Liquefied Natural Gas (LNG)
Liquefied natural gas is natural gas that has been cooled cryogenically to -260°F. The gas in its liquefied state occupies 1/600 of its original volume. Meaning that 600 cubic feet of natural gas will occupy 1 cubic foot as a liquid. LNG is stored in double-walled, vacuum insulated tanks. The high density of LNG makes it ideal for long distance transport and it is the primary method for shipping natural gas overseas. One of the primary concerns with shipping and storing LNG is boil-off. The temperature of the LNG slowly begins to rise above the boiling point and the liquid beings to evaporate into gas in small amounts. Pressure release valves open to release the gas to a separate tank where it can be used as fuel or compressed into CNG or if the pressure exceeds a certain level the gas is vented to the atmosphere. Once LNG reaches its final destination it is regasified and ready for use as a fuel.
Safety precautions must be made when handling LNG equipment. Personnel performing the transfer of LNG from one tank to another need proper training and safety equipment to reduce incidents related to cryogenic temperatures, such as frostbite. Regular maintenance should be performed as well, especially on components that come into contact with LNG to prevent leaks or equipment malfunction. Embrittlement occurs when LNG comes in contact with plastics, rubbers, and certain metals. The final safety concern related to LNG is rapid phase transition (RPT). RPT occurs when LNG comes into contact with water and the result is an event that resembles and explosion.
Compressed Natural Gas (CNG)
Compressed natural gas is gas that has been compressed to between 3000 psi and 3600 psi. Compression reduces the volume of natural gas to 1/100 of its original volume. CNG is stored in a series of cylinders composed of high strength material(s). Natural gas is delivered to compression stations via pipeline where the gas is then compressed and stored in the cylinders. There are two types of compression stations, fast fill and slow fill. Fast fill stations offer filling times comparable to diesel or gasoline and slow fill stations fill a tank over a period of time, usually overnight. CNG tanks require regular inspection, maintenance, and certification to stay in operation and to reduce incidents and leaks.
Pipeline gas, also known as sales gas, has been processed to remove the heavier hydrocarbons, water, other gases, and corrosives. The first step in processing is separation of the liquids from the gases, typically done at the wellhead. The liquids are processed further separately from the gas stream. The gas stream undergoes either glycol dehydration or solid desiccation to remove water vapor. Gaseous heavier hydrocarbons, butane and pentane, are removed next via oil absorption. Next, the lighter hydrocarbons, ethane and propane, are separated out by turbo expansion of the gas stream. Finally, hydrogen sulfide and carbon dioxide are removed by bubbling the natural gas stream through an amine solution. Further processing can be done after the amine step; however, the gas is at pipeline quality after the amine step. The natural gas can now be transported through pipelines or as LNG or CNG. This could be used as a source of fuel for operations if a pipeline were constructed connecting the site to the main pipeline. The gas is primarily methane and the price is generally lower than the price of CNG or LNG.
Field gas, or wellhead gas, is increasingly becoming a source of fuel for both drilling and hydraulic fracturing treatments. The maximum amount of cost savings can be achieved by using field gas since transportation and processing expenses are not included. Onsite processing equipment might be required depending on the quality of natural gas that the well is producing. The natural gas needs to have a high percentage of methane; otherwise engine knock and damage will occur. One method that can be used to determine if engine knock will occur is by determining the methane number either by calculation or by experiment. Low methane numbers are generally associated with engine knock and low performance whereas high methane numbers are indicative of high performance. To increase the methane number and reduce engine wear the gas needs to have all of the liquids and corrosives removed. Using produced or associated gas as a fuel for operations also provides and alternative to flaring since the gas can be put to use.
For more information contact:
Carolyn LaFleur, clafleur@HARCResearch.org, 281-364-4035