The number of wells required to exploit the hydrocarbon reservoir varies with size of the reservoir and its geology. Large oilfields can require a hundred or more wells to be drilled, whereas smaller fields may only require ten or so. The drilling procedure involves similar techniques to those described for exploration; however, with a larger number of wells being drilled, the level of activity obviously increases in proportion.
The well sites will be occupied for long and support services – workforce accommodation, water supply, waste management, and other services – will correspondingly increase. As each well is drilled it has to be prepared for production before the drilling rig departs.
The heavy drill pipe is replaced by lighter weight tubing in the well and occasionally one well may carry two or three strings of tubing, each one produced from different layers of reservoir rock.
Following development drilling and well completion, a “Christmas tree” is placed on each wellhead to control flow of the formation fluids to the surface. Hydrocarbons may flow freely from the wells if the underground formation pressures are adequate, but additional pressure may be required such as a sub-surface pump or the injection of gas or water through dedicated injection wells to maintain reservoir pressure.
Depending on reservoir conditions, various substances (steam, nitrogen, carbon dioxide, and surfactants) may be injected into the reservoir to remove more oil from the pore spaces, increase production, and extend well life.
Production from most wells follows a predictable pattern called a decline curve where production increases relatively rapidly to a peak, and then follows a long, slow decline.
Operators may periodically perform well workovers to clean out the wellbore, allowing oil or gas to move more easily to the surface. Other measures to increase production include fracturing and treating the bottom of the wellbore with acid to create better pathways for the oil and gas to move to the surface. Formation fluids are then separated into oil, gas and water at a central production facility, designed and constructed depending on the reservoir size and location.
The production facility processes the hydrocarbon fluids. Crude oil processing essentially involves the removal of gas and water before export. Gas processing involves the removal of liquids and other impurities such as carbon dioxide, nitrogen and hydrogen sulfide. Oil and gas terminal facilities receive hydrocarbons from outside locations sometimes offshore and process and store the hydrocarbons before they are exported.
There are several types of hydrocarbon terminals, including inland pipeline terminals, onshore / coastal marine receiving terminals (from offshore production), barge shipping, or receiving terminals. The infrastructure required for development drilling in onshore operations is similar to the one described above for exploration. However, once drilling is completed, the individual wellhead assemblies and well sites occupy considerably lesser space than when the drill rig was on site.
Typically, each well requires an area of some 10 square meter (m2) surrounded by a security fence. Often the well sites are concentrated within a central area, which includes processing facilities, offices and workshops, and this would typically occupy an area of several hectares, depending upon the capacity of the field.
Since the production operation is a long-term development, the temporary facilities used in exploration are replaced by permanent facilities and are subject to detailed planning, design, engineering and construction. The temporary workforce associated with exploration activity is replaced by a permanent workforce, usually accommodated in the local area and, where desirable, fully integrated with the local community: indeed a large proportion of the workforce may be recruited locally and receive specialized training, but if we are honest about it, often these jobs are really just for unskilled or semi-skilled labour, with skilled labour being imported into the project from other countries.
Similarly, the local infrastructure provides a variety of requirements in addition to labour, such as material supplies, education, and medical, etc.
In offshore production developments, permanent structures are necessary to support the required facilities, since typical exploration units are not designed for full scale production operations. This will involve the installation of an offshore drilling and production platform that is self sufficient in terms of energy and water needs for workforce and for drilling wells and processing hydrocarbons ready for export.
There are many types of offshore platforms, including:
- Fixed platforms: Used in water depths up to 500 m. They consist of steel or concrete legs (jacket) secured directly to the seabed by steel piles that support a steel deck. Drilling equipment, production facilities and accommodation are typically housed on the deck.
- Compliant towers: Used in water depths ranging from 500m to 1,000m. These towers consist of a narrow, flexible tower on a piled foundation supporting a conventional deck.
- Tension leg platforms: Used in water depths of up to 2,000m and consists of a floating facility moored to the seabed and fixed in place by anchors. Mini tension leg platforms (Seastars) exist that are used in water depths between 200m and 1,000m.
- Jack-up platforms: Used in shallower water up to 100m and transported to location where the legs are lowered by hydraulic jacks into position to support the deck.
Normally, a steel platform is installed to serve as the gathering and processing centre and more than 40 wells may be drilled directionally from this platform. Concrete platforms are sometimes used. If the field is large enough, additional ‘satellite’ platforms may be needed, linked by subsea flow lines to the central facility.
In shallow waters, typically a central processing facility is supported by a number of smaller wellhead platforms. Recent technological developments, aimed at optimizing operations, include remotely operated subsea systems, which remove the requirement for satellite platforms. This technology is also being used in deep water where platforms are unsuitable, and for marginal fields where platforms would be uneconomic. In these cases, floating systems – ships and semi-submersibles – ‘service’ the subsea wells on a regular basis.
Produced oil and gas may be exported by pipeline, trucks, tankers or rail tank cars. Gasto-liquids is an area of technology development that allows natural gas to be converted toliquid. Gas is often exported, after liquefaction, as liquefied natural gas (LNG) using specially constructed tankers. Pipelines are constructed in a sequential process, including staking of the right-of-way (ROW) and pipeline centerline; ROW clearing and grading; trenching (for buried pipeline); pipe laying, welding, and bending; field coating of welded joints; testing; lowering; trench backfilling; and ROW reinstatement.
Pumps or compressors are used to transport liquids or gas from the oil and gas fields to downstream or export facilities. During commissioning, flow lines, pipelines, and associated facilities (e.g., block valves and meters, regulators and relief devices, pump stations, pigging stations, storage tanks) are filled with water and hydro tested to ensure integrity.
Pipeline operation usually requires frequent inspections (ground and aerial surveillance, and facility inspections) and periodic ROW and facility maintenance. Production and pipeline operation is usually monitored and controlled from a central location through a supervisory control and data acquisition system (SCADA) which allows field operating variables to be monitored such as flow rate, pressure, and temperature and to open and close valves.
Recent advances in horizontal drilling have enhanced directional drilling as a means of concentrating operations at one site and reducing the ‘footprint’ on land of production operations and the number of platforms offshore. The technology now enables access to a reservoir up to several kms from the drill rig, while technology is developing to permit even wider range. This further minimizes the ‘footprint’ by reducing the need for satellite wells. It also offers more flexibility in selecting a drill site, particularly where environmental concerns are raised.