Coals form a surprisingly heterogeneous group of fossil fuel, much more diverse than crude oils and natural gases. Considerable ranges apply to the presence of moisture (from <1% to > 40%), ash (incombustible minerals, from <1% to > 40%) and sulfur (from a trace to more than 5%), and even energy density, one of the two key variables that determines power density of coal resources, has more than a three-fold range. Anthracites (essentially carbon with minimal impurities) have energy density (all rates are higher heating values) of about 30 GJ/t, high-quality bituminous coals range from 24-30 GJ/t, sub-bituminous coal (commonly used for electricity generation) range between 18-24 GJ/t and, due to their high moisture and ash content, lignites (brown coals) span an even wider range, all the way to less than 10 GJ/t (Smil 2008).
The other key variable in appraising coal’s recoverable deposits is the thickness of coal seams: it ranges from less than 30 cm, too thin to be exploited by modern mining methods (but commonly extracted with the help of simple tools by hard and dangerous manual labor in traditional underground operations), to stunning near-surface accumulations of coal that are not just tens of meters but in some places (such as in Victoria’s Latrobe Valley) individual seams are up to 165 m and stacked coal seams are up to 250 m thick (Australian Government 2012). In contrast, modal values for seams that can be extracted only by underground methods are only between 0.5-2 m; for example Northern Appalachian Pittsburgh coal bed has extensive blocks up to 1.9-2.4 m thick (USGS 2013). Combination of these two variables determines the theoretical extremes of power densities of coal extraction. A thin 1-m thick seam of German Braunkohle, low-quality lignite with 8.5 GJ/t and specific density of 1.2 t/m3, stores just over 10 GJ/m2 while a 30-m thick seam of high-quality bituminous coal (28 GJ/t, 1.35 t/m3) will contain about 1.1 TJ/m2.
Most of the basins have multiple exploitable seams and hence even with the fuel of lower quality their deposits will average tens of GJ under every square meter and the richest basins (some of them with low-quality fuel in multiple seams) will have stores an order of magnitude higher. US data show that the average for five richest Appalachian coal beds (Pittsburgh, Upper Freeport, Fire lay, Pond Creek and Pocahontas 3) is almost 200 GJ/m2 (USGS 2013). The latest evaluation of coal resources in the Powder River Basin in Montana and Wyoming –- where the Wyodak seam, with energy density of 20.3 GJ/t, is between 9.1-24.4 m thick –- had original stores up to 400 GJ/m2 (Scott and Luppens 2013).
In Germany’s brown coal mining region, in the Rheinland between Cologne and Aaachen, the deposit exploited by Fortuna-Garsdorf mine, yielded about 400 GJ/m2 during the 1980s, while the region’s three mines operating in 2013, Inden, Garzweiler and Hambach, extract seams with combined thickness of up to 45 m that contains about 450 GJ/m2 (RWE Power 2013). World records belong to Australian coal deposits: in the Latrobe Valley of Victoria's Gippsland Basin brown coal seams that are up to 100-m thick store more than 1 TJ/m2, and at least 50% more when inferred resources are added (Geoscience Australia 2012). And the thickest parts of the Number 3 seam of high-quality bituminous coal (32 m; 24.5 GJ/t) in Queensland's Blair Athol, whose production is now winding down and should cease in 2016 (Rio Tinto 2013), had also stored at least 1 TJ/m2.
Underground mining Actual power densities of coal production depend not only on the energy stored in seams but also on the modes of extraction. Traditional underground room-and-pillar technique had to leave about half of all coal in place to support the tunnel roofs, but longwall mining has made it possible to nearly double that rate. This technique, pioneered in Europe and spread to the US only during the 1970s, uses large shearers (drum-shaped cutting heads) to cut coal from the length of a seam (some longwalls are now 300 m long) and dump it onto conveyors while the miners stay protected under a jack-supported steel roof that advances along a seam as the cutting progresses and the roof behind it is left to cave in (Osborne 2013). Longwall mining can recover more than 90% of coal in place as long as the seams are reasonably level or slightly inclined. This technique boosted productivity to such an extent that the two largest underground US coal mines (Bailey and Enlow Fork in Pennsylvania) now produce close to 10 Mt/year (USEIA 2011).
