Vapor Recovery System
An excellent alternative to waste gas flaring, the vapor recovery system is a series of small gas compressors that collect and compress small gas volumes for use as fuel on site or added to gas sales.
Gas Sales Compressor
This machine increases the pressure on gas by decreasing the volume inside a piston cylinder. This comes with an increase in gas temperature which must be controlled with a discharge cooler. Compressors normally come with one, two, or three pistons acting in series to increase gas pressure without overloading the machine. Power can come from an internal combustion engine or an electric motor.
A portion of a facility processing more than one well is dedicated to collecting test data for individual wells. A series of valves called a manifold are used to isolate the flow from one well and direct it to the test system while the remaining wells continue to flow together into the main facility. The test system can be made of one gravity separator where the gas, oil, and water are separated and metered before returning the volumes to the main streams separated in the main facility. More complicated combinations of test devices can be used, making the test system a small version of the main facility. This provides the most accurate test information, but at a greater cost and larger space requirement. Without a test system it is impossible to know what individual wells are producing.
Hydrocarbons and Water
Steel (for hydrocarbons) and fiberglass (for water) tanks used for temporary storage of liquid prior to sale or disposal. Tanks have an enclosed space hazard and should not be entered without using correct safety measures. It is also possible under rare conditions to have a static electric discharge cause a tank explosion. Punctures, over flows, fires, and theft are the main problems with tanks.
Gas meters are normally orifice meters, which measure the gas volume flowing past a point over a set period of time. They can be used at any point where gas flow rate is required, but certainly at the point of gas sales. Meters are calibrated on a schedule to ensure accuracy.
Liquid spills should be contained by the fire wall or berm around the facility. Offshore the facilities have a drain system where by spills can be collected and sent to the surge or slop oil tanks to be reprocessed. Spills or equipment upsets should be reported and cleaned up in the manner required by law. Any oil that reaches a waterway must be handled using methods appropriate to the waterway.
A Lease Automatic Custody and Transfer (LACT) system is a way to meter oil, collect an oil sample, verify the oil quality, and sell the oil into an oil pipeline without continuous interaction by the operator. These systems are generally believed to improve sales volume accuracy and to minimize the time people must intervene in oil sales.
Electricity may be generated on site as is typical in an off shore facility. The electricity is distributed to the various motors or heaters through a distribution network of transformers and power wires. The electricity is generated using diesel or natural gas powered generators. Excess power can be sold to the power grid if a connection is available. This is a good way to use waste natural gas as fuel to power electric generators that in turn sell the power to the grid.
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Air emissions come from vented hydrocarbon gasses and combustion exhaust gasses. Venting can occur when tank hatches are opened or through the normal “breathing” of connections and fittings and the exhaust of pneumatic controls. Combustion gasses include carbon dioxide, nitric oxides and unburned portions of hydrocarbons. These gasses are now regulated, with the exception of carbon dioxide. Without a special permit, unrestricted natural gas flaring is no longer allowed beyond a period defined by regulation
Corrosion, Poison Gas, Oil Sweetening, Water Polishing
Chemicals are normally injected in the produced stream to prevent corrosion, reduce the sulfur content in oil and to reduce the carbon dioxide and hydrogen sulfide content in natural gas. Chemical treatment is expensive and most operators use it only as required.
Pipelines are sections of steel pipe welded together to provide a flow conduit for fluids. Compressor stations may be needed for gas and pumping stations for liquids to keep the pressure inside the pipeline high enough to maintain the desired flow rate. Pressure limits on the pipeline are set by the building regulations or code in force where the line is built. Routine inspection and cleaning of the pipeline should be done to insure the safe operation of the pipe. Pigs are cylindrical shaped objects that are placed in a pipeline at a point using a pig launcher and pumped through the pipe with pressure to be removed at a pig catcher. The pig can be used to push out any debris that may collect inside the pipe or to inspect the inner surface of the pipe if the pig is instrumented (making it a “smart” pig).
Point of Sale
The point where product custody is changed. Volumes sold and product quality verified at this point. Beyond the point of sale the responsibility for the product passes from the seller to the purchaser.
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A variety of vessels are used to break oil in water emulsions to allow for the separation of the oil. These are vessels that heat the emulsion, subject the emulsion to an electric voltage, inject emulsion-breaking chemicals or peform combinations of these techniques. Once the emulsion breaks, the vessels act as gravity separators to create a relatively oil-free water, water-free oil, and a small volume of gas.
