An oil well is a boring into the Earth that is
designed to bring petroleum oil hydrocarbons
to the surface. Usually some natural gas is produced along with the oil. A well that
is designed to produce mainly or only gas may be termed a gas well.
History
The earliest known oil wells were drilled in China in 347 [AD]. These
wells had depths of up to about 240 metres (790 ft) and were drilled using
bits
attached to bamboo
poles. The oil was burned to evaporate brine and produce salt.
By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The
ancient records of China and Japan are said to contain many allusions to the use of natural
gas for lighting and heating. Petroleum was known as burning water in
Japan in the 7th century.
According to Kasem Ajram, petroleum was distilled
by the Persian alchemist Muhammad ibn Zakarīya Rāzi (Rhazes) in
the 9th century, producing chemicals such as kerosene in the
alembic (al-ambiq),
and which was mainly used for kerosene
lamps. Arab and Persian chemists also
distilled crude oil in order to produce flammable
products for military purposes. Through Islamic Spain,
distillation became available in Western
Europe by the 12th century.
Some sources claim that from the 9th century, oil fields
were exploited in the area around modern Baku, Azerbaijan,
to produce naphtha for the petroleum industry. These fields were described
by Marco
Polo in the 13th century, who described the output of those oil wells as
hundreds of shiploads. When Marco Polo in 1264 visited the Azerbaijani city of
Baku, on the shores of the Caspian Sea, he saw oil being collected from seeps.
He wrote that "on the confines toward Geirgine there is a fountain from
which oil springs in great abundance, in as much as a hundred shiploads might
be taken from it at one time."
In the North America, the first commercial oil well entered
operation in Oil Springs, Ontario in 1858, while the first
offshore oil well was drilled in 1896 at the Summerland Oil Field on the California Coast.
The earliest oil wells in modern times were drilled
percussively, by repeatedly raising and dropping a cable tool into the earth. In the 20th Century cable
tools were largely replaced with rotary
drilling, which could drill boreholes to much greater depths and in less
time. The record-depth Kola Borehole used non-rotary mud motor
drilling to achieve a depth of over 12,000 metres (39,000 ft).
Until the 1970s, most oil wells were vertical, although lithological
and mechanical imperfections cause most wells to deviate at least slightly from
true vertical. However, modern directional drilling technologies allow for
strongly deviated wells which can, given sufficient depth and with the proper
tools, actually become horizontal. This is of great value as the reservoir
rocks which contain hydrocarbons are usually horizontal, or sub-horizontal; a
horizontal wellbore placed in a production zone has more surface area in the
production zone than a vertical well, resulting in a higher production rate.
The use of deviated and horizontal drilling has also made it possible to reach
reservoirs several kilometers or miles away from the drilling location (extended
reach drilling), allowing for the production of hydrocarbons located below
locations that are either difficult to place a drilling rig on, environmentally
sensitive, or populated.
Life of a well
A schematic of a typical oil well being produced by a pumpjack, which
is used to produce the remaining recoverable oil after natural pressure is no
longer sufficient to raise oil to the surface
The creation and life of a well can be divided up into five
segments:
- Planning
- Drilling
- Completion
- Production
- Abandonment
Drilling
The well is created by drilling a hole
12 cm to 1 meter (5 in to 40 in) in diameter into the earth with
a drilling rig that rotates a drill
string with a bit attached. After the hole is drilled, sections of steel
pipe (casing), slightly smaller in diameter than the
borehole, are placed in the hole. Cement may be placed between the outside of
the casing and the borehole. The casing provides structural integrity to the
newly drilled wellbore, in addition to isolating potentially dangerous high
pressure zones from each other and from the surface.
With these zones safely isolated and the formation protected
by the casing, the well can be drilled deeper (into potentially more-unstable
and violent formations) with a smaller bit, and also cased with a smaller size
casing. Modern wells often have two to five sets of subsequently smaller hole
sizes drilled inside one another, each cemented with casing.