Coal mining land claims fall into two obvious categories. Land occupied by permanent structures includes all buildings containing stationary operating equipment, maintenance shops, parts stores, parking lots, facilities to process (wash, sort, crush) raw coal before marketing and load-out arrangements to transport the fuel. The extent of this claim may change with time as a particular operation grows or declines, and in the case of underground mining it can be fairly limited. Above-ground structures of underground mines include buildings to operate hoisting and ventilation machinery, offices, machine shops and storages. For a mine producing high-quality coal that needs to be only crushed to uniform sizes before marketing the only other permanent structures could be a crusher and a silo to store coal and loading facilities for trucks or railroad cars: all of these could occupy only about 1 ha in a mine producing 1 Mt/year.
But most coals need preparation (washing, rock removal, and crushing to specific sizes requested by buyers) and hence most underground mines have on-site facilities to perform those tasks (Leonard and Leonard 1991; Arnold, Klima and Bethel 2007). The waste generated by coal preparation makes up the second, incremental, category of land claims as more space is taken every year by the disposal of rock separated from coal (in the past they were often stored in tall conical heaps that signalled the presence of mines from afar) and by often extensive tailing ponds (fine particular waste suspended in water). Surface mining may generate the same kind of claims to store waste products, but most of its incremental space demand will be due to the necessity to remove and reposition often large volumes of the overburden (sands, clays and rocks) that covers coal seams.
Even so, large underground coal mines make relatively limited land claims, an obvious consequence of concentrating the output from a large area of underground corridors into a fairly small area of the mine’s above-ground operating and fuel processing facilities. The total for Bailey mine (Waynesburg, southwestern Pennsylvania) is about 60 ha, and with annual output of 9.8 Mt (at 30 GJ/t) that translates to a very high power density of about 15,000 W/m2. Similarly, even after counting all land claimed by past storage of mining waste and tailings, the Enlow Fork Mine, the second largest underground operation in the US, also in Pennsylvania, occupies roughly the same areas and its extraction power density is about 14,000 W/m2. Rates for smaller mines with poorer coal in thinner seams will be an order of magnitude lower, but commonly more than 2,000 W/m2.
But in many relatively shallow mines the tunnelling has additional surface impacts far away from a mine shaft due to ground subsidence. For example, by the end of 2009 underground operations of the just mentioned Bailey and Enlow mines in Pennsylvania extended over, respectively, over 12,600 ha and 14,000 ha, with further areas intended for expanded longwall mining (Schmid & Company 2010). Surface subduction caused by underground mining is sometimes a continuous slow process that damages streets and buildings, in other cases it is a sudden catastrophic collapse. Impact on local water resources is another surficial impact of underground mining that is of obvious concern to communities relying on those water supplies.
Surface mines Removal of the overburden in surface (open-cast) has become an increasingly ambitious enterprise. Only in exceptional cases there are thick coal seams hidden beneath only 10-20 m of overburden: in Australia’s Latrobe Valley brown coal seams are up to 100 m thick and in places multiple seams create a nearly continuous coal layer up to 230 m thick under only 10-20 m of easily removed sandy overburden (Department of Primary Industries Victoria 2010). With improving technical capabilities surface coal mining has moved to exploit reserves whose overburden:coal seam ratio is higher than 3:1, in a few cases even surpassing 6:1, with the deepest mines (such as Rhineland’s Hambach) now extending to as much as 370 m below the surface.
Topsoil from fields is put aside for future reuse or is used in ongoing recultivation projects on the site, entire villages or small towns may have to be relocated or their inhabitants resetlled elsewhere (Michel 2005), and the overburden (usually sedimentary clays and rocks) is removed by some of world’s largest electric machines: stripping is done either by giant bucket excavators (common in German mines and deployed to create terraced cuts) or by walking draglines (used in the US). German practice is to deposit the overburden by spreaders behind the cut or carry it further away by conveyor belts and use it to backfill older mined-out sites or create artificial table-top accumulations. In the US surface mining in the West, including the world’s largest open-cast mines in Wyoming, is done by terraced trenches or pits in ways similar to German operations.
In the East open mines in the Appalachia states follow the contours of hills following the outcropping seams, or they proceed in a much more destructive way: since the 1980s mountaintop removal has already levelled more than 500 peaks, mostly in Kentucky and West Virginia (Copeland 2013; Perks 2009; McQuaid 2009). Summits or ridges above coal seams are removed by blasting away layers of rock often more than 150 m thick. Laws require to return this overburden to the mined areas but the spoils are commonly dumped into adjacent valleys creating massive fills that bury streams, may be more than 300 m wide and more than 1.5 km long with volume of more than 200 Mm3. Extensive land claims of mountaintop mining are also illustrated by averages of land occupied per mining unit in West Virginia: for underground mines it is just over 16 ha, for mountaintop operations it is about 140 ha.
