Advanced Technology

Leonard LeBlanc Houston Flow chart You name it in the hydrocarbon family - natural gas can be converted to almost anything desired. While the threshold price needed for competitive operation of a conversion facility has finally dropped down to the $19-22/bbl WTI level, the flexibility of the output is a distinct advantage for producers with large isolated gas reserves. In fact, the new processes are likely to lead to the booking of new gas reserves, heretofore considered non-commercial.

Leonard LeBlanc
Houston

Gas-to-liquids technology will yield new reserves bookings

You name it in the hydrocarbon family - natural gas can be converted to almost anything desired. While the threshold price needed for competitive operation of a conversion facility has finally dropped down to the $19-22/bbl WTI level, the flexibility of the output is a distinct advantage for producers with large isolated gas reserves. In fact, the new processes are likely to lead to the booking of new gas reserves, heretofore considered non-commercial.

The process of converting gas has been known for over 50 years (Germany in World War II, Sasoil conversion in South Africa, Gulf Oil's Badger process, Shell's synthetics generation, Mobil's MTG system), but the cost of the catalysts (use and recovery) has always been very high. Recently, Syntroleum and Texaco announced a much less expensive process to catalytically convert gas. This past month, Exxon jumped on board with its own fluid bed process, known as Advanced Gas Conversion (AGC-21).

The Exxon process reacts gas, steam, oxygen and catalysts to obtain hydrogen and carbon monoxide, which are in turn combined in a slurry reactor with additional catalysts. The feedstock product can then be upgraded by isomerization to cat-cracker feedstock. Besides direct sales of the products, the converted liquids can be blended to improve heavy hydrocarbons.

Reportedly, Exxon can take an output of 500-1,000 MMcf/d and convert it into 50,000-100,000 b/d of stable liquids (distillate, diesel, jet fuel, oil). Just as with the Syntroleum-Texaco process, the AGC-21 output is free of sulfur, metals, and contaminants. Exxon is considering the process for isolated gas reserves in Qatar (North), Indonesia (Natuna), and Alaska (North Slope).

The process costs remain a slight stumbling block at the moment because crude oil prices can slip easily below $20/bbl for a period of time, however, further technical development and volume throughput savings could render the process competitive at firm $18/bbl oil prices. If oil prices remain above $20/bbl in the coming years, look for these technologies to completely change the business of exploration and production around the globe.

Gas-loaded ocean a hazard, unless it's done purposefully

The amount of gas or air in the ocean determines the fluid's density, and thus how high or low in the water a displacement vessel will float (otherwise known as freeboard). In the past, there were real fears that a floating drilling unit would sink if huge plumes of gas from a blowout surfaced under the hull. In fact, rumors of ship disappearances in the Bermuda Triangle are still based in part on massive methane releases from the seabed.

Today, we know that, while possible in theory, such an event is not likely, and has never occurred to a drilling vessel. Apparently, a uniformly high volume of gas is needed to bring a vessel down to the gunwales. Jackup drilling units have been damaged in the past when the seabed sediments supporting the legs became aerated as a result of gas escaping outside the well casing.

At the same time, a directed flow of air or gas in small volumes beneath a planing hull can produce very desirable effects. Aerated water is slippery. When a gas is introduced beneath a planing hull while underway, a boundary layer is created between the hull and the water. Friction losses drop and the hull experiences a gain in speed by as much as 15%. Displacement losses are marginal because the air volume flow is small. The larger the hull surface-water contact, the greater the benefit derived from the air boundary.

The principle seems to work best with high-speed passenger ferries, which are now being fabricated with air injection systems on the hull steps, but other planing vessels may be considered for the process. No installations have been reported for crew vessels within the petroleum industry - yet. The technology was developed in Russia years ago for military applications.

Hydrate forms with heavier compounds at higher temperatures

Not many methane hydrate crystalline structures are alike, but most form at predictable temperature/pressure regimes. However, researchers have found at least one hydrate, structure H, that crystallizes with heavier compounds and at higher temperatures than all others. The finding alters the blockage risks associated with subsea product transfer.

The UK's Health & Safety Executive is studying the ramifications of hydrate formation, and especially structure H since it is somewhat different. Operators are finding ideal hydrate forming temperatures and pressures in deepwater West of Scotland, and it holds implications not only for production operations, but the risks of hydrate blockage in drilling safety equipment and hydraulic lines.

Explanation developed for some rogue waves

Mariners and offshore workers may go an entire lifetime at sea without ever experiencing a rogue wave, but once they do, the event is usually unforgettable, if they survive. A rogue wave is one starkly out of character for surrounding conditions or exceeding 100-year return storm wave heights. However, the wave may not be very distinguishable from the surrounding sea until a very deep trough ahead of the wave arrives.

Although most rogue waves are believed to be associated with subsea seismic displacements, three scientists believe there is another equally common cause. Bengt Fornberg of the University of Colorado, Benjamin White of Exxon Research and Engineering, and Marius Gerber of Stellenbosch University in South Africa have developed the mathematics and models showing how ocean currents concentrate ocean swells to form isolated rogue waves.

The model was designed initially to suit the south-flowing Agulhas Current off southeast South Africa, but it can apply to the Gulf Stream's movement through the North Atlantic. When the ocean swells are moving in the opposite direction of the current, the interaction reduces the spacing between the incoming waves and changes their direction. This results in isolated waves, characterized by a deep trough ahead, following by a steep forward face on the rogue wave itself..

The researchers are working on models that can predict the location and conditions by which rogue waves are likely to arise.'

Reference:

"Rough math: Focusing on rogue waves at sea," Science News, November 23, 1996.

Copyright 1997 Offshore. All Rights Reserved.

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