question: What happens to an oil field after the oil is pumped out? Does it just leave a void/cavern? Isn't it today to leave that because it could collapse?
comment: Oil isn't in a big cavern underground. It's seeped into the rock, and when a hole is drilled through the pressure of the Earth compressing the rock squeezes the oil out. So when there's no oil left, there's still plenty of rock down there. In some formations depleting the oil/gas can cause some ground subsidence, but usually nothing dangerous.
passages: While neither field is producing in the present day, periodic attempts have been made to find and cap leaking wells from the operations in the late 19th and early 20th century. In those early years of the petroleum industry, there was little to no regulation of operating practice, and abandoned wells were often filled with whatever was available – earth, wooden poles, old mattresses and other rubbish were commonly stuffed down the holes, but these methods are ineffective for long-term plugging of an oil well. Since petroleum migrates upwards from pressurized source regions, over time it will find any open pathways to the surface; in the case of natural gas, this can cause an explosion hazard. Modern well abandonment techniques involve cutting off a well casing well below ground surface and capping with concrete, forming a permanent seal. Between 1960 and 1968, eighty old wells were located and capped in an attempt to stop the slow leaking of oil into the ocean. More wells were located, cut, filled, and capped in 1975 and 1993.
   During the "primary recovery stage" (also called the 'Kareem Stage'), reservoir drive comes from a number of natural mechanisms. These include: natural water displacing oil downward into the well, expansion of the natural gas at the top of the reservoir, expansion of gas initially dissolved in the crude oil, and gravity drainage resulting from the movement of oil within the reservoir from the upper to the lower parts where the wells are located. Recovery factor during the primary recovery stage is typically 5-15%.
   Section::::Environmental impact.:Seismicity.:Subsidence.
  
  There is no documented evidence of hydraulic fracturing leading to subsidence. Operations are commonly monitored with tiltmeters, and no compaction issues have been documented. Given the mechanical properties of unconventional rocks (their densities, low porosities, low Biot coefficients, and high stiffness), compaction is very unlikely to occur during hydrocarbon extraction.
  
  Section::::Environmental impact.:Seismicity.:Insurance.
   To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world, such as the La Brea tar pits in California, and numerous seeps in Trinidad.
  
  Section::::Drive mechanisms.
  
  A virgin reservoir may be under sufficient pressure to push hydrocarbons to surface. As the fluids are produced, the pressure will often decline, and production will falter. The reservoir may respond to the withdrawal of fluid in a way that tends to maintain the pressure. Artificial drive methods may be necessary.
  
  Section::::Drive mechanisms.:Solution-gas drive.
   The operators of the Coalinga field attained peak production in 1912 – of oil – a value which was to decline steadily for the next several decades.
  
  During the 1960s and 1970s, enhanced recovery technologies such as water flooding, steam flooding, fire flooding, and polymer flooding were employed to increase the declining productivity of the field, and to reach and recover previously submarginal deposits. Even with such methods, the current oil output of the field has declined considerably from the early part of the 20th century: in 2006, the field's operators reported of oil pumped.
  
  Section::::References.
   Many of the deeper pools were found in the 1960s, and some of them proved not only to have higher quality oil, but were more productive. In the Northeast Area, the Phacoides, Oceanic, and Point of Rocks pools vary from below ground surface; the temperature of the oil varies from at that depth, and the initial reservoir pressures were also high, up to , pressures that would have caused major blowouts and gushers during the early part of the century, but can be better managed with modern technology.
   The amount of oil that is recoverable is determined by a number of factors, including the permeability of the rock, the strength of natural drives (the gas present, pressure from adjacent water or gravity), porosity of the reservoir rock, i.e. the rock storage capacity, and the viscosity of the oil. When the reservoir rocks are "tight", as in shale, oil generally cannot flow through, but when they are permeable, as in sandstone, oil flows freely.
  
  Section::::Estimated ultimate recovery.
   In the early 1960s, in an effort to restore declining production, several enhanced recovery methods were employed. The first waterflooding operation began in 1963 in the Portals-Fairfax area, and in 1964 both waterflooding and cyclic steam were in use in several areas of the field, with the steam recovery being used on the heavy oil reservoirs such as Racetrack Hill, where a steam system is still in place.
   Some enhanced recovery technologies have been used since the 1960s, particularly in the Miocene and Pliocene producing horizons. Steamflooding of the Pliocene commenced in 1973, a method useful for decreasing viscosity of heavy oil and encouraging it to flow to pumping wells. The Miocene pool has been subject to cyclic steam, steamflooding, waterflooding, and fireflooding.
   One variation of the cold production method is called Cold Heavy Oil Production with Sand (CHOPS). CHOPS creates a wormhole or void where oil gets pulled from the surrounding rocks towards the wellbore. These methods are termed "cold production," since they are used at reservoir ambient temperature. When natural lift pressure does not generate sufficient underground pressure or when the pressure declines and is no longer sufficient to move oil through the wellbore, primary production has reached its extraction limit, to be succeeded by secondary recovery.
  