Coal is usually mined by giant electric shovels (with bucket able to scoop more than 100 t) and its recovery rates are high: in the world’s largest mines Wyoming they average 91% (WSGS 2013). Cut coal is transported from the coal face either by conveyor belts (in Europe) or it is hauled away by huge off-road Liebherr, Komatsu or Caterpillar trucks capable of carrying loads of more than 300 t (in the US). In some mines coal is moved by conveyor belts directly from the mining face to a nearby mine-mouth electricity generating plant, but in most cases more land is claimed by loading facilities to transport the fuel by railroads: for long-distance deliveries it is rapidly loaded into railway cars coupled in unit trains of up to 150 cars (Khaira 2009).
Total loads of these trains are usually in excess of 10,000 t and their loading is done in less than two hours. In large open-cast Wyoming mines these load-out arrangements consist of a railway loop, tall coal silos and automatic loading dispensers that operate 24/7: in Black Thunder this rapid load-out (including the railroad loop) occupies less than 70 ha, in Cordero mine less than 20 ha. Unit trains constantly peddle between a mine and their destinations, large thermal electricity-generating plants that might be hundreds, even thousands of km away, and coastal loading terminals where coal is transferred to large bulk carriers for intercontinental shipping (mostly from Australia, Indonesia, South Africa). Some long-distance transport is also done very inefficiently by trucks (commonly in China as railroads are already overburdened by coal shipping) and very efficiently by river barges (in the US on the Mississippi).
Some open-cast coal mines publish their cumulative and annual land claims, and satellite imagery makes it easy to assess not only the extent of the areas directly affected by ongoing open-cast extraction but usually also of the previously exploited areas that have been simply abandoned, replanted with grasses and trees or converted into water reservoirs. Calculations based on the company’s data (RWE Power 2013) show the following annual power densities of lignite extraction in the three Rheinland open-cast mines: Inden 2,400 W/m2, Garzweiler about 2,800 W/m2 and Hambach approximately 5,000 W/m2.
For the world’s two largest open-cast mines, North Antelope Rochelle mine complex (Peabody Powder River Mining, near Wright, Wyoming) and Black Thunder complex (owned by Arch Coal Company, operating six pits), recent power densities of coal extraction have been almost 12,000 W/m2 (WSGS 2013; Arch Coal 2013). In 2010 and 2011 Queensland’s Blair Athol extracted coal with densities around 2,400 W/m2 when counting only incremental land claims (Rio Tinto 2013). China, the world’s largest coal producer, extracts most of its coal from underground mines, but is largest open-cast operation, Heidaigou mine south of Junggar in Nei Monggol produces sub-bituminous coal (16 GJ/t), from a site that now covers about 15 km2, with power density of less than 1,000 W/m2 (Zhang and Cotterill 2008; Google Earth 2014).
In contrast to pit or trench extraction, mountaintop removal is, undoubtedly, the most space-demanding method of coal extraction (as well as one causing great harm to streams, water quality and biodiversity) as scores of these operations cover more than 500 ha and as the largest mountaintop mine can claim more than 2,500 ha and result in dumping some 750 Mm3 of spoils. Extraction in some of these mines –- where massive volumes of rock are blasted away just to get at some thin seams –- proceeds with power densities lower than 200 W/m2 or even less than 50 W/m2, or the same order of magnitude as PV conversions.
I have chosen extreme cases for these calculations in order to establish the full range of power densities of coal production, and there is a helpful way to verify the results by using aggregate US data on land disturbed by coal mining. According to the annual reports of the US Office of Surface Mining and Reclamation between 1996 and 2009 surface mining in Wyoming disturbed about 25,700 ha for a cumulative output of 4.37 Gt (Source Watch 2011). Converting the output with average energy density of 20.3 GJ/t yields high power density of almost exactly 11,000 W/m2 for Wyoming coal extraction. In contrast, in 2009 power density for surface coal mining in Tennessee was only about 350 W/m2, a very low rate caused by the state’s extensive mountaintop extraction. Land claims in other states fall between these two extremes and, as expected, nationwide totals of 624,400 ha and 8.69 Gt for the years 1996-2009 translate to a considerably lower average than the Wyoming rate: in fact, at just over 1,000 W/m2 they are an order of magnitude lower.
