
Unconventional (oil and gas) reservoirs, or unconventional resources (resource plays) are
accumulations where oil and gas
phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
*Phase space, a mathematica ...
s are tightly bound to the rock fabric by strong
capillary forces, requiring specialized measures for evaluation and
extraction.
Conventional reservoir
Oil and gas are
generated naturally at depths of around 4 or 5 km below Earth’s
surface
A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
. Being lighter than the water-
saturated rocks below the
water table
The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...
, the oil and gas are driven by
buoyancy
Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...
up through
aquifer
An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
pathways towards Earth's surface over time. Some of the oil and gas percolate all the way to the surface as natural
seepages, either on land or on the sea floor. The rest remains trapped underground by
geological barriers in a variety of
trap geometries. In this way, underground
pockets of oil and gas accumulate by displacing water in
porous rock. If the pockets are
permeable, they are referred to as ''conventional
reservoir
A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation.
Reservoirs are created by controlling a watercourse that drains an existing body of wa ...
s''. Wells are drilled into these reservoirs to create a path for oil and gas to reach the surface. When
pressure differences are relatively high, oil and gas rise to the
well bore naturally through buoyancy. Where the pressures are low, flow can be assisted with pumps (e.g.
nodding donkeys).
History
In the early days of the
oil industry
The petroleum industry, also known as the oil industry, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The largest volume products ...
, there was no need for
stimulation
Stimulation is the encouragement of development or the cause of activity in general. For example, "The press provides stimulation of political discourse." An interesting or fun activity can be described as "stimulating", regardless of its physic ...
to improve
recovery efficiency, because supply vastly outstripped demand and leaving "difficult" oil in the ground was economically expedient. Two
world war
A world war is an international War, conflict that involves most or all of the world's major powers. Conventionally, the term is reserved for two major international conflicts that occurred during the first half of the 20th century, World War I ...
s, followed by huge economic growth resulted in surging demand for cheap portable energy, while the availability of new conventional oil and gas resources declined. The industry initially sought to enhance
recovery of trapped oil and gas, using techniques like ''restricted'', or low volume hydraulic fracturing to stimulate the reservoir further, thereby reducing the volume of oil and gas left in the ground to an economic minimum. By the turn of the
millennium
A millennium () is a period of one thousand years, one hundred decades, or ten centuries, sometimes called a kiloannum (ka), or kiloyear (ky). Normally, the word is used specifically for periods of a thousand years that begin at the starting ...
, a new kind of
energy resource
Energy development is the field of activities focused on obtaining sources of energy from Natural resource, natural resources. These activities include the production of Renewable energy, renewable, nuclear power, nuclear, and fossil fuel derive ...
was required, particularly by the USA, who were driven to achieve
energy independence. The USA turned to ''unconventional reservoirs'' to achieve their goals, which had been known about for decades but had previously been too costly to be economically attractive. Today, unconventional reservoirs include
basin-centered gas,
shale gas
Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s, a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and ...
,
coalbed methane (CBM),
gas hydrates,
tar sands,
light tight oil
Tight oil (also known as shale oil, shale-hosted oil or light tight oil, abbreviated LTO) is light crude oil contained in unconventional petroleum-bearing formations of low permeability, often shale or tight sandstone.
Economic production from ...
and
oil shale
Oil shale is an organic-rich Granularity, fine-grained sedimentary rock containing kerogen (a solid mixture of Organic compound, organic chemical compounds) from which liquid hydrocarbons can be produced. In addition to kerogen, general compos ...
, mostly from North America.
Essential differences between conventional and unconventional reservoirs
The distinction between conventional and unconventional
''resources'' reflects differences in the qualities of the
reservoir
A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation.
Reservoirs are created by controlling a watercourse that drains an existing body of wa ...
and/or the physical properties of the oil and gas (''i.e.''
permeability and/or
viscosity
Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
). These characteristics significantly impact predictability (risk to find, appraise and develop) and in turn the methods of extraction from those reservoirs such as
fracking
Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of formations in bedrock by a pressurized liquid. The process involves the high-pressure inje ...
.
Conventional oil & gas accumulations are concentrated by buoyancy driven
aquifer pathways into
discrete
Discrete may refer to:
*Discrete particle or quantum in physics, for example in quantum theory
* Discrete device, an electronic component with just one circuit element, either passive or active, other than an integrated circuit
* Discrete group, ...
geological traps, which are detectable from the surface. These traps constitute relatively small but high resource density
fields. Most conventional oil or gas fields initially flow naturally by buoyancy alone into the well bore, with their limits defined by
fluid mechanics
Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasma (physics), plasmas) and the forces on them.
