The current state of development of methane hydrate resources – ‘from Mallik to the Nankai Trough' – was examined by an international symposium in Japan late last year. Two of the participants, Professor Andrew Palmer and Simon Falser from the National University of Singapore's Centre of Offshore Research & Engineering, look back over the proceedings, discuss their own research efforts to date and set the possibility of hydrates production in the broader context of the offshore scene.
Hydrates are snow-like solid compounds of water and hydrocarbons, stable at low temperatures and high pressures. Figure 1 is a phase diagram for water and methane: a useful way to remember where the stability boundary is located is that at a pressure of 4MPa methane hydrate is stable if the temperature is lower than 4°C. Methane molecules are trapped within a cage of water molecules. Dissociating 1m3 of solid hydrate turns it into 164m3 of gas and 0.9m3 of water. The presence of higher hydrocarbons shifts the boundary to the right.
Offshore engineers generally run into hydrates in the context of flow assurance, because they can bung up pipelines and subsea systems. One of the schemes deployed to stop flow from the Macondo well failed because of hydrates (as any first-year petroleum engineering student could have told the operator – but never mind). Hydrates also exist naturally in many parts of the world, but most widely under deep water and in the Arctic, as one would expect. The quantities are enormous: one estimate is that more than 88,000tcf1 of methane are locked up in hydrates, which correspond to more than 800 years of gas production at the current rate.
Last November's Tokyo symposium on methane hydrate resources was an opportunity to examine the current state of development. Japan's MH21 programme is far and away the most credible and developed programme. Canada and the US are some way behind (also Korea, but they are very cagy), and Taiwan, India, and New Zealand are at early stages. China and Russia were not represented at the conference, probably for political reasons, but it is known that the Russians produced gas from hydrates at the Messoyakh field2 in the early 1970s.
Hydrates are much too deep to mine economically, and the obvious way to produce the gas is to dissociate in place, by reducing the pressure, raising the temperature, adding an inhibitor that moves the phase boundary, or displacing the methane by another substance whose molecules preferentially occupy the cages.
All these methods are being studied, but depressurisation is the most popular, though there are difficulties. The most promising fields have porefilling hydrates in relatively coarsegrained sands, with good porosity and permeability and hydrates saturations of 60% or more.
Mallik is an onshore hydrate field in the Mackenzie Delta, 100km north of Inuvik. The hydrate zones are at 900-1110m depth, underlying about 600m of permafrost. It is easier and cheaper to carry out exploratory tests onshore rather than offshore, and Japan and Canada have been working together at Mallik since 2002. A 2007 test produced 800m3 of gas by depressurisation by an ESP, but then failed because of excessive sand production. A test the next year controlled sanding by screens, and produced 13,000m3.
It is possible – but far from certain – that there will be further tests at Mallik, though it is highly unlikely that it will be an early candidate for production on a serious scale. A gas pipeline from the Mackenzie Delta has been talked about for 40 years, and in the 1970s the Berger Commission ruled against it, on political grounds related to First Nations land claims. The project languished when the gas price drooped, was revived, and then languished again, in part because of the advent of shale gas in large quantities, but also because the price tag jumped from $9 billion to $16 billion. Yet another environmental impact assessment is under way.
Nankai promise
The Nankai Trough is much more promising and exciting.
Japan has a huge energy demand and little fossil fuel of its own. The trough lies parallel to the south coast of Honshu and Shikoku, at a distance of 100km or so, in 800-1400m deep water, well within the capability of the offshore industry. Much research is being done, both in Japan and elsewhere, on the thermodynamics and kinetics of hydrate dissociation, on the flow of fluids and heat within the formation, on well design, petroleum engineering questions, and on environmental impacts (such as seabed subsidence and a possibility that largescale submarine flowslides might be triggered).
Several groups have developed numerical models to examine the complex coupled thermal and geotechnical changes that accompany dissociation. Hydrate reservoirs are very different from conventional gas reservoirs, not least because the initial permeability is very low, because pore-filling hydrate blocks the flow paths between particles. The temperature changes can have harmful effects. Dissociation is an endothermic process that absorbs heat, and so if dissociation is to continue heat has to be supplied from somewhere, either by conduction and convection from the surrounding formation or by artificial supply from the wellbore.
Other groups are studying hydrates on a laboratory scale. Figure 2 shows burning hydrate formed in our laboratory at NUS. Figure 3 shows an experiment on formation and dissociation in an apparatus with cylindrical symmetry, which simulates both a production scheme and a possible system for in situ site investigation. A body of hydrate in sand is formed within a cooled pressure vessel, and dissociation is initiated by reducing the pressure on the axis. The diffusion of temperature, pressure and density change can then be observed by thermocouples, pressure transducers and gamma-ray transmission.
Figure 4 plots the temperature history at a radius four times the wellbore radius for a water- and gas-saturated sample. Temperatures in the gas-saturated sample drop rapidly due to Joule-Thomson cooling of the free gas; the endothermic effect is more obvious for the water saturated hydrate sample, for which the temperature declines by about 3°K over 90 minutes.
The Japanese plan is for a production test in 2012. The shortlisted sites are the a- and â fields in the eastern Nankai Trough, and a final decision will be made this year.
German researchers are looking into a synergistic link between gas production from hydrates and carbon dioxide capture and storage. Carbon dioxide displaces methane from methane hydrates, so that the gas is liberated and can be flowed to the surface, while the carbon dioxide stays behind as a carbon dioxide hydrate. An advantage of this option is that the replacement of methane by carbon dioxide is weakly exothermic, and that the heat generated can help to increase dissociation.
A possible problem is that the pores at an advancing carbon dioxide front become blocked with the new carbon dioxide hydrate, so that further flow is impeded.
Offshore production of gas from hydrates is not going to happen this year or next year. The general opinion is that serious production will begin in one or two decades, depending in part on what happens to the gas price, and on competing energy sources such as shale gas, coal bed methane and nuclear energy. OE
References
1. A Milkov. ‘Global estimates of hydrate-bound gas in marine sediments: How much is really out there?' Earth Science Reviews, 2004. 66(3-4): p183-197. 2. I Makogon. ‘Hydrates of hydrocarbons'. 1997, Pennwell Corp.
About the Authors
Andrew Palmer has divided his career equally between practice as a consulting engineer and university teaching. In 1975 he joined RJ Brown & Associates, and in 1985 he founded consulting engineers Andrew Palmer & Associates. In 1996 he returned to research and university teaching as research professor of petroleum engineering at Cambridge University. He is currently Keppel Professor at the National University of Singapore. Prof Palmer is the author of three books and more than 200 papers on pipelines, offshore engineering, geotechnics and ice.
Simon Falser is currently a research scholar at the National University of Singapore. He is researching different gas production methods from hydrate bearing sediments and their associated geomechanical behaviour. Prior to that, he worked with Schlumberger on the feasibility of a new concept of well intervention through a coiled tubing riser. Falser graduated with a MEng from NUS (2009) and Dipl-Ing from Innsbruck (2008).
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