[h=2]What activities occur during the production phase?[/h] The primary activity conducted during the production phase is pumping hydrocarbons to the surface. During this phase, additional wells may be drilled within the development area to enhance hydrocarbon recovery. Once the fluid starts flowing, it must be separated into its components (oil, gas, and water). Other activities that occur during production phase include production enhancement, well servicing (routine maintenance such as replacing worn or malfunctioning equipment), and well workover (a more extensive equipment repair). The production phase may last for a number of decades. During this phase, wells and associated facilities are routinely monitored.
The first step of the production phase after the well is completed is to start the oil and gas flowing to the surface. To release coal bed methane from coal, its partial pressure must be reduced by removing the water from the coal seam. Once the pressure is lowered, gas and water can move through the coal bed and up the
borehole
. To release gas from the gas shale formation, fractures that can conduct the gas must be created. Various methods may be required to initiate or to improve the flow of hydrocarbons into the well. One such method is known as
hydraulic fracturing
or "fraccing" (rhymes with "cracking"). "Fraccing" is used to create rock fractures so that hydrocarbons can flow into the wellbore. For
fracturing
fluids to perform ideally, they must have the following qualities:
- Be viscous enough to create a fracture of adequate width,
- Maximize fluid travel distances to extend fracture length,
- Be able to transport large amounts of proppants
into the fracture, and
- Require minimal gelling agent for easier degradation and reduce cost.
In limestone reservoir rock, acidic fraccing fluids are pumped through the perforations. The acid dissolves channels in the limestone that allow oil or gas to flow into the wellbore. For other reservoir rock (e.g., sandstones, coal), specially blended fraccing fluids containing proppants (sand, walnut shells, aluminum or ceramic pellets) are pumped down the borehole and out the perforations. The pressure from the fluid makes small fractures in the formation that allow the hydrocarbons to flow into the well, while the proppant holds the fractures open. Fraccing and acidizing are sometimes performed simultaneously, in an acid fracture treatment. Eventually the formation will not be able to absorb the fluid as quickly as it is injected. At this point the pressure created causes the formation to crack or fracture. The fractures are held open by the proppants and the oil or gas is then able to flow through the fractures to the well.
The main types of hydraulic fracturing fluids are water and potassium chloride, gelled fluids, foamed gels, or a combination of these fluids. These fluids can contain potentially toxic substances such as diesel fuel (which contains benzene, ethylbenzene, toluene, xylenes, naphthalene, and other chemicals),
polycyclic aromatic hydrocarbons (PAHs)
, methanol, formaldehyde, ethylene glycol, glycol ethers, hydrochloric acid, and sodium hydroxide. Since some aspects of a hydraulic fracturing operation are considered proprietary, information about the specific constituents used in a given hydrofracturing operation, their concentrations, and their final distribution in subsurface formations is typically not available causing some concerns over the risks presented by this practice. Some of the fraccing fluids are pumped out of the well during the process of producing hydrocarbons and
produced water
. About 20 to 40% of the fraccing fluids may remain underground. In situations where production wells lack integrity (for example, if the well casing or well cement has deteriorated), any underground sources of water located in proximity to the compromised well could be impacted by fraccing fluids.
Besides hydraulic fracturing, a less practiced procedure called cavitation can be used to stimulate coal bed methane flow. In the cavitation process, water and air or foam are pumped into the well to increase the pressure in the reservoir. Shortly afterwards, the pressure is suddenly released, and the well violently blows out, spewing gas, water, coal and rock fragments out of the well (called surging) and is accompanied by a jet engine-like noise that can last up to 15 minutes). The coal fragments and gas that escape are directed to an earthen
berm
, which supposedly prevents the materials from entering the environment. The gas is flared or burned and the coal fines and fluids are initially collected at the base of the berm. The loose rock and coal that remain in the well are cleaned out by circulating eater (often with a soap solution or surfactant) within the well and pumping the material into a pit. The coal refuse is then burned onsite in a pit (referred to as a burn pit or blooie pit). The cavitation process is repeated several dozen times over a two-week period. Cavitation results in an enlargement of the initially drilled wellbore by as mush as 16 feet in diameter in the coal seam, as well as fractures that extend from the wellbore. If the cavitation fractures connect to natural fractures in the coal, they provide channels for gas to more easily flow to the well.
At shallow depths (e.g., ‹ 500 ft), coal seam fractures are sometimes open enough to produce the flow of gas and water into the wellbore without the need for fraccing.
Most hydrocarbons are initially produced by natural lift production methods - referred to as primary recovery. Primary recovery relies on underground pressure to drive fluids to the surface. Once underground pressure dissipates, a pump is needed to lift the oil out of the reservoir. Wells can be pumped from the surface by the familiar rising and falling "horse head" pump jacks or by long slender submersible pumps that operate deep inside the wellbore. Most oil reservoirs in the U.S. are produced using some type of artificial lift – known as secondary recovery. Water that is produced and separated from the oil during primary recovery may be injected back into the oil-bearing formation to increase formation pressure and effectively push more hydrocarbons towards the producing wells. Water injection, which is also considered a secondary recovery method, can boost hydrocarbon recovery by an additional 20% and also disposes of the
wastewater
. However, water co-produced with coal bed methane is not reinjected into the producing formation to enhance recovery as it is in conventional gas fields. Tertiary or enhanced recovery techniques are then used to mobilize the remaining hydrocarbons. The common approaches include:
- Thermal recovery where steam is injected into the formation. The heat from the steam makes the oil flow more easily, and the increased pressure forces it to the surface.