Those open-cast operations following the strictest rules may cause only temporary disruption as the land can be contoured to approximate its natural state and replanted with grasses, shrubs and trees or converted to water surfaces within 3-5 years after the mining had ended. On the other hand, in every coal-mining country has large areas of old open excavations, overburden heaps, rock waste from underground mining and mine tailings that have never been reclaimed. Since 1977 when the US Congress passed the Surface Mining Control and Reclamation Act the program reclaimed nearly 100,000 ha of land affected by coal mining but that still leaves almost 5,200 coal-related abandoned mine sites to be fully reclaimed (AMLP 2013).
Coal transportation Adding land needed for coal transportation outside of mine loading facilities and unloading structures at power plants of ports could be done easily only in the case of a dedicated line used for no other shipments –- but then it might not turn out to be a significant addition. For example, a double-track line used only to move coal between an open cast mine (extracting 10 Mt/year with power density of 2,500 W/m2) and a 4 GWe power plant 500 km away would occupy (even when assuming, liberally, right-of-way of 20 m for two tracks) 1,000 ha –- but during the 30 years of extraction the mine would disturb more than 11,000 ha.
In reality, a large mine usually supplies many customers and unit trains travel over many lines and share them with other traffic (Kaplan 2007). For example, coal from the Black Thunder complex is transported via Burlington Northern Santa Fe and Union Pacific railroads to some 115 power plants in more than 20 states, as well as to Europe (BNSF 2013). While it might be possible to calculate the share of the right-of-ways attributable to coal that has originated from that mine since it began its coal extraction (on the basis of a detailed breakdown of annually carried cargo) it would, when prorated over many decades, represent a negligible addition to the fixed and incremental space claims made by that large open-cast mine in Wyoming’s Powder Basin. Similar consideration apply to land claimed by large railway terminals where coal is received and loaded for overseas export: Lamberts Point Coal Terminal in Norfolk, VA occupies more than 150 ha but because it can handle up to 44 Mt of exports a year (Dinville 2013) power density of its throughput prorates to at least 24,000 W/m2, a very high rate whose proportional attribution to coal production from individual mines would add a negligible amount to their overall land claim.
Perhaps the most notable conclusion of this survey of power densities of modern coal extraction is their wide range. The most productive underground mines using longwalls to exploit relatively thick seams of high-quality coal have small surface footprints and produce the fuel with power densities in excess of 10,000 W/m2. In contrast, underground mines exploiting thinner seams of poor quality coal have a fairly large surface footprint, mostly due to the accumulation of incombustible waste and tailings produced by the requisite coal cleaning, and their power densities are often well below 1,000 W/m2. And the differences are even greater for surface mines: some mountaintop removals in the Appalachia have power densities, well below 100 W/m2, while the rates in some of the world’s largest mines exploiting the world’s richest coal seams in Wyoming’s Powder River basin or Australia’s Latrobe Valley exceed 10,000 or even 15,000 W/m2. Power densities of coal extraction thus span two orders of magnitude, from rates nearly as low as those of PV electricity generation to the rates that surpass those of rich hydrocarbon fields.
Crude oils, refining and long-distance deliveries
Crude oils are much more energy dense than coals (most of them cluster tightly around the mean of 42 GJ/t), their share of incombustible constituents is negligible but their sulfur content is often high (>2%) and it serves to divide the liquids into sweet (low-S) and sour (high-S) streams (Smil 2008). Another key division is the fuels’ specific density, with light oils containing higher share of lighter fractions and heavy oils requiring expensive catalytic cracking to produce higher shares of the most valuable transportation fuels (gasoline and kerosene). Worldwide demand for crude oil has been easier to satisfy not only because of the presence of such extraordinarily rich resources as the Middle Easter, Western Siberia or Texan oilfields but also because of relatively inexpensive ways of long-distance delivery, by pipelines on land and by tankers for intercontinental trade.
