Oil and Gas Production Activities
Impacts from oil and gas production can result from activities that occur during each project phase: exploration, drilling/development, production, and decommissioning/reclamation.
The major activities that occur during the exploration phase include: (1) seismic surveys and (2) exploratory well drilling. Field activities that occur during exploration include:
Access roads may need to be constructed or upgraded to support exploration phase activities.
To identify potential production areas both remote sensing (e.g., photography, radar, infrared images, and microwave frequency receivers) and geophysical exploration (e.g., seismic tests) are used. Seismic exploration (the most important tool for discovering oil and gas reserves) involves exploding dynamite in a hole drilled several hundred feet in the ground, dropping a heavy object from a truck onto a hard surface such as a paved road, or shaking the ground with a mechanism known as a vibrasizer. Seismic waves from these procedures travel downward and outward and then bounce back from subsurface features (e.g., faults, formation boundaries) at different rates and strengths depending on what underground substances the waves pass through. These waves are analyzed to determine the location of oil and gas deposits. Coal seems must be at least 20 feet thick to produce economically viable coal bed methane.
Exploratory drilling is required to verify that there are accumulations of hydrocarbons and that the site can produce enough oil or gas to make it economically viable to develop. This stage includes building roads for access to the drilling area; clearing vegetation and leveling the drilling area; constructing a drill pad and pits to hold water and drilling wastes; and installing the drill rig and associated engines, pumps and equipment.
Exploratory drilling involves:
Conventional oil and gas wells generally range from 3,500 to 10,000 feet deep, whereas shale gas and coal bed methane wells are generally 1,000 to 4,000 feet deep. Drilling continues in stages: drill, run and cement new casings, then drill again. The final well depth is indicated when the rock cuttings reveal oil sand from the reservoir rock. At this stage, the drilling apparatus is removed from the hole and several tests are performed to confirm this finding:
Wells are completed for production if the value of the recoverable hydrocarbons is greater than the cost of drilling, producing, and delivery to market. If not, the exploratory well would be plugged, all drilling equipment and materials would be removed from the drill site, and the site would be restored as near as possible to its original condition. If enough hydrocarbons are present to possibly warrant commercial production, additional exploratory wells would be drilled to test the production conditions and further delineate the boundaries of the reservoir. Further information on drilling activities is provided in the description of the drilling/development phase below.
Exploration and development costs are significantly lower for coal bed methane than for conventional oil and gas exploration because there is a wealth of information available on coal beds that make it much easier to find prospective gas producing coal bed reservoirs and there are generally fewer dry holes in coal beds compared to conventional gas reservoirs.
What activities occur during the drilling/development phase?
During the drilling/development phase, full field development occurs. This involves the construction of well pads, access roads, gathering pipelines, and other ancillary facilities (e.g., wellhead compressors, separators, dehydrators, storage tanks, reserve pits, flare pits, and so forth) and the drilling and completion of wells.
As the wellbore is drilled, casing is placed in the well to stabilize the hole and to isolate water bearing and hydrocarbon bearing zones. Three to four separate casing strings – lengths of tubing of a give diameter – may be used. A conductor casing is used where surface soils may cave in during drilling. It extends 20 to 100 feet from the surface and is often placed before drilling using a pile driver. The next string (surface casing) begins at the surface and may penetrate down to two to three thousand feet. It primarily protects the surrounding freshwater aquifers from incursion of oil or brine from greater depths. The intermediate string begins at the surface and extends to within a couple thousand feet of the bottom of the wellbore. This casing string prevents the hole from caving in and facilitates the movement of equipment used in the hole. The final production string extends the full length of the wellbore and encases the downhole production equipment. Shallow wells may only have two casing strings, while deeper wells may have multiple intermediate casings. After each casing string has been installed, cement is forced out the bottom of the casing up the annulus (space between the casing and the side or the wellbore or between two casing strings) to hold it in place.
The general drilling sequence for coal bed methane wells involves drilling an 8.75 in. hole that is drilled to a minimum depth of 160 ft, where a 7 in. steel casing is run and cemented into place. A 6-7/8 in. hole is subsequently drilled to a depth of 2,000 to 7,500 ft, depending upon the basin, and a 5.5 in. production casing is run between the bottom of the wellbore and the surface. A pumpjack or pumping unit is then installed to pump water from the coal bed to the surface.
Conventional wells are drilled vertically using sections of rigid pipe to form the drill string. In some situations a coil tubing technology can replace the typical drill string with a continuous length of pipe stored on a large spool. This approach can reduce drilling waste and minimize the equipment footprint. This technology is best used to re-entering existing wells and when multiple casing wells are unnecessary. Other advanced drilling techniques include:
These advanced technologies can significantly reduce the environmental impacts of traditional vertical wells by increasing the amount of hydrocarbons recovered per well and by supporting the development of multiple wells from a single well pad. Conventional vertical oil or gas well takes 3 to 10 days to drill, but directional drilling could extend this time to a month or more. Coal bed methane wells may only take a few days to drill and a few more to complete.
Fluid is required during drilling to: (1) cool and lubricate the drill bit, (2) remove the rock fragments (drill cuttings) from the drilling area and transport them to the surface, (3) counterbalance formation pressure to prevent formation fluids (i.e., oil, gas, and water) from entering the wellbore prematurely, and (4) prevent the open (uncased) wellbore from caving in. Drilling fluids may be gas or foam, but liquid-based fluids (drilling muds) are used most of the time. In addition to liquids, drilling muds usually contain bentonite clay that increases the viscosity and alters the density of the fluid. Water is most frequently used, but oil-based (diesel oil or mineral oil) and synthetic muds are usually used in deep holes or high-angle directional drilling where water does not provide enough lubrication.
As drilling mud is brought to the surface, it is run through a sieve to remove the drill cuttings (pulverized rock) before the mud is recycled down into the well. Drill cuttings are periodically collected for microscopic analysis to determine the type of rock being drilled and specific formation, how porous it is, and whether hydrocarbons are present. The drilling mud is also analyzed with sensors to see if trace amounts of oil or natural gas are present. During well logging, a special bit is used to cut cylindrical piece of rock that can be brought to the surface for analysis. This core is analyzed to determine porosity and permeability, and provides a good indication of how well oil or natural gas would flow through the rock.
Once the drilling has been completed, several steps are required before production begins:
What activities occur during the production phase?
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:
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:
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.
What activities occur during the decommissioning/reclamation phase?
Decommissioning/reclamation activities would include:
At the well site, the casing would be filled with cement and wellhead, pump jacks, tanks, pipes, facilities, and other equipment would be removed. The wellbore is plugged to prevent underground fluids from getting into groundwater. Well plugging involves a number of steps:
Finally, the casing is cut off below the surface and capped with a steel plate welded to the casing. Surface reclamation should then be undertaken to restore the natural soil consistency and plant cover. Waste-handling pits, if present, are properly closed. All areas disturbed by the project would be restored to preproject conditions and/or to conditions acceptable to regulatory agencies, landowners, or other stakeholders. Where the soil has been contaminated with hydrocarbons, the soils would be transported to a licensed landfill or they could be restored using bioremediation with microorganisms that digest the hydrocarbons. Rather than being plugged, some wells that may be converted for use as either for disposal of the produced water from other wells or as part of oil enhancement operations in the production field.