  Section::::Oil production techniques.:Secondary recovery.
   Tripping out of the hole means that the entire volume of steel (of drillstring) is being removed, or has been removed, from the well. This displacement of the drill string (the steel) will leave out a volume of space that must be replaced with an equal volume of mud. If the replacement is not done, the fluid level in the wellbore will drop, resulting in a loss of hydrostatic pressure (HSP) and bottom hole pressure (BHP). If this bottom hole pressure reduction goes below the formation pressure, a kick will definitely occur.
  
  Section::::Kicks.:Causes of kicks.:Swabbing while tripping.
   A peculiarity of the Lost Hills operations is the pronounced subsidence of the ground surface as it collapses into the area vacated by the petroleum after being pumped out. Portions of the hills overlying the oil field have subsided up to in the central region of operations, and subsidence occurs field-wide at a rate of about per year. The dropping land surface causes operational problems, including fractures of well casings, and sometimes complete well failures. Waterflooding – the practice of filling the reservoir with water to push petroleum to recovery wells, and thereby also reoccupying the space vacated by oil and gas – has partially mitigated the problem. Some wells have actually disappeared into craters: in 1976, a Getty Oil well blew out, and quickly collapsed into a crater over deep and , taking with it the concrete pad, casing, and pumping unit. Yet another Getty well suffered the same fate in 1978.
   Carbon dioxide enhanced oil recovery or CO2-EOR increases the amount of oil recovered from an underground oil reservoir. By pumping into an oil reservoir, previously unrecoverable oil is pushed up to where the oil can be reached. The US Department of Energy states that this can produce an additional 30 to 60 percent of the original amount of recoverable oil. Once all of the recoverable oil has been reached, the depleted reservoir can act as a storage site for the .
   When a normally pressured formation is raised toward the surface while prevented from losing pore fluid in the process, it changes from normal pressure (at a greater depth) to abnormal pressure (at a shallower depth). When this happens, and then one drills into the formation, mud weights of up to 20 ppg (2397 kg/m ³) may be required for control. This process accounts for many of the shallow, abnormally pressured zones in the world. In areas where faulting is present, salt layers or domes are predicted, or excessive geothermal gradients are known, drilling operations may encounter abnormal pressure. 
   Drilling for oil and gas has been going on in Pennsylvania since 1859, and there are an estimated 300,000 to 500,000 wells drilled before the state kept track of the wells, or required them to be properly plugged. The Pennsylvania Department of Environmental Protection (DEP) has a program to locate and plug old wells. A 2014 study examined 19 abandoned wells, 14 of which had never been plugged, and only one of which was known to the state. Methane leakage rates were measured, and extrapolations over all the expected orphaned wells in the state indicated that the old wells made up a significant source of methane. A 2019 study explores the long-term ( 30 years) flow and transport of fracturing fluids into overburden layers and groundwater aquifers through a leaky abandoned well. It shows the spatial properties of the abandoned well as well as its distance from the hydraulic fracture are the most important factors influencing the vertical flow of fracturing fluid into groundwater aquifers. The study suggests that even for various field settings, only a limited amount of fracturing fluid can reach the aquifer in a long-term period.
   Sometimes the weight of the overburden, which refers to the rocks and fluids above the formation, will tend to compact the formation, resulting in pressure built-up within the formation if the fluids are trapped in place. The formation in this case will retain its normal pressure only if there is a communication with the surface. Otherwise, an "abnormal formation pressure" will result.
  
  BULLET::::- Abnormal formation pressure
   Removal of oil from the large Kettleman Hills fields, mainly the North Dome, has been suggested as a cause of the 1985 M6.1 earthquake, since the total deformation caused by the quake was exactly that necessary to compensate for the oil removed historically, i.e. by filling the vacated space. The same mechanism has been suggested for the 1983 Coalinga earthquake and the 1987 Whittier Narrows earthquake, both of which occurred near the centers of mature oil fields.
  
