Until the s, 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 nearly 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. Before a well is drilled, a geologic target is identified by a geologist or geophysicist to meet the objectives of the well. The target the end point of the well will be matched with a surface location the starting point of the well , and a trajectory between the two will be designed.
When the well path is identified, a team of geoscientists and engineers will develop a set of presumed properties of the subsurface that will be drilled through to reach the target. These properties include pore pressure , fracture gradient, wellbore stability, porosity , permeability , lithology , faults , and clay content.
This set of assumptions is used by a well engineering team to perform the casing design and completion design for the well, and then detailed planning, where, for example, the drill bits are selected, a BHA is designed, the drilling fluid is selected, and step-by-step procedures are written to provide instruction for executing the well in a safe and cost-efficient manner.
With the interplay with many of the elements in a well design and making a change to one will have a knock on effect on many other things, often trajectories and designs go through several iterations before a plan is finalised. 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 known as the annulus. 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. 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. 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.
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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.
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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.
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 CO 2 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.
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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 ". A well is said to reach an "economic limit" when its most efficient production rate does not cover the operating expenses, including taxes.
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 expensive.
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Traditionally elastomer and cement plugs have been used with varying degrees of success and reliability. Over time, they may deteriorate, particularly in corrosive environments, due to the materials from which they are manufactured. Conventional bridge plugs also have very small expansion ratios, limiting them for use in wells with restrictions.
Alternatively, high expansion plugs, such as inflatable packers, do not have the differential pressure capabilities required for many well abandonments, nor do they provide a gas-tight seal. New tools have been developed that make re-entry easier, these tools offer higher expansion rations than conventional bridge plugs and higher differential pressure ratings than inflatable packers, all while providing a V0 rated, gas tight seal that cement cannot provide.
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 where the carbon dioxide effervesces.
Unwanted natural gas can be a disposal problem at the well site. If it escapes into the atmosphere it becomes known as fugitive gas. 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.
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