CNG Storage – How Is It Done?

How is CNG stored?

Natural gas is able to be stored for an imprecise period of time. The natural gas storage activity such as the exploration, production, and transportation takes time. The natural gas which reaches its destination however is not always needed on the spot; therefore it is injected into underground storage facilities. This storage facility is known to be located near market centers which do not have a ready supply of locally produced natural gas.

Basically, natural gas is known to be a seasonal fuel as the demand for natural gas is usually higher during the winter; this may be partly because it is used for heat in residential and commercial settings. Stored natural gas plays a major role in ensuring that any excess supply delivered during the summer months is able to meet the increased demand during the winter months. However, it can’t be denied that the demand for natural gas during the summer months had also been increasing due to electric generation via gas fired power plants. Natural gas in storage also plays a role in ensuring against any unforeseen circumstances such as accidents or natural disasters that may affect the production or delivery of natural gas.

Besides that, natural gas storage plays a vital role in maintaining the reliability of meeting regulatory obligations which is important in meeting the demands of consumers. Historically, natural gas storage was just a part of the bundled product sold by the pipelines to distribution utilities. However, this changed in 1992 prior to the introduction of the Federal Energy Regulatory Commission’s (FERC) Order 636, which opened up the natural gas market to deregulation. Therefore, natural gas was now available to anyone who seeked storage for commercial purposes or operational requirements. Storage was once known to serve only as a buffer between transportation and distribution to ensure that adequate supplies of natural gas were in order for seasonal demand shifts, and unexpected demand surges. But now, natural gas storage is also used by producers and marketers as a speculative tool where they store gas when they believe prices will rise in the future and then sell it when it reaches those levels.

The most important type of gas storage is known to be in the underground reservoirs. There are basically three main types of underground natural gas storage facilities which consist of depleted gas reservoirs, salt cavern reservoirs and aquifer reservoirs. Each of these types posses a different physical and economic characteristic which determines the suitability of a particular type of storage for a given application. Depleted gas reservoirs are the most well-known form of underground storage which consist the formations of natural gas fields which have produced all their economically recoverable gas. The exhausted reservoir formation has the capability of holding injected natural gas. Facilities such as this is economically attractive as it allows the re-use of an already developed reservoir which means it allows the use of the extraction and distribution equipment which was left over when the field was productive and hence is able to reduce the cost of converting a depleted reservoir into a storage facility. Consequently, depleted reservoirs are known to be the cheapest, easiest to develop and operate among these three types of underground storage.

The suitability of a depleted reservoir as an economically viable storage facility is determined by the geographical and geological factor. Geographically, depleted reservoirs have to be relatively close to consuming regions as they must also be close to transportation infrastructure which includes trunk pipelines and distribution systems. As for the geological factor, it is preferred that depleted reservoir formations have high porosity and permeability. The porosity of the formation determines the amount of natural gas that it may hold, while its permeability measures the rate at which natural gas flows through the formation and ultimately determines the rate of injection and withdrawal of gas from storage. In order to maintain working pressures in a depleted reservoir, about 50 percent of the natural gas in the formation has to be kept as cushion gas. However, depleted reservoirs that had already been filled with natural gas and hydrocarbons do not require the injection of gas which would become physically unrecoverable as this is already present in the formation. This facility is believed to provide a future economic boost when the cost of gas tends to get higher.

Besides that, an underground salt formation also offers another option for the storage of natural gas. Salt caverns allows very little amount of injected natural gas to escape from storage unless it’s specifically extracted. The wall of a salt cavern has structural strength of steel which makes it resilient against reservoir degradation. Once a suitable salt dome is discovered and found to be suitable for the development of a gas storage facility, it is a necessity to develop a ‘salt cavern’ within the formation Essentially, this is done by the process of cavern leaching where fresh water is pumped down a borehole into the salt. This water tends to dissolve some of the salt in the deposit, where it is then cycled back up the well hence leaving a large empty space that the salt used to occupy. This process is continued until the cavern reaches the desired size. Once created, a salt cavern offers an underground natural gas storage vessel with very high deliverability. Cushion gas requirements are known to be the lowest of all the three storage types, as salts caverns only requires typically about 33 percent of total gas capacity to be used as cushion gas. Salt caverns are usually much smaller compared to depleted gas reservoir and aquifer storage facilities. A salt cavern facility usually takes up about one one-hundredth of the acreage taken up by a depleted gas reservoir.  Consequently, a salt cavern is not able to hold the large volumes of gas necessary to meet base load storage requirements. However, deliverability from salt caverns is typically much higher than for either aquifers or depleted reservoirs. Therefore, this allows the gas stored in a salt cavern to be withdrawn and replenished more readily and quickly compared to the other types of storage facilities which is useful during emergency situations or during short terms of unexpected demand surges.

Furthermore, aquifers are also known as natural gas storage. Aquifers can be defined as layers of permeable rock, sand, or gravel through which ground water flows, containing enough water to supply wells and springs. However, these water containing formations may be reconditioned and used as natural gas storage facilities in certain situation. These facilities are usually operated on a single annual cycle as with depleted reservoirs. The aquifer formation had always been known for its good geological and physical characteristic. However, it can’t be denied that considerable investment is needed for investigating these and determining the aquifer’s suitability for natural gas storage. Once an aquifer is determined to be suitable, the entire associated infrastructure has to be developed from scratch. This increases the development costs compared to depleted reservoirs. The development cost includes installation of wells, dehydration facilities, extraction equipment, pipelines and possibly compression equipment.  Aquifer storage requires more cushion gas than depleted reservoirs which takes up to 80% of the total gas volume. Most aquifer storage facilities are usually developed when the price of natural gas is low as aquifer storage becomes more expensive to develop when the price of natural gas begins to increase. Based on the fact that developing aquifer storage requires a high cost and time consumption, it is therefore generally the least desirable and most expensive type of natural gas storage facility.

Many research are being conducted on many fronts in the gas storage field to help identify new and improved ways to store gas. For instance, research conducted by the US Energy department shows that salt formations can be chilled to allow more gas to be stored and hence reduce the size of the formation needed to be treated, and have salt extracted from it. This would help to lower the development costs for salt formation storage facility type. Another aspect being looked at, are other formations that may hold gas. Therefore, it’s just a matter of time where there would be more enhanced, developed and economical ways to store gas in the near future.

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