Originally applied to water (hydromechanics), it found applications in a wide range of discipl ...
measurable from the well bore (''e.g.'' fluid pressure,
OWC/GWC ''etc.''). In general, the technical and commercial risk associated with discrete conventional reservoirs can be reduced using relatively inexpensive remote techniques such as
reflection seismology
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflection (physics), reflected seismic waves. The method requir ...
and extracted with relatively few appraisal and development wells.
Unconventional reservoirs, in contrast, are regionally dispersed over large areas with no indicative trap geometry that can be used for predictive purposes. The oil and gas in unconventional reservoirs are generally low density resources, frequently trapped in the rock by strong capillary forces incapable of flowing naturally through buoyancy. The limits of an unconventional field are therefore usually defined by relatively expensive well testing for delivery. Extraction from unconventional reservoirs requires changing the physical properties of the reservoir, or the flow characteristics of the fluid, using techniques such as
fracking
Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of formations in bedrock by a pressurized liquid. The process involves the high-pressure inje ...
or
steam injection. The technical and commercial risk associated with unconventional reservoirs is generally higher than conventional reservoirs owing to the lack of predictability of the trap extent and of the reservoir quality, which requires extensive well placement and testing to determine the economic
reserves/well limit defined by
well delivery.
Environmental differences
As with all forms of
fossil fuel
A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geolog ...
, there are established issues with
greenhouse gas emissions
Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (), from burning fossil fuels such as coal, petroleum, oil, and natural gas, is the main cause of climate chan ...
through export (distribution) as well as consumption (combustion), which are identical whether the oil or gas are derived from conventional or unconventional reservoirs. Their
carbon footprint
A carbon footprint (or greenhouse gas footprint) is a calculated value or index that makes it possible to compare the total amount of greenhouse gases that an activity, product, company or country Greenhouse gas emissions, adds to the atmospher ...
s, however, are radically different: conventional reservoirs use the natural energy in the environment to flow oil and gas to the surface unaided; unconventional reservoirs require putting energy into the ground for extraction, either as heat (''e.g.'' tar sands and oil shales) or as pressure (''e.g.'' shale gas and
CBM). The artificial transfer of heat and pressure require the use of large volumes of
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...
creating
supply and disposal issues. The distribution of the resource over large areas creates land use issues, with implications for local communities on infrastructure, freight traffic and local economies. Impact on the environment is an unavoidable consequence of all human activity but the difference between the impact of conventional reservoirs compared with unconventional is significant, measurable and predictable.
See also
*
Source rock
In petroleum geology, source rock is a sedimentary rock which has generated hydrocarbons or which has the potential to generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sedim ...
*
Petroleum trap
*
Fracking in the United States
*
Environmental impact of fracking
*
Coalbed methane
*
Methane clathrate
Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large a ...
(gas hydrate)
*
Shale gas
Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s, a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and ...
*
Synthetic natural gas, such as
oil shale gas
*
Tight gas
*
Oil sand
*
Tight oil
Tight oil (also known as shale oil, shale-hosted oil or light tight oil, abbreviated LTO) is light crude oil contained in Unconventional (oil & gas) reservoir, unconventional petroleum-bearing formations of low Permeability (earth sciences), perme ...
*
Extreme energy
*
Renewable energy
Renewable energy (also called green energy) is energy made from renewable resource, renewable natural resources that are replenished on a human lifetime, human timescale. The most widely used renewable energy types are solar energy, wind pow ...
*
Future energy development
The future is the time after the past and present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the apparent nature of reality and the unavoidability of the future, everything that currently ...
*
Hubbert peak
The Hubbert peak theory says that for any given geographical area, from an individual oil-producing region to the planet as a whole, the rate of petroleum production tends to follow a bell-shaped curve. It is one of the primary theories on peak ...
*
Energy development
Energy development is the field of activities focused on obtaining sources of energy from natural resources. These activities include the production of renewable, nuclear, and fossil fuel derived sources of energy, and for the recovery and re ...
*
Alternative fuels
*
World energy resources and consumption
World energy supply and consumption refers to the global supply of energy resources and its consumption. The system of global energy supply consists of the energy development, refinement, and trade of energy. Energy supplies may exist in var ...
*
Oil megaprojects
Oil megaprojects are large oil field
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is cr ...
*
Creveney shale mining operation
References and notes
Notes
Abbreviated definitions
Petroleum industry
Unconventional oil
Unconventional gas
Peak oil
Petroleum production
Petroleum geology
Reservoir rock formations
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