To drill the well
The drill bit, aided by the weight
of thick walled pipes called "drill collars" above it, cuts into the
rock. There are different types of drill bit; some cause the rock to
disintegrate by compressive failure, while others shear slices off the rock as
the bit turns.
- Drilling fluid, a.k.a. "mud", is pumped down the inside of the drill pipe and exits at the drill bit. The principal components of drilling fluid are usually water and clay, but it also typically contains a complex mixture of fluids, solids and chemicals that must be carefully tailored to provide the correct physical and chemical characteristics required to safely drill the well. Particular functions of the drilling mud include cooling the bit, lifting rock cuttings to the surface, preventing destabilisation of the rock in the wellbore walls and overcoming the pressure of fluids inside the rock so that these fluids do not enter the wellbore. Some oil wells are drilled with air or foam as the drilling fluid.
- The generated rock "cuttings" are swept up by the drilling fluid as it circulates back to surface outside the drill pipe. The fluid then goes through "shakers" which strain the cuttings from the good fluid which is returned to the pit. Watching for abnormalities in the returning cuttings and monitoring pit volume or rate of returning fluid are imperative to catch "kicks" early. A "kick" is when the formation pressure at the depth of the bit is more than the hydrostatic head of the mud above, which if not controlled temporarily by closing the blowout preventers and ultimately by increasing the density of the drilling fluid would allow formation fluids and mud to come up through the annulus uncontrollably.
- The pipe or drill string to which the bit is attached is gradually lengthened as the well gets deeper by screwing in additional 9 m (30 ft) sections or "joints" of pipe under the kelly or topdrive at the surface. This process is called making a connection, or "tripping". Joints can be combined for more efficient tripping when pulling out of the whole by creating stands of multiple joints. A conventional triple, for example, would pull pipe out of the hole three joints at a time and stack them in the derrick. Many modern rigs, called "super singles," trip pipe one at a time, laying it out on racks as they go.
This process is all facilitated by a drilling
rig which contains all necessary equipment to circulate the drilling fluid,
hoist and turn the pipe, control downhole, remove cuttings from the drilling
fluid, and generate on-site power for these operations.
Completion
After drilling and casing the well, it must be 'completed'.
Completion is the process in which the well is enabled to produce oil or gas.
In a cased-hole completion, small holes called perforations are made in the portion of the casing which passed through the production zone,
to provide a path for the oil to flow from the surrounding rock into the
production tubing. In open hole completion, often 'sand screens' or a 'gravel
pack' is installed in the last drilled, uncased reservoir section. These
maintain structural integrity of the wellbore in the absence of casing, while
still allowing flow from the reservoir into the wellbore. Screens also control
the migration of formation sands into production tubulars and surface
equipment, which can cause washouts and other problems, particularly from
unconsolidated sand formations of offshore fields.
After a flow path is made, acids and fracturing fluids may
be pumped into the well to fracture, clean, or otherwise prepare and
stimulate the reservoir rock to optimally produce hydrocarbons into the
wellbore. Finally, the area above the reservoir section of the well is packed
off inside the casing, and connected to the surface via a smaller diameter pipe
called tubing. This arrangement provides a redundant barrier to leaks of
hydrocarbons as well as allowing damaged sections to be replaced. Also, the
smaller cross-sectional area of the tubing produces reservoir fluids at an
increased velocity in order to minimize liquid fallback that would create
additional back pressure, and shields the casing from corrosive well fluids.
In many wells, the natural pressure of the subsurface reservoir is high enough for the oil or gas to flow to the surface. However, this is not always the case, especially in depleted fields where the pressures have been lowered by other producing wells, or in low permeability oil reservoirs. Installing a smaller diameter tubing may be enough to help the production, but artificial lift methods may also be needed. Common solutions include downhole pumps, gas lift, or surface pump jacks. Many new systems in the last ten years have been introduced for well completion. Multiple packer systems with frac ports or port collars in an all in one system have cut completion costs and improved production, especially in the case of horizontal wells. These new systems allow casings to run into the lateral zone with proper packer/frac port placement for optimal hydrocarbon recovery.
