Ingo Pecher
Pending projects and future initiatives
Last updated: 9 Jan. 2008
Proposal to NERC: The Upper Edge of Gas Hydrate Stability and its Role in Seafloor Erosion and Methane Release: Laboratory Studies, Modelling and Oceanographic Analyses
(Ingo A. Pecher, Bahman Tohidi, Gary Couples, Ross Anderson, Melissa Bowen, and Jinhai Yang, Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, UK)
Gas hydrates are ice-like crystalline solids composed of water and gases, most commonly methane. Methane hydrate forms under low temperatures and moderate pressures and is commonly found in the first 10s to 100s of meters beneath the seafloor of deep ocean basins. The dissociation, or melting, of solid hydrate to water and gas poses a major geohazard because it may lead to tsunamogenic submarine slides. The release of methane, which is a potent greenhouse gas, from melting hydrates is also thought to have played a critical role in the fluctuations of Earth's climate and may do so again in the future.
Most research into these topics has focussed on the base of gas hydrate stability, which is often located hundreds of meters beneath the seafloor. At these depths, overpressured gas from melting hydrates may trigger submarine slides and methane release. It is now becoming evident that gas hydrate deposits near the seafloor at water depths close to the top of gas hydrate stability (TGHS), the upper edge of hydrate stability in the ocean, are most susceptible to environmental changes. An increase of water temperature pushes the level of the TGHS downward because higher pressure is required for gas hydrate to be stable. This may cause vast areas of the seafloor to cross the boundary of hydrate stability; any gas hydrate close to the seafloor will melt.
We have discovered the probably most direct evidence to-date that links gas hydrates to seafloor weakening, occurring at the TGHS. Seismic and oceanographic data show that seafloor ridges offshore New Zealand are eroded at water depths were temperatures repeatedly cross the boundary for hydrate stability. We hypothesise that similar to frost heave, freeze-thaw cycles of gas hydrates may weaken the seafloor. These cycles may also control the often episodic release of methane at these depths, which we need to understand to assess its potential effect on the Earth’s climate.
Most studies into the link between gas hydrates and sediment strength have focussed on overpressure from gas released during hydrate melting. Pore-scale capillary forces have been largely ignored. Similar to water ice, the growth of hydrate crystals in tight spaces, such as pores or cracks, exerts significant stress on the sediment frame. For ice, it is well established that this stress leads to cracking of even hard material like concrete.
We propose to study in the laboratory how hydrate freeze-thaw cycles damage sediments. Based on samples from the study area, we will select synthetic rocks, form and dissociate hydrate under varying conditions, and investigate microscopic damage to the rock fabric. Results will be used for modelling pore-scale hydrate formation and compare capillary forces to gas pressure. Several of these experiments will be conducted in an ultrasonic cell to calibrate seismic models and a triaxial cell for geotechnical models to guide possible future field experiments.
We also propose an oceanographic analysis to investigate the global significance of this process by predicting the depth of the TGHS using water temperatures from the World Ocean Database. We will investigate where this depth coincides with known gas hydrate deposits and search for geologic data evidence for seafloor weakening.
We then plan sensitivity analyses to study where hydrates close to the TGHS are most susceptible to water temperature changes. Results will form an invaluable basis for possible future studies linking ocean and hydrate models to predict the response of the gas hydrates to ocean warming.
Our proposed project brings together experts in gas hydrates, mudrocks, and observational oceanography. It is part of a broader initiative to study the significance of the upper edge of gas hydrate stability for seafloor hazards and the Earth’s climate. Discussions are currently under way to conduct a new international field campaign in our study area perhaps as early as 2009.
Proposal to EPSRC: Ultrasonic Properties of Sediments During Gas Hydrate Growth and Dissociation - Laboratory Studies and Rock Physics Modelling
(Ingo Pecher, Bahman Tohidi, Ross Anderson, Jinhai Yang, and Colin MacBeth, Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS)
Gas hydrates are ice-like crystalline solids composed of water and gases, most commonly methane. Methane hydrate forms under low temperatures and moderate pressures and is commonly found in the first 10s to 100s of meters beneath the seafloor of deep ocean basins. The dissociation, or melting, of solid hydrate to gas and water can weaken the stability of sediments and poses a major hazard to seafloor installations such as subsea pipelines, cables and wellbores. Gas hydrate dissociation, whether caused by direct human activities, such as the pumping of warm oil through hydrate-bearing sediments, or environmental variations, such as ocean warming, is also of major significance for geoscientists because weakening of the seafloor may lead to tsunamogenic submarine slides. The release of methane, which is a potent greenhouse gas, from melting gas hydrates is thought to have played a critical role in the fluctuations of Earth's climate and may do so again in the future.
One of the best ways of detecting melting gas hydrates is through seismic methods similar to those for oil and gas exploration: sound waves are generated in the water using compressed air and recorded with hydrophones. The gas from melting hydrates causes strong reflections of elastic waves that can be identified in the seismic records. Additional information about the stiffness of the sediment, which is likely to decrease during gas hydrate dissociation, can be extracted from the speed at which the sound waves travel.
