Jeremy Beckman - Editor, Europe
Associated gas can hinder investment in remote offshore oilfields for a host of reasons. If volumes are low, re-injection may be feasible, depending on the risk to reservoir integrity. But the added well construction costs may render the development uneconomic.
Flaring excess gas is no longer politically acceptable. As for disposal via existing offshore trunklines to LNG terminals or power plants, cost is again the major issue. Moderate volumes of associated gas may justify construction of spur lines, and the resultant tariffs, but this is rarely an option for a smaller, remote field with a limited productive life.
Another solution involves stationing a standalone gas processing plant alongside the offshore oil production facilities. However, most gas-to-liquids or floating LNG schemes are capital-intensive, demanding a steady, strong gas stream. In general, these technologies are better suited to large reserves of stranded gas than to modest volumes of associated gas.
CompactGTL has devised an alternative that focuses less on monetizing the gas than on moving the oilfield development forward. Its proprietary, gas-to-liquids process converts associated gas to synthetic crude oil (syncrude) offshore at the point of production for blending with the field’s main crude stream. The equipment for this task is designed to easily integrate into a newbuild FPSO or similar floating facility.
The first commercial application should be offshore Brazil. In 2006, the company was contracted for a pilot syncrude plant which is due to undergo trials next year at Petrobras’ R&D center in Aracaju, northeast Brazil. The equipment will receive unprocessed gas which it will convert to syncrude at a rate of 20 b/d for an initial six-month test program.
Petrobras plans to conduct extended well tests on the ultra deepwater Tupi discovery in the Santos basin. It views the CompactGTL process as a way around Brazil’s restrictions on flaring of gas from exploratory/appraisal wells over a period of more than 30 days. This time-frame is too tight for modeling the performance of sub-salt reservoirs generating complex seismic data.
Research centers
The basics for the CGTL process were developed in 1999-2000 by nuclear industry specialists AEA Technology at Harwell, UK. AEA had the idea of transferring the building blocks for small-scale hydrogen production to the newly emerging gas-to-liquids industry. The idea was taken up by FMC Technologies, which formed a short-lived joint venture with Accentus plc, the commercial arm of AEA Technology, called GTL Microsystems.
In 2005, Coller Capital, a private equity fund, bought a portfolio of assets from Accentus which included the GTL technology. Within a year later, Coller also bought out FMC’s share of GTL Microsystems and formed CompactGTL plc as a new entity. CompactGTL was established at a new, purpose-built laboratory/office complex in Abingdon, where R&D focussed on improving the process and its sub-components via experiments with laboratory-scale catalytic reactors, reaction kinetics studies, and so on.
Management then constructed a larger pilot plant test facility in Wilton, northeast England, which started up in mid-2008. The plant, housed in a 40-ft (12.2-m) container, can produce 0.2 b/d of syncrude from simulated associated gas, allowing testing of intermediate-scale gas-to-liquid reactors.
How the process works
The CGTL process uses a steam methane reformer (SMR) to convert produced gas to syngas (a mixture of carbon monoxide and hydrogen). The syngas then is compressed and conveyed to a Fischer-Tropsch reactor (FTR) from which it emerges as synthetic crude oil, water, and a tail-gas stream of hydrogen, carbon monoxide, and light hydrocarbon gases.
Both the SMR and the FTR are similar in design, each comprising two sets of mini-channels packed with Fecraloy foils coated in a catalyst. The configuration and construction of the SMR and FT reactors is based on plate-and-fin heat exchanger technology.
One set of channels in the SMR combusts gas to generate heat at a temperature of around 1,292º F (700º C), which is directed to the other set of channels to trigger the steam/methane reforming reaction. Both sets of channels are in closely alternating layers, to optimize the heat transfer needed to intensify the process, and also to allow the reactor size to be compact.
For the FTR – because in this case the reaction is exothermic, as opposed to endothermic for the SMR – the second set of channels carries cooling water.
The process is designed to be modular, allowing it to be sized according to an oil field’s needs. As an example, a bank of seven reactor modules could convert 15 MMcf/d of associated gas to 1,500 b/d of syncrude. As the field’s production declines, these modules can be progressively removed. With conventional GTL processes, single giant reactors achieve economies of scale, but these rely on a constant gas stream.
CompactGTL says its technology suits a largely self-contained plant operating a stable process which does not require a supply of oxygen, and handles small volumes of fluids. The system is claimed, therefore, to be insensitive to sloshing/wave motions in an offshore setting on an FPSO, which is not the case for most giant reactors.
“Another point to bear in mind,” says CEO Nicholas Gay, “is that most existing gas processing technologies require the facilities to be designed with a certain bandwidth of gas flow. We, however, offer a modular process: we can adapt our technology according to the gas supply. No one else offers this flexible gas processing capacity. Even with the giant LNG plants, there is always a need to find more gas to maintain operations at the same level.”
Tests at Wilton have proven the technology. Management has ensured that the manufacturing techniques applied to the pilot-plant in operation here also suit scaled-up to commercial, 200-b/d reactors weighing less than 25 metric tons (27.6 tons). This has been achieved through developing strategic alliances with established manufacturing groups worldwide
Additionally, CompactGTL has contracted Technip subsidiary Genesis Oil & Gas in London for conceptual study work for commercial plants, some of which has been client-funded. Bayer Technical Services in Germany has provided theoretical reactor modeling and process design support; the Paul Scherer Institute in Switzerland has supported catalytic combustion studies; and The Welding Institute (TWI) in the UK has assisted with mechanical integrity studies.
Generic floater design
The $45-million budget, 20 b/d CGTL test plant for Petrobras, under construction by Zeton in Toronto, is due to start operating at Aracaju next July. The plant will be equipped with all the required gas pre-treatment packages and all necessary utility systems, including steam generation.
According to CompactGTL, a commercial-scale plant converting 10 MMcf/d of associated gas to 1,000 b/d syncrude plant on a conventional FPSO or well-test vessel would occupy a footprint of 20 x 40 m (65.6 x 131 ft) on two levels. A generic design for a plant of this scale was conceived in cooperation with SBM Offshore in Monaco.
CompactGTL claims cost and complexity advantages compared with offshore LNG or a methanol-conversion process, or compressed natural gas proposals, as all of these require a more complicated logistical spread or separate storage.
The GTL solution will not suit all compositions of associated gas in all situations, notably very gassy crude or condensate. Also, in line with all other GTL processes, the plant must always incorporate a comprehensive gas treatment and sulfur removal package.
Gay adds “Petrobras is planning a commercial-scale plant, probably for start-up during 2012-13. In the meantime, we are developing other prospects and contracts. Theoretically, we could be selling plant capacity of 25-30,000 b/d per year of syncrude during 2015-18. However, we do not want to make a big song and dance before we have taken further steps towards the first commercial plants.”
