Fire from Ice: Gas Production from Methane Hydrates
Prof AC Palmer (Centre for Offshore Research and Engineering, Department of Civil and Environmental Engineering)
Hydrates are snow-like solid compounds of water and hydrocarbons, stable at low temperatures and high pressures. A useful way to remember where the stability boundary is located is that at a pressure of 4 MPa methane hydrate is stable if the temperature is lower than 4ºC. A cage of water molecules traps each methane molecule, and dissociating 1m3 of solid hydrate turns it into 164 m3 of gas and 0.9 m3 of water. Hydrates 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 2×1015 m3 of methane are locked up in hydrates, which corresponds to more than 800 years of gas production at the current rate.
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. We are working in a programme led by Professor Tan Thiam Soon, Vice-Provost (Education) and a Professor in Civil and Environmental Engineering. It has two objectives: to find good ways of producing gas from hydrate fields, and to find how to investigate their properties by in-situ tests. We are collaborating with Cambridge and Southampton universities in the UK
Trials of gas production have been carried out at two onshore hydrate fields, Messoyakh in Siberia and Mallik in the Mackenzie Delta, north of Inuvik in Arctic Canada. However, the most promising and exciting prospect is the Nankai Trough, which lies parallel to the south coasts of Honshū and Shikoku in Japan, at a distance of 100 km and in 800 to 1400 m 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 impact. Our research combines experiments and numerical simulation, and Figure 1 from our laboratory at NUS 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.
One of our conclusions has a significant impact on planning for production. Depressurisation is the simplest and most obvious way to dissociate, but the results are disappointing, because the dissociation process absorbs heat and that cools down the hydrate and brings dissociation almost to a halt. A much better option is to combine depressurisation with downhole heating. That is energy-efficient, and the heat that needs to be supplied is small by comparison with the energy liberated.
A November 2010 symposium in Tokyo and a July 2011 conference in Edinburgh provided an opportunity to compare what we are doing with what others are doing. We concluded that NUS is well-placed to make a significant contribution, because we do experiments on real hydrates, whereas most researchers just do computations.
This work was done in collaboration with Research Scholar Simon Falser and Research Engineer Matilda Loh (CORE, Department of Civil and Environmental Engineering.