Offshore Conversion of Associated Economic Option for Deep Water and Gas to Synthetic Crude Oil: An Marginal Fields.
Authors: Antia David D.J. , Seddon Duncan 
Published: October 2008

A low cost modular Fischer Tropsch gas conversion approach which can add value to associated gas in both deep water and marginal fields is described. The process is suitable for location on vessels (FPSO's), jackups, platforms and semisubmersibles. It can economically convert &gt;1 to &gt;200 MMCF/D to syncrude. The modules can be used to:Strip value from gas prior to injectionRemove a reinvention or flaring requirementReduce carbon dioxide emissions from the field

Conversion ratios depend on the catalyst and process configuration used. They range from 70 - &gt; 170 Bbls/MMCF.

Introduction

The technology to convert natural gas into syncrude dates from developments in the 1920s by Fischer and Tropsch, Early commercial plants (first in production in the early 1930s) used coal as a feedstock and produced a synthesis - gas (water - as) from the combustion of coke with alternate air and steam blasts. Synthesis gas is composed of hydrogen and carbon monoxide. The synthesis - gas was passed over a cobalt catalyst to make a synthetic oil which was then further refined to transport fuels. Since these early plants, there have been many commercial, semi-commercial (very large demonstration) plants in many parts of the world using a variety of types of reactors and catalysts and producing a range of products, The products range from the production of synthetic natural gas (SNG; researched and used extensively in Japan) to heavy wax (produced at the Sasolburg plants of Sasol).

These coal technologies are readily adapted to natural gas (as a synthesis - gas feedstock) with the following major benefits:-Considerably lower cost gasification plant (Partial oxidation) is required to produce the synthesis gas.Higher purity feedstock requiring little or no purification,

In addition natural gas can be reformed into a synthesis - gas using steam alone and a catalyst (steam reforming), or a combination of steam reforming and partial oxidation.. This process is widely used in the chemicals industry, but for Fischer - Tropsch synthesis, the synthesis - gas would require extensive Yurther treatment to adjust the hydrogen to carbon monoxide ratio and to remove excess carbon dioxide; these treatments are not required using the older partial oxidation technologies.

Currently four commercial plants are in existence. Two use coal (Sasolburg and Secunda, RSA) and two use natural gas (Mossgas, RSA, and Shell's MDS plant at Bintulu, Malaysia). In all four operations, the synthetic oil and off-gases are intermediates in extensive refinery operations producing transport fuels and refined wax.

The main reason why these plants have not been used extensively is the high cost of feedstock natural gas. A gas price of $2/MCF equates to a syncrude price of between &lt; $11 and &gt;3 25/Bbl. The exact operating cost translation is a function of the conversion efficiency used. This is a function of process configuration and catalyst used.

This high feedstock rice is a major barrier to usage. However, in marginal and deepwater fields there may be insufficient gas to justify the development of a gas export infrastructure.