Estimates of oil resources in place are inherently uncertain but when using the best available totals for the world’s major oilfields and prorating them per unit of surface area that corresponds to the underground extent of the reservoirs the resulting power densities are very similar to those of coal deposits. For many smaller fields the rates are less than 1 GJ/m2 and even some giant /oilfields (the designation applies to any field that contains 500 million barrels, or 79,000,000 m3, of ultimately recoverable oil) store less than 10 GJ/m2. Only the richest fields (both spatially extensive giants and unusually concentrated formations) store 101-102 GJ/m2. As many fields also contain substantial volumes of associated natural gas their original storage should be reported for the combined content of the two hydrocarbon fuels.
Here are a few well-known examples of original storages in the ascending order of energy densities (Nehring 1978; Robelius 2007; Li 2011). Algerian Hassi Messaoud, known for its exceptionally light and sweet crude, rates 35 GJ/m2. Saudi Shayba, a supergiant in the desolate sands of Rub’ al’Khālī, stores about 130GJ/m2. Samotlor –- Russia’s largest oil field (and number six in the global ranking) covering 1,752 km2 in the Tuymen region of Western Siberia –- had original stores of about 180 GJ/m2. Saudi al-Ghawār –- the world’s richest oilfield and a true super-giant that extends over 220,000 ha in the Eastern Province of Saudi Arabia –- originally contained about 220 GJ/m2 of crude oil and 40 GJ/m2 of natural gas for the total of 260 GJ/m2. Alaska’s Prudhoe Bay (North Slope), the largest (85,417 ha) and the richest hydrocarbon field in North America, had originally in place about 145 EJ of crude oil and 44 EJ of natural gas (BP 2006) and hence the aggregate energy storage density of about 220 GJ/m2. And the Greater al-Burqān –- Kuwait’s largest field and the world’s largest petroliferous sandstone formation, just south of the Kuwait City, (780 km2 and about 440 EJ of oil and 65 EJ of natural gas originally in place) –- stored about 560 GJ/m2.
Most of oil in the Earth’s crust is not in liquid form but it is interspersed in sands or shales, and storage density of these non-conventional oil resources rivals that of the richest classical fields. This is hardly surprising as the shares of oil in these rocks are low (commonly less than 10%) but the oil-bearing strata can be very thick. The world's largest concentration of oil interspersed in the shales is the Green River formation that underlies parts of Colorado, Utah and Wyoming where the oil-bearing layers are up to 150 m thick. Piceance Basin in Colorado is the world’s largest deposit of shale oil: there are roughly 210 Gt of oil locked in the rock under its roughly 18,200 km2 (Johnson et al. 2010), prorating to nearly 500 GJ/m2 of energy originally in place. That is more than twice the density in the world’s richest field that produces liquid oil -– but the technical challenges of extraction oil from shale and cost of such operations are incomparably greater than getting oil from giant oilfields that are initially under high natural pressure while the net energy returns of such endeavors are considerably lower.
Power densities of oil production Extraction densities depend on the richness of exploited reservoirs, methods of oil recovery (free flowing wells, artificial lift aided by water flooding or injection of gases), density of wells (uncontrolled in early decades of oil era, resulting in veritable forests of crowded oil rigs; now carefully planned to optimize the output) and their productivity (that, too, is deliberately controlled either to extend the production span of an oilfield or to match the available pipeline capacity). All power densities in this book are standardized in terms of annual fluxes but I will make an exception here and offer approximations of maximum daily power densities for the world’s most famous oil well gushers (SJVG 2010).
The earliest gushers were in the Baku oilfields: Bibi Eibat drilled on September 27, 1886 flowed at 84,000 barrels per day (bpd) which is (with 5.8 GJ/barrel) about 5.6 GW; starting at October 15, 1927 Baba Gurgur near Iraqi Kirkuk spewed 95,000 bpd for eight and half days (6.4 GW); famous Spindletop near Beaumont, Texas, produced 100,000 bpd on January 10, 1901 (6.7 GW); and the recorded maximum is for Cerro Azul No. 4 well near Tampico (Veracruz, Mexico) that blew on February 16, 1916 and when it was capped three days later its February 19 flow was 260,858 bpd or 17.5 GW. (For comparison, BP’s Macondo well, that spilled nearly 700,000 t of oil into the waters of the Gulf of Mexico between April 20 and July 15, 2010, had the maximum flow of about 62,000 bpd, or 4.2 GW.)