Gravity causes heavy (dense) liquids to collect at the bottom of a separator vessel, light liquids like oil to collect in the middle, and gas to collect at the top. In this way, fluids segregate as a function of their density and can be drawn off for sale or disposal independently. If the hydrocarbons meet sales contract requirements (e.g. B.S.& W. 2% or less, which is the ratio of basic sediment and water) they may be sold directly. If the fluids do not meet these standards, additional treatment will be needed.
Gas with water vapor content bubbles through a contact tower filled with an absorbent such as glycol. The glycol absorbs the water vapor, and the lean gas exits the top of the tower. The exiting gas will now have a lower water vapor content allowing the gas to meet the specifications of the sales contract. When the absorbent is saturated with water, operators boil the fluid to vaporize the water. Operators reuse the newly water-lean absorbent and dispose of the extracted water after it condenses. They can use any gas vapors exiting the boiler as fuel. Any trace liquid hydrocarbons that condense with the water can be separated in a scrubber and sold or used as a part of the fuel for the boiler.
Pneumatic, Hydraulic, Electric
Controls of automatic shutdown valves, vessel levels, pressures at various points in the facility, and data transmission to remote locations requires power. Electric power can be obtained from the power grid or generated on site. Hydraulic power can be created by the use of a pressurized hydraulic fluid with the pressure supplied by a pump. Pneumatic control power comes from pressurized gas, such as natural gas taken off a separator. The pressure for pneumatic supply is set by a pressure regulator. Alternatives to field natural gas are air (not normally used due to the water vapor content and potential freezing), or nitrogen from supply bottles on site.
Vapor Recovery System
Necessary to fight small fires, warn of safety hazards, and treat poison gas. These include signs, fences, flags, wind socks, fire extinguishers, automatic safety shut down valves, chemical injection, and waste gas flares.
Extra gas treatment beyond simply taking the gas off the top of the gravity separator. This may include dehydration to remove water vapor, hydrogen sulfide and carbon dioxide removal, and compression. These extra steps are taken to meet the contract specifications for sale of the gas.
Gas Sales Compressor
A variety of machines and techniques to create water of a desired quality. Gravity devices such as skim tanks or parallel plate coalescers are used to separate trace oil amounts from water. Hydrocyclones separate dense fluids from less dense fluids by applying centrifugal forces and can be used to separate trace oil volumes from water. Chemicals are sometimes used to help oil coalesce into large and more easily separated droplets or to help solids flocculate for easier separation. Gas flotation units are used to separate very small amounts of oil from water.
A flare is a safe point to combust waste gas. Unrestricted gas flaring is not allowed in most places without a special permit.
Point of Sale
The Sales Contract
The legal document outlining the quality of a product and the price to be paid for the product is paramount among other articles in the document. Quality restrictions for gas can be the gas heating content (BTU/volume), the maximum allowed water vapor, hydrogen sulfide and carbon dioxide concentrations, and the gas specific gravity. Quality restrictions for hydrocarbon liquids include the specific gravity (API gravity), sulfur content, and water and sediment content. The price can fluctuate depending on changes in the product quality.
Pumpjacks create artificial lift when the pressure of the producing reservoir drops below that which is required to raise hydrocarbons to the surface during production.
Indirect heaters where natural gas is burned to create heat which is used to heat a water bath to a set temperature. The water bath heats coils of piping inside the heater through which flows the fluid to be heated (natural gas or liquid hydrocarbons). This is necessary for gas that has sufficient water vapor to condense liquid water that could freeze and block flow or for oil where paraffin may be deposited causing a blockage. Clearly line heaters are useful on shore or production facilities above the ocean. For places where heat cannot easily be provided such as subsea flow lines the injection of an antifreeze such as methanol is used with gas or a paraffin suppressant can be injected with oil.
A Christmas tree is a series of valves and fittings that sits above the top of the wellhead to control pressure. Pipelines leaving the Christmas tree carry produced fluids to processing.
Water Injection Well
Operators inject produced fluids and wastewater into underground formations for disposal through Underground Injection Control Class II wells.
Oil Transfer, Fluid Recirculation, Water Injection
Liquid pumps are used to provide pressure to a liquid to allow for the required flow rate of the liquid through a given pipe or portion of the facility. Pumps can be low volume and low pressure, perhaps to recirculate contaminated liquid back through the separators. Or pumps can be high pressure and large volume for pipe line or water injection well use. Centrifugal pumps are often used in low pressure service and positive displacement (piston) pumps used for high pressure service. Pumps can be powered by electric motors or internal combustion engines. Low pressure pumps can be powered pneumatically using the supply gas from the separators.