  Section::::History, production, and operations.
   The volume of water used to hydraulically fracture wells varies according to the hydraulic fracturing technique. In the United States, the average volume of water used per hydraulic fracture has been reported as nearly 7,375 gallons for vertical oil and gas wells prior to 1953, nearly 197,000 gallons for vertical oil and gas wells between 2000–2010, and nearly 3 million gallons for horizontal gas wells between 2000–2010.
   The advent of tertiary recovery techniques in the 1960s made possible another pulse of development of the field. In 1963, several cyclic steam projects commenced, followed by steam flooding, and in some cases fire flooding – all techniques for mobilizing heavy, viscous oil relatively close to the surface. At the end of 2009, all of the cyclic steam and steam flood wells were reported to be shut in, while water flood and water disposal wells remained in use on the field.
   Contamination of groundwater by methane can occur after the abandonment of the well: "Gas and other contaminants may accumulate slowly in these cracks, enter shallow strata or even leak at the surface many years after production or well abandonment. Even the presence of surface casing provides no assurance against gas leakage at the surface from the surrounding ground."
  
  Section::::Environmental impact.:Land usage.
   When marginal wells are prematurely abandoned, significant quantities of oil remain behind. In most instances, the remaining reserves are not easily accessible when oil prices subsequently rise again: when marginal fields are abandoned, the surface infrastructure – the pumps, piping, storage vessels, and other processing equipment – is removed and the lease forfeited. Since much of this equipment was probably installed over many years, replacing it over a short period should oil prices jump upward is cost prohibitive. Oil prices would have to rise beyond their historic highs and remain at elevated levels for many years before there would be sufficient economic justification to bring many marginal fields back into production.
   The existence of an offshore field was suspected for a long time, largely due to the persistent natural seepage of oil from the sea floor. The Coal Oil Point seep field is now one of the most actively studied seep zones in the world. In 1966, ARCO built Platform Holly, in of water approximately two miles southwest of Coal Oil Point, and began drilling wells into the various zones in the South Ellwood Offshore field. Peak production from the field was in 1984. Mobil operated Platform Holly until 1997, at which point Venoco, Inc. acquired all rights to the field. Currently three pipelines – one oil, one gas, and one for utilities – connect the platform to the processing plant on the mainland. In addition, an oil pipeline transports oil from "tents" constructed over some of the natural seeps on the ocean floor to the processing plant. Leakage from the natural seeps near Platform Holly has decreased substantially, probably from the decrease in reservoir pressure from the oil and gas produced at the platform.
   Enhanced recovery methods such as water flooding, steam flooding, or CO flooding may be used to increase reservoir pressure and provide a "sweep" effect to push hydrocarbons out of the reservoir. Such methods require the use of injection wells (often chosen from old production wells in a carefully determined pattern), and are used when facing problems with reservoir pressure depletion, high oil viscosity, or can even be employed early in a field's life. In certain cases – depending on the reservoir's geomechanics – reservoir engineers may determine that ultimate recoverable oil may be increased by applying a waterflooding strategy early in the field's development rather than later. Such enhanced recovery techniques are often called "tertiary recovery".
   Coalbed methane wells, which first began to be drilled in the 1980s, are commonly hydraulically fractured to increase the flow rates to the well. Hydraulic fracturing is commonly used in some coalbed methane areas, such as the Black Warrior Basin and the Raton Basin, but not in others, such as the Powder River Basin, depending on the local geology. Injected volumes tend to be much smaller than those of either tight gas wells or shale gas wells; a 2004 EPA study found a median injected volume of for coalbed methane wells.
  
  Section::::History.:Horizontal wells.
   This seep also releases on the order of of liquid petroleum per day. The field produces about 9 cubic meters of natural gas per barrel of petroleum.
  
  Leakage from the natural seeps near Platform Holly, the production platform for the South Ellwood Offshore oilfield, has decreased substantially, probably from the decrease in reservoir pressure due to the oil and gas produced at the platform.
  
  Section::::History.
  
presuppositions: Oil resides in a cavern or void.  Oil is located all together in one place .
corrections: Oil does not reside in a cavern or void, however it's seeped directly into rock.  Oil is not in one place but is seeped into rock, which is then left behind after oil is squeezed out. 
raw_presuppositions: Oil resides in a cavern or void.  Oil is located all together in one place . leaves a void/cavern.
raw_corrections: Oil does not reside in a cavern or void, however it's seeped directly into rock.  Oil is not in one place but is seeped into rock, which is then left behind after oil is squeezed out.  There's still plenty of rock down there after getting the oil. 