NGH saves 20% over LNG
Ben C. Gerwick Inc.
Close to 5,000 tcf of natural gas worldwide is stranded. The huge potential value of the gas explains why considerable effort is being made throughout the industry to reduce transport costs so stranded reserves can be brought to market. One solution is to transport it as natural gas hydrates (NGH).
NGH technology has the potential in some cases to drive the cost of natural gas transport down to a level comparable to the cost of transporting crude oil. And transporting hydrates could be cheaper than transporting LNG. Japanese industry officials have estimated that the cost of producing, storing, and transporting NGH will be at least 20%-30% less than that of carrying out the same process for LNG for moderate-sized fields relatively close to market.
It is important to understand how NGH technology works in order to evaluate its viability. Briefly stated, hydrate systems would use gas hydrates to reduce the volume of natural gas by approximately 169 times (the theoretical limit is over 180 times) and would store the hydrates within a range of potential temperatures and associated pressures. Hydrate transport systems are attractive for fields that are relatively close to markets. Such areas include the deepwater Gulf of Mexico, Alaska, Eastern Canada, Venezuela, Trinidad & Tobago, Egypt, and Indonesia.
Why move gas in the form of hydrates? Because there are several unique advantages, including low cost. Large quantities of stranded associated natural gas have been identified and in many cases represent a significant production cost. To recover the crude oil, the associated gas in most cases must be re-injected. Exporting the gas in hydrate form would allow marketing of even large proven reserves of stranded associated gas with very little additional capital investment beyond that already committed for the recovery of the crude oil reserves.
Easy and safe transportation and storage represent another advantage. With relatively minor modifications, existing oil tankers could accommodate supplemental steel hydrate containment pipes that would allow transport of previously stranded associated natural gas. Intermediate storage facilities for the hydrates at both the production and re-gasification ends of the transport route could be relatively safe, simple banks of steel pipes located onshore or underwater offshore. The storage, transport, and handling of natural gas in hydrate form is inherently safer than that of either cryogenically liquefied or compressed gas.
Economics, of course, also constitutes an advantage. Hydrates exhibit a low melting rate, which means minimal gas cargo loss. The remaining question is, "Can hydrates be produced and sold profitably?" And the answer is yes. Commercial production of gas hydrates has been studied and found to be fully practicable.
The photograph shows NGH slurry. Hydrates are formed when small ice particles come in contact with natural gas. This method of hydrate formation moves a large portion of the cooling function away from the formation reactor. Pneumatic transport of the hydrate particles also solves the water removal problem. (Photo: BG Group)
Storage technology fundamentals
The theoretical storage capacity of the proposed system depends partially on the type of hydrate crystal to be used, which in turn depends on the composition of the natural gas to be transported and the additives used to promote hydrate formation. The two most likely types of crystals to be used are Structure II or Structure H. After accounting for such factors as incomplete cage filling, occlusions/impurities, and incomplete packing of the hydrate crystals, the storage capacity of Structure II is somewhat over 160 times.
One transport option, the storage of NGH in its meta-stable range of temperature/pressure, may be desirable because it allows the hydrates to be transported at atmospheric pressures at a relatively high (but sub-freezing) temperature around 23° F. This type of system could make good use of pneumatic conveyance to an onboard compactor. While pneumatic formation/conveyance does not require water separation and avoids the need for pressure vessels, it places greater demands on the insulation system because NGH dissociates relatively rapidly if the hydrates warm above the meta-stable range.
Another suggestion, which is limited to associated gas, is to cool the NGH to within -4° to +14° F and to put the NGH into the similarly cooled crude hydrocarbon liquids within the hold of an oil tanker.
A third option is to store and transport pressurized natural gas. At a temperature of 35° F, natural gas hydrates typically are in equilibrium at pressures between 200 and 390 psia, depending on the gas composition. A very large crude carrier could be modified to carry gas hydrates in long insulated steel deck-mounted pipes to transport associated gas together with crude oil in the tanker to market. For non-associated stranded gas, steel pipes within the carrier's hull could be used for transport. Such a system requires the use of inert nitrogen gas within the hull of the vessel to address the potential for natural gas leaks. Of course, numerous other equilibrium conditions are also practicable.
Using a pressurized NGH system with containment pipes makes the transport of associated natural gas on existing crude oil tankers economically feasible.