And all of these GW-sized blow-outs spewed from pipes whose area was no more than 0.01 m2! Even if were to prorate such flows across a large drill site of 2 ha (200x100 m) we would get power densities on the order of 105 W/m2 (300,000 W/m2 for a 6-GW gusher). No other short-term power densities of energy extraction are even remotely close to these extraordinary, but necessarily short-lived, sudden releases of accumulated stores of high energy-density fossil fuels. Obviously, such flows can last only for hours or days, and the output of every naturally pressurized well will decline with time, usually following a fairly predictable course. Secondary recovery methods will improve declining flows but they will claim more land for new injection wells as well as for the delivery of injected water or gases.
Perhaps the most representative examination of land claimed by conventional crude oil production comes from the dataset of 301 California oil fields covering some 3,000 km2 and containing at least 58,000 production wells, 22,000 shut-in wells and 25,000 injection wells, and average density of 31 wells/km2 (Yeh et al. 2010). Calculations of land disturbed per well were based on satellite image analysis of three fields and they resulted in the range of 0.33-1.8 ha/well with the average of 1.1 ha/well and that rate includes not only the cleared or occupied area surrounding a well but also all access roads and other facilities found in each image. The authors used the identical approach to assess land claims of Alberta’s conventional oil fields whose much lower density of only 0.3-2.5 wells/km2 translated into larger land claims, ranging from 1.6 to 7.1 ha/well and an average of 3.3 ha/well (Yeh et al. 2010).
Satellite images of sufficiently high resolution (showing objects smaller than 5 m) are now readily available on Google Earth and they indicate that in the world’s most productive fields the extent of land disturbance per well is closer to the just noted Californian mean rather than to the Albertan average. Saudis have been hardly forthcoming with detailed information about the development and production of the world’s largest super-giant oilfield but we know that al-Ghawār has about 3,400 wells that dot its elongated (along NNE-SSW axis) shape in the Eastern Arabian desert between Fazrān and Haradh (Afifi 2005), and that these wells are typically rectangular areas of 100-150 m (1.5 ha) graded in sand (Google Earth 2014). The field also has many water injection wells –- with water brought by pipelines from the Persian Gulf –- as water flooding has become increasingly necessary to keep up its productivity. Samotlor and its wells dotting the Siberian forest and wetlands claim mostly between 0.5-2 ha.
And a detailed study of land disturbance associated with oil and gas development in the Upper Colorado Basin found that the lifetime average pad size in Utah, Colorado and Wyoming was 1.04 ha (Buto, Kenney and Gerner 2010). But considerably more land could be disturbed if there is a need to build new long access roads and to clear land for larger storage yards. As for the oil well densities, their maxima are prescribed in many jurisdictions. For example, in the East Texas field rules require minimum distance of 200 m between adjacent wells: this implies maximum of 25 wells/km2; West Texas field also has more than 20 wells/km2, Oklahoma assigns 16 ha as the area drained by shallower wells and 32 ha for deeper wells (implying densities of, respectively, 6.25 and 3.12 wells/km2 while some smaller fields in the US have more than 50, others less than 5 well/km2.
Lack of accurate information makes it impossible to offer reliable calculations of peak and short-term extraction densities for the world’s largest super-giant oil fields. Assuming that al-Ghawār’s output peaked at 250 Mt/year its maximum extraction power density would have been only on the order of 150 W/m2 when that output is prorated over an entire footprint (2,200 km2) of the reservoir, but it rises to about 5,000 W/m2 when only areas disturbed by wells and roads are considered. Most of al-Burqān’s 350 wells (famously set on fire by Saddam Hussain’s retreating army in 1991) have areas of about 1 ha, but the total reservoir area is given either as 500 km2 or as much as 820 km2 (when two smaller associated formations are included), and the field’s peak output was either 120 or 140 Mt/year in 1971 or 1972 (Sorkhabi 2012). Consequently, the field’s maximum operating power density would be as much as 370 W/m2 (prorated over the reservoir’s smallest area) and more than 20,000 W/m2 when only areas disturbed by wells and roads are considered.
But in order to convert land claims to truly representative power densities of oil extraction we need to know long-term productivities of oil wells. These rates have a very wide range, from (as already noted) famous gushers of early exploration years whose output (after controlling their enormous initial bursts) remained high for many years, to marginal wells whose initial flow is barely economical. Inevitably, all natural well flows decline with time and hence power densities for the early years of extraction will be substantially higher than in later years or decades. Again, no accurate rates can be offered for the largest Middle Eastern reservoir because the total mass of their ultimately recoverable oil and the complete duration of their exploitation are uncertain. For example, the total for al-Burqān are is as low as 6 Gt and as high as 10.5 Gt and the field is still producing at about half of its peak rate nearly 70 years after the beginning of its exploitation.