Production
The production stage is the most important stage of a well's
life, when the oil and gas are produced. By this time, the oil rigs and workover rigs
used to drill and complete the well have moved off the wellbore, and the top is
usually outfitted with a collection of valves called a Christmas tree or production tree. These
valves regulate pressures, control flows, and allow access to the wellbore in
case further completion work is needed. From the outlet valve of the production
tree, the flow can be connected to a distribution network of pipelines and
tanks to supply the product to refineries, natural gas compressor stations, or
oil export terminals.
As long as the pressure in the reservoir remains high
enough, the production tree is all that is required to produce the well. If the
pressure depletes and it is considered economically viable, an artificial lift
method mentioned in the completions section can be employed.
Workovers are often necessary in older wells, which may need
smaller diameter tubing, scale or paraffin removal, acid matrix jobs, or
completing new zones of interest in a shallower reservoir. Such remedial work
can be performed using workover rigs – also known as pulling units, completion
rigs or "service rigs" – to pull and replace tubing, or by the
use of well intervention techniques utilizing coiled
tubing. Depending on the type of lift system and wellhead a rod rig or
flushby can be used to change a pump without pulling the tubing.
Enhanced recovery methods such as water flooding, steam
flooding, or CO2 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".
Abandonment
A well is said to reach an "economic limit" when
its most efficient production rate does not cover the operating expenses,
including taxes.
The economic limit for oil and gas wells can be expressed
using these formulae:
When the economic limit is raised, the life of the well is
shortened and proven oil reserves are lost. Conversely, when the economic limit
is lowered, the life of the well is lengthened.
When the economic limit is reached, the well becomes a
liability and is abandoned. In this process, tubing is removed from the well
and sections of well bore are filled with concrete to isolate the flow path
between gas and water zones from each other, as well as the surface. Completely
filling the well bore with concrete is costly and unnecessary. The surface
around the wellhead is then excavated, and the wellhead and casing are cut off,
a cap is welded in place and then buried.
At the economic limit there often is still a significant
amount of unrecoverable oil left in the reservoir. It might be tempting to
defer physical abandonment for an extended period of time, hoping that the oil
price will go up or that new supplemental recovery techniques will be
perfected. In these cases, temporary plugs will be placed downhole and locks
attached to the wellhead to prevent tampering. There are thousands of
"abandoned" wells throughout North America, waiting to see what the
market will do before "permanent" abandonment. Often, lease
provisions and governmental regulations usually require quick abandonment;
liability and tax concerns also may favor abandonment.
In theory an abandoned well can be reentered and restored to
production (or converted to injection service for supplemental recovery or for
downhole hydrocarbons storage), but reentry often proves to be difficult
mechanically and not cost effective.
Types of wells
Fossil-fuel wells come in many varieties. By produced fluid,
there can be wells that produce oil, wells that produce oil and natural gas,
or wells that only produce natural gas. Natural gas is almost always a
byproduct of producing oil, since the small, light gas carbon chains come out
of solution as they undergo pressure reduction from the reservoir to the surface, similar to uncapping
a bottle of soda pop where the carbon dioxide effervesces.
Unwanted natural gas can be a disposal problem at the well site. If there is
not a market for natural gas near the wellhead it is
virtually valueless since it must be piped to the end user. Until recently,
such unwanted gas was burned off at the wellsite, but due to environmental
concerns this practice is becoming less common. Often, unwanted (or 'stranded' gas without a
market) gas is pumped back into the reservoir with an 'injection' well for
disposal or repressurizing the producing formation. Another solution is to
export the natural gas as a liquid. Gas to liquid, (GTL) is a developing technology that
converts stranded natural gas into synthetic gasoline, diesel or jet fuel
through the Fischer-Tropsch process developed in World War II
Germany. Such fuels can be transported through conventional pipelines and
tankers to users. Proponents claim GTL fuels burn cleaner than comparable petroleum
fuels. Most major international oil companies are in advanced development
stages of GTL production, e.g. the 140,000 bbl/d (22,000 m3/d)
Pearl GTL
plant in Qatar, scheduled to come online in 2011. In locations such as the
United States with a high natural gas demand, pipelines are constructed to take
the gas from the wellsite to the end
consumer.