The associated gas market is one of the most important initial markets for NGH. Proven gas resources are available without exploration costs. Little capital expenditure is required because producing platforms and transport vessels are already available. Oper-ating cost is low compared to other options. And savings – or a greenhouse gas credit for not flaring – result from not re-injecting the associated gas. Another advantage is that the development schedule for the field can be optimized. Associated gas export can be adapted to a range of production rates. Further, the technology is well suited to offshore, using processing equipment with a small footprint, and is insensitive to motions/accelerations.
The containment system should be moderately pressurized. Hydrates can be stabilized above the freezing point of water, which simplifies the process engineering and allows existing technology to be used to handle ice slurries. In this scenario, containment pipes not only serve as pressure vessels, but also facilitate handling the NGH product. Also, the relatively high storage temperatures allowed by the pressurization simplify the refrigeration and insulation requirements.
Finally, there is a belief that NGH stored above the freezing point of water can be re-gasified more readily than NGH stored below freezing. It appears that the meta-stability of NGH below the freezing point of water could hinder re-gasification because of the formation of thin ice films, which are impermeable to gas molecules, that form on the surface of low temperature hydrates.
Compressed NGH pellets. (Photo: MES)
A number of methods have been used to produce natural gas hydrates. These include:
- Natural gas infusion into ice crystals
- Natural gas infusion into propane or iso-butane hydrate crystals
- Dynamic mixing of natural gas and water – with and without surfactants
- Using emulsions and/or immiscible liquids such as mineral oil as a carrier medium for the hydrate crystals.
There are three primary hydrate formation options for the purpose of transport.
The slurry method forms methane hydrates within a flowing water/hydrate slurry. Hydrate is formed within an aqueous slurry, with the heat of hydrate formation removed using coolant in a jacket. The gas is removed and recycled. Then the hydrate is separated from the bulk liquid.
The wet hydrate is further dewatered in a compactor and formed for shipment. For such a system, it should be practicable to ship the ice to the gas field within the emptied hydrate storage pipes. It should be possible to use an ammonia heat exchanger to recover some of the heat energy used to re-gasify the hydrate particles to assist with the ice formation process.
A second option is a hybrid slurry system, which feeds ice water in a slurry to the reactor. Since the majority of the heat of hydrate formation consists of the heat of fusion of the water, feeding ice to the reactor helps to move some of the cooling duty into conventional icemaking equipment.
A third option is an ice formation system, in which the hydrate is formed directly from ice and gas. This option has appeal because it moves the act of cooling to an icemaker and because it minimizes the amount of water retained in the resulting hydrate.
In 2001 Mitsui Engineering & Shipbuilding Co. Ltd. (MES) began the continuous manufacturing of NGH. The company also began experimental pelletization of NGH. MES's hydrate formation process mixes water and natural gas at a temperature of approximately 35° F at a pressure of 750 psia. The company estimates that the production, transportation, and re-gasification process could be operating by 2007.
Meanwhile, Mitsubishi Heavy Industries Ltd. has made headway in manufacturing NGH by spraying water into pressurized natural gas.
Any hydrate transportation system has to be economically competitive with similar gas transport systems such as LNG, gas-to-liquids (GTL), and compressed natural gas (CNG) systems. NGH has cost and application advantages when compared to LNG, GTL, and CNG alternatives.
NGH systems have the lowest initial development cost. The NGH option also offers the lowest operating costs when transporting distances are less than 6,000 nautical miles. Furthermore, NGH is flexible. Transport can be readily tailored to carry the required volume of gas for developments of virtually any size. The volume of gas carried by the NGH systems can be increased or decreased incrementally over the life of a field to match the preferred field development schedule. NGH systems can readily handle any combination of rich and lean gas.
Safety is another serious consideration. Storing natural gas in hydrate form is inherently safer than any of the alternates because NGH have relatively high latent heat, require moderate storage conditions, and have low toxicity. These characteristics lead to low combustibility and less serious consequences if a containment vessel ruptures.
NGH systems are well suited for offshore fields and can allow gas that would otherwise be flared during exploratory drilling to be stored for later collection and regasification.
In addition to all of the other advantages, an NGH system can be implemented quickly because it uses an existing infrastructure. While NGH is low-cost, it is also low-risk because it uses proven technology. And size does not matter. NGH technology is well suited for small and medium-sized gas field that would be otherwise uneconomical to develop.
Of course, the final degree to which natural gas hydrates can make natural gas a portable product is dependent on the cost of the feedstock gas, the transport distance, the mode of transport, and the current market price of natural gas. While the industry considers the viability of capitalizing stranded gas fields using NGH technology, 5,000 tcf in identified stranded reserves is lying below the earth's surface waiting to be produced.