The emissions of hydrocarbons in the atmosphere must be addressed due to loss of value, as well as, concerns about the environment.
One of the main sources of hydrocarbon emissions at oil and gas production facilities is the emission of natural gas liquids (NGL) in gaseous form. In a well, there is generally a mix of a wide range of hydrocarbon molecules. For this discussion, large molecules that remain a liquid at ambient temperature and pressure are "oil"; the smaller molecules which tend to be a gas at higher pressure, are "natural gas."
Flash Gas Control technology addresses the emission of molecules of intermediate size that tend to be a gas at ambient pressure but are liquids at high pressure. These intermediate molecules can be called NGLs or liquefied petroleum gas (LPG). Here, we will focus on hydrocarbons between three to eight carbon atoms (propane to octane).
On a well site, the stream containing water, liquid hydrocarbons (oil), and gaseous hydrocarbons (natural gas) are introduced at high pressure to a three phase separator where the water, oil and gas are separated into three streams. The water is sent to an atmospheric storage tank for future disposal. This oil is sent to a separate atmospheric storage tank and the natural gas is usually dehydrated (water vapor removed) and is sent to the natural gas pipeline.
When oil is sent to an atmospheric pressure storage tank, the pressure drops from the well pressure to atmospheric pressure and the gases that have been dissolved in the oil due to the high pressure are released. In this case the gases are methane, ethane, and NGLs. A similar and more familiar reaction occurs when opening a soda and the gases escape from the liquid due to reduced pressure.
In some installations, these gases are sent to flare. However, the flow rate of gases is often too high resulting in a tank pressure relief valve opening and allowing the gases to escape into the atmosphere. These gases are also inadvertently, yet regularly, released when storage tanks are gauged (opened) to visually check fluid levels.
Currently, vapor recovery units are often used to try to address this environmental and financial problem. The low pressure vapors from the atmospheric tanks are compressed to the natural gas pipeline pressure by using a vapor recovery unit. The methane and ethane proceed to the pipeline (methane emissions) yet most of the larger hydrocarbon molecules condense once again and return to the storage tank at atmospheric pressure, where they will re-vaporize and have to be re-compressed (and never captured).
An underlying problem remains: the hydrocarbons are not liquid at atmospheric pressure and so cannot be sold as oil; they are liquids at pipeline pressures so they cannot be sold as natural gas. The conditions described above, including the continuous recycling of these hydrocarbons, results in valuable commodities being wasted by venting or flaring.