Unconventional (oil and gas) reservoirs, orunconventional resources (resource plays) areaccumulations where oil and gasphases are tightly bound to the rock fabric by strongcapillary forces, requiring specialized measures for evaluation andextraction.[1]
Oil and gas aregenerated naturally at depths of around 4 or 5 km below Earth’ssurface.[a] Being lighter than the water-saturated rocks below thewater table, the oil and gas are driven bybuoyancy up throughaquifer pathways towards Earth's surface over time. Some of the oil and gas percolate all the way to the surface as naturalseepages, either on land or on the sea floor. The rest remains trapped underground bygeological barriers[b] in a variety oftrap geometries. In this way, undergroundpockets of oil and gas accumulate by displacing water inporous rock. If the pockets arepermeable, they are referred to asconventionalreservoirs. Wells are drilled into these reservoirs to create a path for oil and gas to reach the surface. Whenpressure differences are relatively high, oil and gas rise to thewell bore naturally through buoyancy.[c] Where the pressures are low, flow can be assisted with pumps (e.g.nodding donkeys).[2]
In the early days of theoil industry, there was no need forstimulation to improverecovery efficiency, because supply vastly outstripped demand and leaving "difficult" oil and in the ground was economically expedient.[3] Twoworld wars, followed by huge economic growth resulted in surging demand for cheap portable energy,[4] while the availability of new conventional oil and gas resources declined.[5][6][d] The industry initially sought to enhancerecovery of trapped oil and gas, using techniques likerestricted, or low volume hydraulic fracturing to stimulate the reservoir further,[e] thereby reducing the volume of oil and gas left in the ground to an economic minimum.[7][f]
Around 1976, theUnited States Department of Energy directed groundbreaking research that catalyzed several industrial innovations:[8]
By the turn of themillennium, a new kind ofenergy resource was required, particularly by the USA, who were driven to achieveenergy independence. The USA turned tounconventional reservoirs to achieve their goals,[9] which had been known about for decades but had previously been too costly to be economically attractive. Today, unconventional reservoirs includebasin-centered gas,shale gas,coalbed methane (CBM),gas hydrates,tar sands,light tight oil andoil shale, mostly from North America.[10][11]
The distinction between conventional and unconventionalresources reflects differences in the qualities of thereservoir and/or the physical properties of the oil and gas (i.e.permeability and/orviscosity).[12][13][14] These characteristics significantly impact predictability (risk to find, appraise and develop) and in turn the methods of extraction from those reservoirs such asfracking.
Conventional oil & gas accumulations are concentrated by buoyancy drivenaquifer pathways intodiscrete geological traps, which are detectable from the surface. These traps constitute relatively small but high resource densityfields. Most conventional oil or gas fields initially flow naturally by buoyancy alone into the well bore, with their limits defined byfluid mechanics measurable from the well bore (e.g. fluid pressure,OWC/GWCetc.). In general, the technical and commercial risk associated with discrete conventional reservoirs can be reduced using relatively inexpensive remote techniques such asreflection seismology and extracted with relatively few appraisal and development wells.[2]
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.[15] 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,[g] using techniques such asfracking orsteam 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 economicreserves/well limit defined bywell delivery.[1][h]
| Reservoir | Phase | Density[D 1] | Flow[i] | Main predictors[D 2] | Min extraction[D 3] |
|---|---|---|---|---|---|
| Conventional[D 4] | Oil & gas | high | buoyancy | Well bore pressure;Reflection seismic | Well bore |
| Basin-centered gas[D 5] | gas | low | capillary | drilling | well bore (fracking) |
| Shale gas[D 6] | gas | low | capillary | drilling | well bore (fracking) |
| Coalbed Methane[D 7] | gas | high | adsorption | drilling | well bore (depressurization) |
| Gas hydrates[D 8] | gas | high | ?buoyancy? | Reflection seismic; drilling | ?mining/well bore? |
| Tar sands[D 9] | oil | high | ?capillary? | drilling/mining | steam flood |
| Light Tight Oil[D 10] | oil | low | capillary | drilling | well bore (fracking) |
| Oil shales[D 11] | oil | high | bonded | mining | retort (sub mature) |
As with all forms offossil fuel, there are established issues withgreenhouse 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.[16] Theircarbon 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 andCBM). The artificial transfer of heat and pressure require the use of large volumes offresh water creatingsupply 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.[17][18]