- Gas injection using either miscible or immiscible gases. Miscible gases dissolve carbon dioxide
, propane, methane
or other gasses in the oil to lower its viscosity and increase flow; while immiscible gases do not mix with the oil, but increase pressure in the gas cap in a reservoir to drive additional oil to the well bore.
- Chemical flooding involves mixing dense, water-soluble polymers with water and injecting the mixture into the field. The water pushes the oil out of the formation and into the well bore.
Enhanced recovery techniques can bring as much as 60% of the resource to the surface.
During production, oil comes out of the well mixed with water and, often, some natural gas; while natural gas often comes out mixed with
water vapor
and other gases. These components must be separated before pipeline quality oil and/or natural gas can be transported. To remove water and natural gas from oil, the mixture is passed through a device that removes the gas and sends it to a separate line. The remaining oil, gas, and water mixture goes into a heater/treater unit. This helps to break up the mixture so that oil separates from water that is denser. Any remaining gas rises to the top and is removed for processing or burning; water is removed and stored for further treatment. Additional separation of oil from wastewater is accomplished using hydrocyclones that spin the oil/water mixture. Water is forced to the outside of the hydrocyclone where it is removed. The water is injected back into the well, usually into the same formation where the oil and water came from, helping to force more oil out of the reservoir. More than 7 barrels of produced water could be produced for every barrel of oil. Towards the end of the production life for an oil well, as much as 98% of the material brought to the surface is produced water.
Conventional natural gas wells produce much lower volumes of produced waters than do oil wells. Natural gas conditioning is sometimes necessary to remove impurities from the gas stream so that it is of high enough quality to pass through the transportation system. The most significant impurity is
hydrogen sulfide
which is toxic to humans and corrosive for pipes. Water also must be removed. Nitrogen and other gases that may be mixed with the natural gas (methane) must also be removed prior to its sale.
Produced water is primarily disposed of through underground injection (either as a waste disposal method [36%] or to use as part of a water flooding effort for enhanced recovery [57%]). When used for enhanced recovery, it must be thoroughly treated to remove solids, bacteria, and oxygen before being reinjected. About 1% of produced water is used for road spreading (dust suppression, road oils, deicing materials, or road compaction). About 4% is used for irrigation. About 2% is disposed of using evaporation or percolation pits. About 1% are treated and discharged.
Flaring is done at wells that produce only a small amount of natural gas and that have no on-site use for the gas or no pipelines nearby to transport the gas to market. Flaring reduces the health and safety risk of simply venting gas that would be posed by combustible and poisonous gases like methane and hydrogen sulfide. Also, methane is a much more potent
greenhouse gas
than carbon dioxide, the primary product of methane
combustion
.
Most producing wells (about 75%) would be visited daily by one person for visual inspection of equipment, gauges, etc. Access roads would require year round inspections and maintenance and may require snow removal and dust suppression. During a workover, several tasks may be undertaken such as: repairing leaks in the casing or tubing; stimulating the well; cleaning out the wellbore and perforations; perforating a different section of the casing to produce from a different formation; applying corrosion-prevention compounds; and removing accumulated salts (scale) and paraffin from production tubing,
gathering
lines, and valves. In the workover process during well maintenance, scale removal requires strong acids such as hydrochloric or hydrofluoric acids. When carried to the surface in produced water, any acids not neutralized during use must be neutralized before being disposed, usually in a Class II
injection well
. Scale is primarily sodium, calcium, chloride, and carbonate; but may contain trace contaminants such as barium and
NORM
. Corrosion inhibitors and stimulation compounds are also flushed through the well (e.g., zinc carbonate, aluminum bisulfate, and acidic fluids).
Interim reclamation
of areas disturbed during the drilling/development phase that are not longer used is also conducted during this phase.
Water is initially produced in large quantities from coal bed methane wells, while gas is initially produced at low levels. However, water levels decrease and gas production increases over the subsequent several years before a coal bed methane well achieves full-scale gas production. About 7,200 to 28,800 gallons per day are initially pumped from a coal bed methane well to release the methane. The water pumped from coal bed methane wells (produced water) is usually dumped into ponds or injected back into the ground. If the water is high quality, it could be used by ranchers for watering stock or by farmers to irrigate crops. Produced water from coal bed methane wells are saltier the deeper the coal seam. Produced water may also contain nitrate, nitrite, chlorides, other salts, benzene, toluene, ethylbenzene, other minerals, metals, and high levels of total
dissolved solids
. Coal bed methane well gas production will typically increase for up to five years. Production lifetimes of coal bed methane wells are generally longer than conventional gas reservoirs, although average production rates are generally less. A coal bed in the original production zone can produce for 7 to 10 years or more. Multiple coal seams could extend the life of a coal bed methane well site by 10 to 30 years. Conventional gas wells tend to be exhausted after 25 years. Coal bed methane production may range from 50.000 to 500,000 cubic feet per day, while a conventional gas well produces about 1.7 million cubic feet per day. Generally, the higher the quality of coal and the deeper the coal seam, the more methane that is contained in the deposit. Also, the deeper the coal seam, the less water that is present and the sooner the well will begin to produce gas. The water that is present in deeper coal seams is generally of high
salinity
.
Spontaneous combustion and continued burning of completely dewatered coal beds is a concern for coal bed methane development. The most likely location for this to occur is along the edges of coal basins where the coal is closer to the surface and oxygen can most easily enter the coal seam when water is removed
