Analysis of Underground Gas Storages
by
Askeraliev T. and Kadyrov O.
Kazakh
National Technical University after K.I.Satpaev
Republic
of Kazakhstan, Almaty, KazNTU
November
23, 2011
The most essential type of gas storage
is in underground reservoirs. There are three principal types — depleted gas
reservoirs, aquifer reservoirs and salt cavern reservoirs (several
reconditioned mines are also in use as gas storage facilities). Each of these
types has distinct physical and economic characteristics which govern the
suitability of a particular type of storage type for a given application. Two
of the most important characteristics of an underground storage reservoir are
its capability to hold natural gas for future use and the rate at which gas
inventory can be withdrawn–its deliverability rate.
Gas storage is principally used to
meet load variations. It is also used for a variety of secondary purposes,
including:
-maintaining
contractual balance. Shippers use stored gas to maintain the volume they
deliver to the pipeline system and the volume they withdraw. Without access to
such storage facilities, any imbalance situation would result in a hefty
penalty.
-insuring
against any unforeseen accidents. Gas storage can be used as an insurance that
may affect either production or delivery of natural gas. These may include
natural factors such as hurricanes, or malfunction of production or
distribution systems.
-meeting
regulatory obligations. Gas storage ensures to some extent the reliability of
gas supply to the consumer at the lowest cost, as required by the regulatory
body. This is why the regulatory body is monitors storage inventory levels.
-offsetting
changes in natural gas demands. Gas storage facilities are gaining more
importance due changes in natural gas demands. First, traditional supplies that
once met the winter peak demand are now unable to keep pace. Second, there is a
growing summer peak demand on natural gas, due to electric generation via gas fired
power plants[1].
The most important and common form of
underground storage consists of depleted gas reservoirs. Depleted reservoirs
are those formations that have already been tapped of all their recoverable
natural gas. This leaves an underground formation, geologically capable of
holding natural gas. In addition, using an already developed reservoir for
storage purposes allows the use of the extraction and distribution equipment
left over from when the field was productive. Having this extraction network in
place reduces the cost of converting a depleted reservoir into a storage
facility.
Depleted reservoirs are also
attractive because their geological characteristics are already well known. Of
the three types of underground storage, depleted reservoirs, on average, are
the cheapest and easiest to develop, operate, and maintain. The factors that
determine whether or not a depleted reservoir will make a suitable storage
facility are both geographic and geologic. Geographically, depleted reservoirs
must be relatively close to consuming regions. They must also be close to
transportation infrastructure, including trunk pipelines and distribution
systems. While depleted reservoirs are numerous, they are more abundantly
available in producing regions. In regions without depleted reservoirs one of
the other two storage options is required.
Geologically, depleted reservoir
formations must have high permeability and porosity. The porosity of the
formation determines the amount of natural gas that it may hold, while its
permeability determines the rate at which natural gas flows through the
formation, which in turn determines the rate of injection and withdrawal of
working gas. In certain instances, the formation may be stimulated to increase
permeability.
In order to maintain pressure in
depleted reservoirs, about 50 percent of the natural gas in the formation must
be kept as cushion gas. However, depleted reservoirs, having already been
filled with natural gas and hydrocarbons, do not require the injection of what
will become physically unrecoverable gas; that gas already exists in the
formation.
Aquifers are underground porous,
permeable rock formations that act as natural water reservoirs. However, in
certain situations, these water containing formations may be reconditioned and
used as natural gas storage facilities. As they are more expensive to develop
than depleted reservoirs, these types of storage facilities are usually used
only in areas where there are no nearby depleted reservoirs. Traditionally,
these facilities are operated with a single winter withdrawal period, although
they may be used to meet peak load requirements as well.
Aquifers are the least desirable and
most expensive type of natural gas storage facility for a number of reasons.
First, the geological characteristics of aquifer formations are not as
thoroughly known, as with depleted reservoirs. A significant amount of time and
money goes into discovering the geological characteristics of an aquifer, and
determining its suitability as a natural gas storage facility. Seismic testing
must be performed; much like is done for the exploration of potential natural
gas formations. The area of the formation, the composition and porosity of the
formation itself, and the existing formation pressure must all be discovered
prior to development of the formation. In addition, the capacity of the
reservoir is unknown, and may only be determined once the formation is further
developed.
