dimensional shape which is capable of containing the accumulated hydrocarbons and preventing them from migrating elsewhere. This "container" is known as a trap, or structure. For example, the structure can be a dome which was formed by folding the layers of rock.

There are three types of trap.

• Structural traps are formed as a result of the deformation of rock strata in the Earth's crust.
• StratigraphIc traps occur where there is a permeability barrier caused by variation in sedimentary rock types.
• Some traps have both structural and stratigraphic features and are known as combination traps.

78% of the crude oil which has been discovered globally to date is held in structural traps, 13% in stratigraphic traps and 9% in combination traps.

Structural traps

File:Structural Trap (Anticlinal).svg

Fold (structural) trap

File:Structural Trap Fault.svg

Fault (structural) trap

Structural traps are formed by a deformation in the rock layer that contains the hydrocarbons. Domes, anticlines, and folds are common structures. Fault-related features also may be classified as structural traps if a closure is ‎present. Structural traps are the easiest to locate by surface and subsurface geological and geophysical studies. They are the most numerous among traps and have received a greater amount of attention in the search for oil than all other types of traps.

An example of this kind of trap starts when salt is deposited by shallow seas. Later, a sinking seafloor deposits organic-rich shale over the salt, which is in turn covered with layers of sandstone and shale. Deeply buried salt tends to rise unevenly in swells or salt domes, and any oil generated within the sediments is trapped where the sandstones are pushed up over or adjacent to the salt dome.

Stratigraphic traps

Stratigraphic traps are formed when other beds seal a reservoir bed or when the permeability changes (facies change) within the reservoir bed itself. Stratigraphic traps can form against either younger or older time surfaces.

Hydrocarbon Migration and Entrapment

When mature, hydrocarbons are liberated from the source rock and float towards the surface, as they have a lower density than the water that saturates sedimentary rock. If the hydrocarbons encounter a reservoir which has a suitable seal and structure they will be trapped and form an accumulation. If a reservoir is not encountered, the hydrocarbons will simply continue migrating to the surface and escape. Over 95% of mature hydrocarbons have escaped in this manner.

Clearly, the timing of events is crucial - the hydrocarbons must mature and migrate after the formation of the reservoir, seal and structure. These events take place over a very long timescale, an appreciation of which is necessary to understand the way in which hydrocarbon accumulations form.

Estimating reserves

After the discovery of a reservoir, a petroleum engineer will seek to build a better picture of the accumulation. In a simple text book example of a uniform reservoir, the first stage is to conduct a seismic survey to determine the possible size of the trap. Appraisal wells can be used to determine the location of oil-water contact and with it, the height of the oil bearing sands. Often coupled with seismic data, it is possible to estimate the volume of oil bearing reservoir.

The next step is to use information from appraisal wells to estimate the porosity of the rock. The porosity, 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 high pressure, high temperature of the reservoir to "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 (STOIIP). As a result of studying things 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 reserves.

The difficulty is that reservoirs are not uniform. They have variable porosities and permeabilities and may be compartmentalised, 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 which allows simulation of the flow of fluids in the reservoir, leading to an improved estimate of reserves.

Production

To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world, such as the La Brea tar pits in California, and numerous seeps in Trinidad.

Drive mechanisms

A virgin reservoir may be under sufficient pressure to push hydrocarbons to surface. As the fluids are produced, the pressure will often decline, and production will falter. The reservoir may respond to the withdrawal of fluid in a way that tends to maintain the pressure. Artificial drive methods may be necessary.

Solution gas drive

This mechanism (also known as depletion drive) depends on the associated gas of the oil. The virgin reservoir may be entirely liquid, but will be expected to have gaseous hydrocarbons in solution due to the pressure. As the reservoir depletes, the pressure falls below the bubble point, and the gas comes out of solution to form a gas cap at the top. This gas cap pushes down on the liquid helping to maintain pressure.

Gas cap drive

In reservoirs already having a gas cap (the virgin pressure is already below bubble point), the gas cap expands with the depletion of the reservoir, pushing down on the liquid sections applying extra pressure.

Aquifer (water) drive

Below the hydrocarbons may be a ground water aquifer. Water, as with all liquids, is compressible to a small degree. As the hydrocarbons are depleted, the reduction in pressure in the reservoir causes the water to expand slightly. Although this expansion is minute, if the aquifer is large enough, this will translate into a large increase in volume, which will push up on the hydrocarbons, maintaining pressure.

Water and gas injection

Main article: Water injection (oil production)

If the natural drives are insufficient, as they very often are, then the pressure can be artificially maintained by injecting water into the aquifer or gas into the gas cap.

See also

• Drilling
• Oilfield
• Well stimulation