The best way to gauge long-term power densities is to look at the history of those American oilfields that have been exploited for many generations. For example, power density of production from the California oil wells studied by Yeh et al. (2010) averaged about 2,500 W/m2 for the cumulative 1919-2005 output, but only about 1,700 W/m2 for 2005 extraction, while historical (1958-2007) density for Alberta oil wells was roughly 1,100 W/m2 and the marginal rate for 2007 declined to about 640 W/m2. Data for the evolution of West Texas Wasson oilfield near the New Mexico border (Smith 2013) illustrate this process of gradual decline in a greater detail.
The field’s first well drilled in 1936 produced 234 bpd, and if it claimed about 1 ha then its short-term production power density was about 1,500 W/m2; by 1938 201 flowing wells (assuming the same area of 1 ha/well) averaged about 250 W/m2; the field’s peak output in 1948 prorated, with 1,588 wells, to about 320 W/m2. The field is clearly outlined on satellite images by its fairly regularly spaced wells (Google Earth 2014): many are in almost perfectly regular square grid pattern and occupy usually no more than 2,500 m2 (0.25 ha) which means that by 1992 the field’s 2,242 wells (and their service roads) claimed only some 900 ha and addition of several associated gathering and processing facilities raised the total to about 1,000 ha and the power density of the field’s cumulative output over 56 years of extraction would have been about 600 W/m2.
US data illustrate the process of a national scale: average productivity of US oil wells rose from 13 bpd in 1955 to peak at 18.5 bpd in 1972 and since that time it has declined to 10.8 bpd in 2000 and 10.6 bpd in 2011 (USEIA 2012). The decrease would have been much higher without extensive use of secondary recovery, now a standard practice in all aging oil fields that relies on injections of water or gases to boost oil flow (SPE 1999). In contrast, directional and horizontal drilling (now routinely practiced around the world) enables to reach large volumes of oil-bearing layer from a single well and thus spare the land and boost average power densities of crude oil extraction. But these innovations, even when combined with new, highly productive discoveries, could not prevent the global decline of productivity and hence worldwide reductions in power densities of oil extraction.
Oil & Gas Journal has been publishing annual global reviews listing the numbers of producing wells and average production rates for nations as well as for major oilfields and these data make it easy to derive correct orders of magnitude for power densities of oil extraction and their changes over time. In 1972, the last year of inexpensive oil (in the fall of 1973 came the first of OPEC’s large price increases that quintupled the cost of a barrel), Saudi Arabia had only 627 wells and their output (even when assuming 2 ha/well) prorated to about 40,000 W/m2 while average power densities for the entire Middle Eastern oil extraction were nearly 25,000 W/m2, and the worldwide mean (excluding the USSR) was around 500 W/m2. This low global figure was largely due to low productivity of many thousands of old and older US wells: even when assuming just 1 ha/well the US mean in 2012 was only 125 W/m2. Clearly, Mielke’s contemporaneous estimates of land claims by the US onshore oil extraction –- 3.03-6.9 acre-year/1012 Btu, that is 1.2-2.8 ha/PJ or 1,100-2,600 W/m2 (Mielke 1977) –- referred to well sites occupied by highly productive new wells.
Oil & Gas Journal well and output data for the year 2012 show that power densities of oil extraction had fallen everywhere, with the means (even when assuming just 1 ha/well) at about 23,000 W/m2 for Saudi Arabia, less than 9,000 W/m2 for the Middle East and about 100 W/m2 for the US, with the global mean (including all of the world’s countries) at about 650 W/m2 (Oil & Gas Journal 2012). These calculations allow two firm conclusions. First, power densities of oil extraction range between 102 W/m2 for old, or older, oil provinces outside the Middle East (some of them, including Azerbaijan, California and parts of Texas have been exploited for more than a century) to 104 W/m2 for the most productive fields on the Arabian Peninsula. Second, decline of average power densities has been unmistakable, and although this trend has been be slowed-down by secondary recovery it clearly illustrates the maturity, even senility, of many of the world’s major oilfields producing conventional liquid fuel.