It is becoming increasingly obvious that in many settings a mixture of gas and hydrate may coexist during hydrate melting and formation. In recent years, considerable progress has been made in determining the elastic properties of gas-hydrate-bearing sediments; at the Centre for Gas Hydrates Research, Heriot-Watt University, we now routinely measure elastic waves through hydrate-bearing sediments at the ultrasonic frequencies suitable for laboratory measurements. These results, after adjustment from ultrasonic to seismic frequencies, provide us with an increasing knowledge base for predicting the seismic properties of hydrate-bearing sediments for as long as they are gas free. We now propose the first-ever comprehensive study of the elastic properties of sediments that contain coexisting gas and hydrate.
We propose laboratory measurements of the elastic properties during the formation and dissociation of gas hydrates in sand-dominated synthetic and natural sediments at the typical low to intermediate hydrate saturations found in nature. These ultrasonic studies will be accompanied by a series of experiments in micromodels, glass plates that are etched to simulate the sediment pore space. Micromodels allow pore-scale visual observation of gas hydrate formation and dissociation which will enable us to observe the fundamental processes behind coexistence of gas and hydrates. Our laboratory results will be used for modifying elastic rock-physics models of hydrate-bearing sediments to predict the seismic signature of coexisting hydrate and gas. These models, calibrated at ultrasonic frequencies, will then allow an extension of our results to field-seismic frequencies. Based on these models, we plan initial modelling of the propagation of field-seismic waves across melting gas hydrates to investigate if we could detect hydrate melting in seismic surveys.
Our proposed study will already significantly improve methods for predicting the seismic properties of sediments with gas and hydrate during hydrate growth and dissociation for a range of sediment types and hydrate saturations. The results of this project will be the first building block for new comprehensive tools for modelling the seismic properties of sediments containing coexisting gas and hydrates e.g., to predict the seismic response to gas hydrate melting.
Future initiative: Dissociation of offshore permafrost hydrates
Dissociation of permafrost gas hydrates is probably of most immediate concern for climate change related to hydrates because they may release methane almost directly into the atmosphere. However, self-preservation at temperatures below freezing appears to slow gas hydrate dissociation significantly: Dissociating, meta-stable hydrates have been discovered in Siberia, both on- and offshore (Yakushev and Chuvilin, 2000; Yakushev, 2004), and probably the North-American Arctic shelf (Paull et al., 2007). These shallow hydrate deposits may have resided outside the gas hydrate stability field since soon after the end of the last ice age due to warming of bottom waters and/or retreat of glaciers. Offshore permafrost hydrates are of particular interest because their dissociation may contribute to the generally high concentrations of methane in the Arctic Ocean (Shakhova et al., 2005). The Arctic shelf may also be an ideal natural laboratory for studying the impact of possible future flooding of onshore hydrate deposits after sea level rise. We now need to study the extent and effect of the dissociation of offshore permafrost hydrates in order (1) to constrain their current contribution to methane and CO2 release into the ocean and atmosphere and (2) to predict future feedback from flooded permafrost hydrates to global warming. Seismic techniques should be well suited to zoom in on possible candidate sites for more detailed studies by identifying shallow gas-pockets. Geochemical sampling would then allow determining if the gas originates from dissociating hydrates. We are currently in discussion with collaborators at the US Naval Research Laboratory and the Renard Centre for Marine Geology in Belgium about proposing joint field and laboratory studies of offshore permafrost hydrates.
Future initiative: Analysis of 3-D industry data for gas hydrate occurrence
(potential GeoSEAD MSc Individual Project, Ingo Pecher, Heriot-Watt University, John Underhill, University of Edinburgh)
Modern 3-D data acquired by industry for deeper hydrocarbon reservoirs are proving excellently suited for the study of gas hydrates. We are planning to request data from the exploration industry for an analysis of potential gas hydrate occurrences. It is initially planned to offer such a project to one or two GeoSEAD students as an Individual Project. Upon success, we are planning a larger-scale initiative capitalizing on the excellent links of the ECOSSE partners to industry for a comprehensive study of gas hydrate deposits based on 3-D data.
References:
Paull, C. K., W. I. Ussler, S. R. Dallimore, S. M. Blasco, T. D. Lorenson, H. Melling, B. E. Medioli, F. M. Nixon, and F. A. McLaughlin, 2007, Origin of pingo-like features on the Beaufort Sea shelf and their possible relationship to decomposing methane gas hydrates: Geophys. Res. Lett., 34, L01603.
Shakhova, N., I. Semiletov, and G. Panteleev, 2005, The distribution of methane on the Siberian Arctic shelves: implications for the marine methane cycle: Geophys. Res. Lett., 32, L09601.
Yakushev, V., 2004, Intrapermafrost gas hydrates at the north of west siberia: in Proc. AAPG Hedberg Conference, Vancouver, B.C., 4 pp.
Yakushev, V. S., and E. M. Chuvilin, 2000, Natural gas and gas hydrate accumulations within permafrost in Russia: Cold Reg. Sci. Tech., 31, 189-197.