In order to develop a natural aquifer
into an effective natural gas storage facility, all of the associated
infrastructure must also be developed. This includes installation of wells,
extraction equipment, pipelines, dehydration facilities, and possibly
compression equipment. Since aquifers are naturally full of water, in some
instances powerful injection equipment must be used, to allow sufficient
injection pressure to push down the resident water and replace it with natural
gas. While natural gas being stored in aquifers has already undergone all of
its processing, upon extraction from water bearing aquifer formation the gas
typically requires further dehydration prior to transportation, which requires
specialized equipment near the wellhead. Aquifer formations do not have the
same natural gas retention capabilities as depleted reservoirs. This means that
some of the natural gas that is injected escapes from the formation, and must
be gathered and extracted by 'collector' wells, specifically designed to pick
up gas that may escape from the primary aquifer formation.
All of these factors mean that
developing an aquifer formation as a storage facility can be time consuming and
expensive. In some instances, aquifer development can take 4 years, which is
more than twice the time it takes to develop depleted reservoirs as storage
facilities. In addition to the increased time and cost of aquifer storage,
there are also environmental restrictions to using aquifers as natural gas
storage. In the early 1980's the Environmental Protection Agency (EPA) set
certain rules and restrictions on the use of aquifers as natural gas storage
facilities. These restrictions are intended to reduce the possibility of fresh
water contamination.
Underground salt formations offer
another option for natural gas storage. These formations are well suited to
natural gas storage in that salt caverns, once formed, allow little injected
natural gas to escape from the formation unless specifically extracted. The
walls of a salt cavern also have the structural strength of steel, which makes
it very resilient against reservoir degradation over the life of the storage
facility.
Essentially, salt caverns are formed
out of existing salt deposits. These underground salt deposits may exist in two
possible forms: salt domes, and salt beds. Salt domes are thick formations
created from natural salt deposits that, over time, leach up through overlying
sedimentary layers to form large dome-type structures. They can be as large as
a mile in diameter, and 30,000 feet in height. Typically, salt domes used for
natural gas storage are between 6,000 and 1,500 feet beneath the surface,
although in certain circumstances they can come much closer to the surface.
Salt beds are shallower, thinner formations. These formations are usually no
more than 1,000 feet in height. Because salt beds are wide, thin formations,
once a salt cavern is introduced, they are more prone to deterioration, and may
also be more expensive to develop than salt domes.
Once a suitable salt dome or salt bed
deposit is discovered, and deemed suitable for natural gas storage, it is
necessary to develop a 'salt cavern' within the formation. Essentially, this
consists of using water to dissolve and extract a certain amount of salt from
the deposit, leaving a large empty space in the formation. This is done by
drilling a well down into the formation, and cycling large amounts of water
through the completed well. This water will dissolve some of the salt in the
deposit, and be cycled back up the well, leaving a large empty space that the
salt used to occupy. This process is known as 'salt cavern leaching'.
Salt
cavern leaching is used to create caverns in both types of salt deposits, and
can be quite expensive. However, once created, a salt cavern offers an
underground natural gas storage vessel with very high deliverability. In
addition, cushion gas requirements are the lowest of all three storage types,
with salt caverns only requiring about 33 percent of total gas capacity to be
used as cushion gas.
Deliverability from salt caverns is
typically much higher than for either aquifers or depleted reservoirs.
Therefore natural gas stored in a salt cavern may be more readily (and quickly)
withdrawn, and caverns may be replenished with natural gas more quickly than in
either of the other types of storage facilities. Moreover, salt caverns can
readily begin flowing gas on as little as one hour's notice, which is useful in
emergency situations or during unexpected short term demand surges. Salt
caverns may also be replenished more quickly than other types of underground
storage facilities [2].
Reflecting the change in focus within
the natural gas storage industry during recent years, the largest growth in
daily withdrawal capability has been from high-deliverability storage sites,
which are mainly salt cavern storage reservoirs. These facilities can cycle
their inventories–i.e., completely withdraw and refill working gas (or vice
versa)–more rapidly than can other types of storage, a feature more suitable to
the flexible operational needs of today's storage users. The increasing
influence of high-deliverability storage may affect the significance of
inventory
levels.
REFERENCES
1 www.ela.doe.gov
2 « New concepts in underground storage of natural
gas», pipeline research council international, Inc, March 1966