7 Non-conventional sources of petroleum
7.2 Gas hydrates
The temperature and pressure of the deep oceans are controlled, respectively, by deep, cold currents that move from polar latitudes along the sea floor and by the mass of the overlying water column. Consequently, at depths exceeding 300–500 m the sea floor is at a temperature of around 1–2 °C and a pressure that is several hundred times greater than atmospheric pressure. Under these physical conditions, gases such as methane (CH4) and carbon dioxide (CO2) can combine with water to form solid, ice-like crystalline compounds known as gas hydrates (see Section 5.2.2, Figure 14). Clearly, economically interesting gas hydrates are those which contain proportionally far more hydrocarbon gases in their structure than CO2. Depending on the geothermal gradient, the base of the hydrate stability zone may extend to depths of more than 1000 m beneath the ocean floor.
Hydrocarbon gas hydrates can form in deep ocean water but they are not found as a carpet on the sea floor. This is because such hydrates have a lower density than that of seawater (850 kg m−3 compared with 1025 kg m−3). As soon as they form, they float upwards and turn back into methane and water in the lower pressures and warmer temperatures of the upper layers of the ocean.
However, within the sediments just beneath the sea floor, crystals of hydrocarbon gas hydrate form and move upwards buoyantly. Frequently the rising crystals form a ‘log-jam’ within the pore space of the sediment and once this occurs, more gas hydrate crystals become trapped in the pore spaces beneath. Eventually, all available pore spaces in the sediment become completely filled by gas hydrate crystals that readily absorb methane into their lattice. Fully saturated gas hydrates can hold up to 200 times their own volume of methane, creating a zone that is denser than seawater and thus gravitationally stable.
The gas required for formation of gas hydrates comes from two principal sources: biogenic and thermogenic. Biogenic gases are those produced in situ by bacterial breakdown of organic matter contained within the sea-floor sediment. The dominant biogenic gas is CH4 (>99%) with traces of CO2 and H2S. Such gases typically form in oceanic areas that have relatively high rates of sedimentation and plenty of organic matter, such as the coastal margins of North America and the North Pacific Ocean. In contrast, thermogenic gases are those produced by the maturation of kerogen at much greater depths and elevated temperatures, as described in Section 2. Thermogenic gas hydrates contain significant amounts of ethane, propane and butane, and they occur in petroleum-rich provinces, such as the Gulf of Mexico and Caspian Sea, where leakage to the surface is common.
Gas hydrates do not form only in the oceans, but also in deep lake sediments and onshore permafrost zones across Arctic Canada and Russia. Current estimates suggest that gas hydrates globally may contain 1–5 × 1015 m3 of methane, a figure that dwarfs the remaining proven reserves of conventional gas. But there are some issues of both concern and potential, summarised by Figure 21..
Figure 21: Illustration of the major issues concerning gas hydrates in sea-floor sediments. (Left) Earthquakes trigger gas-hydrate instability that in turn triggers massive slumping of sea-floor sediments and tsunamis. Installing large structures on the sea bed might result in rapid release of gas and instability of their foundations. Any release of methane promotes global warming. (Right) The huge potential for developing gas hydrates as resources – they are readily discovered by their distinct ‘signatures’ on seismic sections.
On the positive side, the potential of methane hydrates as a major strategic energy reserve is obvious and much research is being conducted to develop appropriate extraction techniques. This extends to considering whether methane production could be combined with CO2 disposal, thus addressing the twin challenges of this century – reducing the emissions of greenhouse gases and providing a low-carbon fuel to replace oil and coal.
On the negative side, it is now recognised that gas hydrates are a potential geohazard. Dissociation of hydrates at the base of the gas hydrate stability zone (Figure 21) can cause increased pore-fluid pressures in under-consolidated sediments, forming a zone of weakness and a site of potential sea-floor failure. Slope failure can threaten underwater installations and, in extreme cases, generate tsunamis. It has even been suggested that during periods of climatic warming such as we are experiencing at present, onshore hydrates become destabilised, liberate methane to the atmosphere and thus accelerate global warming.