unconventional (oil and gas) reservoir

{{Main|Petroleum reservoir}}

Oil and gas are generated naturally at depths of around 4 or 5 km below Earth’s surface.{{efn|or as little as 2-3 km for thermogenic gas, depending on the Geothermal gradient of Earth's crust, which varies at different locations; less common biogenic methane forms at much shallower depths}} Being lighter than the water-saturated rocks below the water table, the oil and gas are driven by buoyancy up through aquifer 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{{efn|where the capillary entry pressures are higher than the buoyancy pressure of the oil and gas}} 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 reservoirs. 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.{{efn|when oil reaches its bubble point and gas is exsolved, the natural expansion of gas on ascent creates additional energy to lift fluids in the borehole to the surface much faster than by buoyancy alone, producing a blowout if not controlled}} Where the pressures are low, flow can be assisted with pumps (e.g. nodding donkeys).{{cite book |last1=Gluyas |first1=Jon |last2=Swarbrick |first2=Richard|title=Petroleum Geoscience |date=2004 |publisher=Blackwell Publishing |location=UK, USA & Australia |isbn=978-0-632-03767-4 |pages=i-350}}

File:Schematic cross-section of general types of oil and gas resources and the orientations of production wells used in hydraulic fracturing.jpg

In the early days of the oil industry, there was no need for stimulation to improve recovery efficiency, because supply vastly outstripped demand and leaving "difficult" oil in the ground was economically expedient.{{cite news | title=Oil Glut, Price Cuts: How Long Will They Last?| journal=U.S. News & World Report| date= 18 August 1980| volume= 89 |issue = 7 | page= 44 }} Two world wars, followed by huge economic growth resulted in surging demand for cheap portable energy,{{cite book |last1=Black |first1=Brian C. |title=Crude Reality: Petroleum in World History |date=2012 |publisher=Rowman & Littlefield |location=New York |isbn=978-0742556546}} while the availability of new conventional oil and gas resources declined.{{cite web|url=http://www.hubbertpeak.com/Lynch/|title=Michael Lynch Hubbert Peak of Oil Production|publisher=Hubbertpeak.com|access-date=2013-11-03}}{{cite book | title= Oil Crisis | last= Campbell | first=CJ | year= 2005 | isbn= 0-906522-39-0 | page= 90 | publisher= Multi-Science Pub. Co. | location= Brentwood, Essex, England }}{{efn|the expression "conventional resources" refers to oil or gas derived from conventional reservoirs}} 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,{{efn|restricted hydraulic fracturing (aka fracking or fraccing) compensates for formation damage in proximity to the well bore, whereas pervasive or high volume fraccing penetrates deep into the surrounding rock strata. Fraccing works by allowing oil or gas to flow to the well-bore by opening fracture pathways through impermeable rock}} thereby reducing the volume of oil and gas left in the ground to an economic minimum.{{cite book| last = Hyne| first = Norman J.| title = Nontechnical Guide to Petroleum Geology, Exploration, Drilling and Production| url = https://archive.org/details/nontechnicalguid00hyne| url-access = limited| publisher = PennWell Corporation| year = 2001| pages = [https://archive.org/details/nontechnicalguid00hyne/page/n447 431]–449| isbn = 9780878148233}}

{{efn|the costs of enhancing recovery are high}} By the turn of the millennium, 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,US Energy Information Administration, [http://www.eia.gov/naturalgas/data.cfm Natural gas data], accessed March 21, 2014. 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.{{cite web |last1=Erbach |first1=Gregor |title=Unconventional gas and oil in North America |url=https://www.europarl.europa.eu/RegData/bibliotheque/briefing/2014/140815/LDM_BRI%282014%29140815_REV1_EN.pdf |website=EPRS In-depth analysis |publisher=European Parliamentary Research Service}}{{cite news |last1=Anon |title=Leader:America's oil bonanza |url=https://www.economist.com/leaders/2012/11/17/americas-oil-bonanza |access-date=20 November 2022 |agency=The Economist |publisher=The Economist Newspaper Limited |date=17 November 2012}}

The distinction between conventional and unconventional resources reflects differences in the qualities of the reservoir and/or the physical properties of the oil and gas (i.e. permeability and/or viscosity).Bear, Jacob, 1972. Dynamics of Fluids in Porous Media, Dover. {{ISBN|0-486-65675-6}}{{Cite book |last1=Tissot |first1=B.P. |last2=Welte |first2=D.H. |date=1984 |title=Petroleum Formation and Occurrence |page=476|doi=10.1007/978-3-642-87813-8|isbn=978-3-642-87815-2}}{{cite journal |last1=Cander |first1=Harris |title=Abstract:What Are Unconventional Resources? A Simple Definition Using Viscosity and Permeability |journal=AAPG - Poster Presentation Annual Convention and Exhibition |date=2012 |url=https://www.searchanddiscovery.com/documents/2012/80217cander/ndx_cander |access-date=24 November 2022}} 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 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 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.{{cite book |last1=Zee Ma |first1=Y |last2=Holditch |first2=Stephen A.|title=Unconventional Oil and Gas Resources Handbook Evaluation and Development |date=2016 |publisher=Elsevier Inc. |isbn=978-0-12-802238-2}} 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,{{efn|e.g. tar sands and immature oil shales}} 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.{{efn|risking for conventional reservoirs is primarily in finding the resource; in unconventional, it is finding a quality resource, defining the resource limits (measured by the EUR per well), which means the well itself defines the extent of commercial viability}}

As with all forms of fossil fuel, there are established issues with greenhouse gas emissions through export (distribution) as well as consumption (combustion), which are identical whether the oil or gas are derived from conventional or unconventional reservoirs.{{cite web |last1=United Nations |title=IPCC Sixth Assessment Report |url=https://www.ipcc.ch/report/ar6/wg3/ |website=IPCC |publisher=United Nations |access-date=24 November 2022}} 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 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.{{cite book |last1=Ahlbrandt |first1=Thomas S. |last2= Charpentier |first2=Ronald R.|last3=Klett |first3=T.R. |last4= Schmoker |first4=James W.|last5= Schenk |first5=Christopher J.|last6= Ulmishek |first6=Gregory F. |title=Global Resource Estimates from Total Petroleum Systems |date=2005 |publisher=American Association of Petroleum Geologists |isbn=0891813675}}{{cite web|title=Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States|url=http://www.eia.gov/analysis/studies/worldshalegas/pdf/fullreport.pdf|publisher=U.S. Energy Information Administration (EIA)|access-date=June 11, 2013|date=June 2013}}

{{Portal|Energy}}

• Source rock
• Petroleum trap
• Fracking in the United States
• Environmental impact of fracking
• Coalbed methane
• Methane clathrate (gas hydrate)
• Shale gas
• Synthetic natural gas, such as oil shale gas
• Tight gas
• Oil sand
• Tight oil
• Extreme energy
• Renewable energy
• Future energy development
• Hubbert peak
• Energy development
• Alternative fuels
• World energy resources and consumption
• Oil megaprojects
• Creveney shale mining operation

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{{Petroleum industry}}

Category:Petroleum industry

Category:Unconventional oil

Category:Unconventional gas

Category:Peak oil

Category:Petroleum production

Category:Petroleum geology

Category:Reservoir rock formations

{{DEFAULTSORT:Unconventional (oil and gas) reservoir}}