Viscosity Permeability Classification scheme

Unconventional (oil and gas) reservoirs, or unconventional resources (resource plays) are accumulations where oil and gas phases 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. 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). $Schematic cross-section of general types of oil and gas resources and the orientations of production wells used in hydraulic fracturing$

History

In the early days of the oil industry, 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 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, coalbed methane (CBM), gas hydrates, tar sands, light tight oil and oil shale, 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

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. 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 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 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, 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 footprints, 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.

References and notes

Notes

Abbreviated definitions

Petroleum industry Unconventional oil Unconventional gas Peak oil Petroleum production Petroleum geology Reservoir rock formations {{DEFAULTSORT:Unconventional (oil and gas) reservoir