Another obvious way to classify oil wells is by land or
offshore wells. There is very little difference in the well itself. An offshore
well targets a reservoir that happens to be underneath an ocean. Due to
logistics, drilling an offshore well is far more costly than an onshore well.
By far the most common type is the onshore well. These wells dot the Southern
and Central Great Plains, Southwestern United States, and are the most common
wells in the Middle East.
Another way to classify oil wells is by their purpose in
contributing to the development of a resource. They can be characterized as:
- wildcat wells are those drilled outside of and not in the vicinity of known oil or gas fields.
- exploration wells are drilled purely for exploratory (information gathering) purposes in a new area.
- appraisal wells are used to assess characteristics (such as flow rate) of a proven hydrocarbon accumulation.
- production wells are drilled primarily for producing oil or gas, once the producing structure and characteristics are determined.
- Abandoned well are wells permanently plugged in the drilling phase for technical reasons.
At a producing well site, active wells may be further
categorised as:
- oil producers producing predominantly liquid hydrocarbons, but mostly with some associated gas.
- gas producers producing almost entirely gaseous hydrocarbons.
- water injectors injecting water into the formation to maintain reservoir pressure, or simply to dispose of water produced with the hydrocarbons because even after treatment, it would be too oily and too saline to be considered clean for dumping overboard offshore, let alone into a fresh water resource in the case of onshore wells. Water injection into the producing zone frequently has an element of reservoir management; however, often produced water disposal is into shallower zones safely beneath any fresh water zones.
- aquifer producers intentionally producing water for re-injection to manage pressure. If possible this water will come from the reservoir itself. Using aquifer produced water rather than water from other sources is to preclude chemical incompatibility that might lead to reservoir-plugging precipitates. These wells will generally be needed only if produced water from the oil or gas producers is insufficient for reservoir management purposes.
- gas injectors injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.
Lahee classification
- New Field Wildcat (NFW) – far from other producing fields and on a structure that has not previously produced.
- New Pool Wildcat (NPW) – new pools on already producing structure.
- Deeper Pool Test (DPT) – on already producing structure and pool, but on a deeper pay zone.
- Shallower Pool Test (SPT) – on already producing structure and pool, but on a shallower pay zone.
- Outpost (OUT) – usually two or more locations from nearest productive area.
- Development Well (DEV) – can be on the extension of a pay zone, or between existing wells (Infill).
Cost
The cost of a well depends mainly on the daily rate of the
drilling rig, the extra services required to drill the well, the duration of
the well program (including downtime and weather time), and the remoteness of
the location (logistic supply costs).
The daily rates of offshore drilling rigs vary by their
capability, and the market availability. Rig rates reported by industry web
service show that the deepwater water floating drilling rigs are over twice
that of the shallow water fleet, and rates for jackup fleet can vary by factor
of 3 depending upon capability.
With deepwater drilling rig rates in 2010 of around
$420,000/day, and similar additional spread costs, a deep water well of
duration of 100 days can cost around US$100 million.
With high performance jackup rig rates in 2010 of around
$150,000, and similar service costs, a high pressure, high temperature well of
duration 100 days can cost about US$30 million.
Onshore wells can be considerably cheaper, particularly if
the field is at a shallow depth, where costs range from less than $1 million to
$15 million for deep and difficult wells.
The total cost of an oil well mentioned does not include the
costs associated with the risk of explosion and leakage of oil. Those costs
include the cost of protecting against such disasters, the cost of the cleanup
effort, and the hard-to-calculate cost of damage to the company's image.
Reefs
Offshore platforms (the structure supporting the
wells) often provide habitat for marine life. After the wells have been
abandoned, sometimes the platforms can be toppled in place or moved elsewhere
to be dropped to the ocean floor to produce artificial
reefs.
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