Geophysical Aspects Of The Hydrate Zones

The chemistry of methane hydrates is well described in this volume. Geophysically the impact of hydrate formation is that it strengthens the sediment. If this occurs prior to sedimentary lithification, as it often does, then removal of this rigidity on dissociation can affect sediment stability, particularly in the presence of external disturbance. Therefore the main geophysical effect of hydrate formation derives from the rigidity given to the sediment and this is a consequence of the change in physical properties. Both compressional wave velocity (Vp) and shear wave velocity (Vs) depend on the physical properties of the sediment through which they propagate, therefore if:

K is the sediment bulk modulus - the force required to change volume, |i is the shear modulus - the force required to change shape, and p is the density, then

This shows that Vp > Vs and that the existence of p. not only determines Vp but is necessary for Vs to exist.

Vp will increase from ~1.6 km s"1 in normal sediment up to 2.5 km s"1 or more in the presence of hydrate. Vs can rise by orders of magnitude and dramatically affects the acoustic properties of the sediment, particularly its ability to transmit shear waves.

The base of the Hydrate Stability Zone (HSZ) shown in figure 2 is bounded by Vp > 1.6 km s"1 within the hydrate and Vp < 1.6km s"1 below, particularly if there is free methane in the pore spaces. This causes a negative acoustic impedance contrast at the BSR that will reflect seismic energy as an inverse wavelet compared to that of the seafloor and most other reflections. This characteristic and the blanking enables seismic profiling to identify the presence of hydrate and the reflector at the base of a hydrate layer as a BSR.

Below about 450 meters water depth, the temperature - pressure conditions in the sediment are appropriate for hydrate formation, as they are in most ocean basins (Miles, 1995). The geochemical conditions for methane production however are most commonly met along continental margins rather than in the deep oceans and these are well documented in other chapters.

The Hydrate Economic Zone (HEZ, figure 1) is the combined hydrate, methane, and deeper sediment zone from which it is important to characterize methane and the geotechnical properties that bear on the methane recovery (Max and Chandra, 1998). It includes the HSZ and deeper methane and pore fluid zones that are methane rich. In an area where sedimentation has continued over a long period of time, hydrate at the base of the HSZ may become unstable through rising geotherms and dissociate. Methane produced in or below sediment from dissociated hydrate will rise through buoyancy into the HSZ and tend to again form hydrate. The actual process will depend on the local sedimentology and is still a subject of discussion. However this is the zone in which the geotechnical parameters must be extremely well known if safe extraction of methane from methane reservoirs is to be engineered. It is presently estimated that this economic zone is no more than 1.5 to 2 times the thickness of the HSZ. Below this zone, where sediment compaction and

Otfshofe distance flim)

Fig. 2. Hydrate stability P-T phase boundary (dashed) applied to sea temperature and geothermal gradient (solid) offshore Galvaston TX. The calculated thickness of HSZ (Miles, 1995) is the potential maximum depth to that methane hydrate can remain stable, assuming sufficient sediment thickness.

geotechnical properties are more normal, it is likely that no geophysical data need be recovered for economic exploitation of the HEZ.

World-wide the scale and distribution of hydrate deposits appears comparable and controlled by similar circumstances. The sediments in which hydrates form do not appear to exert a significant influence over their nucleation or growth but this has not been quantified. Thus for any geophysical analysis of hydrate the broadest range of sediment types must be anticipated.

3. GEOPHYSICAL SURVEY OBJECTIVES

Concentrated and dispersed hydrate deposits occur in different locations within the seafloor sediments. Hydrate in the lower part of the HSZ could comprise the energy resource potential and create the primary survey interest. This is currently driving research and development interest in both India and Japan. These deposits are near the base of the HSZ where the hydrate will be most concentrated. From a safety aspect, dispersed hydrate at or near the seafloor is seen as a hazard to sediment stability and any engineering operations sited on it. The stability of the seafloor on a geological scale however is important in shaping the continental margin. The redistribution of sediments away from continental margins by sediment mass wasting (Dillon et al., 1995) and the possible formation of Tsunami associated with these events highlight the potential environmental impact. Therefore geophysical surveys will be directed toward the resource, safety or environmental issues, each requiring specific tools determined by the depth of penetration or aerial coverage required.

In order to acquire the basic information required from the HEZ it is necessary to undertake both regional and detailed geophysical surveys that may need to be calibrated by ground-truth sampling. This involves an integrated survey and analysis strategy to bring together the morphology, geology, sedimentology and physical properties of the HEZ.

One of the primary issues for hydrate research is the use and interpretation of acoustic energy to determine the disposition and volume of hydrate and methane in oceanic marine sediments. With a more detailed understanding of the distribution of hydrate and associated methane deposits, a more realistic assessment of the basic process of marine sedimentation and hydrate diagenesis can be made.

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