GEO ExPro

Gas Hydrates - Part II: Rock Physics, an Introduction

Most oceanic gas hydrates are mapped using seismic data. How much do key seismic parameters vary from a hydrate rock to a water-saturated rock? Should the hydrate be considered as part of pore fluid fill or a part of the rock itself? To answer such questions, we need to understand the rock physics of sediments containing gas hydrates.
This article appeared in Vol. 9, No. 4 - 2012

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Source: GEOMAR/Helmholtz Centre for Ocean Research Kiel
“The world is the geologist’s great puzzle-box; he stands before it like the child to whom the separate pieces of his puzzle remain a mystery till he detects their relation and sees where they fit, and then his fragments grow at once into a connected picture beneath his hand.” - Jean Louis Rodolphe Agassiz (1807–1873), Swiss paleontologist, glaciologist, geologist

A major and obvious difference between ice and hydrate is that the hydrate is highly flammable (‘burning ice’).

It is of course the methane content in hydrate that makes it of commercial interest. In fact the difference between ice and methane hydrate is not huge: one could consider hydrate as one dash of methane gas and six dashes of pure ice. Ice has a hexagonal crystalline structure, with a density of roughly 0.92 g/cm3 at 0°C and increases to 0.93 for a temperature of -180°C. The density of methane hydrate is slightly less (0.90 g/ cm3), since there is only 1 mole of methane per 5.75 moles of ice. Another way to formulate this is to say that whereas ice has the formula H2O, the empirical chemical formula for methane hydrate is (CH4)8(H2O)46.

So, if the density difference between ice and hydrate is negligible, what about other crucial seismic parameters, such as velocities?

Rock Physics Experiments

In 2001 Michael Helgerud presented a comprehensive PhD thesis at Stanford where he analyzed wave velocities in gas hydrates and sediments containing gas hydrates.

For pure gas hydrate samples made in the laboratory he measured P-wave (compressional) velocities around 3700 m/s and S-wave (shear) velocities around 1950 m/s. Only small variations with temperature and confining pressure were observed.

The ratio between P-wave velocity of pure hydrate versus ice is 0.98. For the S-wave velocity the corresponding ratio is close to 1.0. This means that, acoustically, pure ice and pure methane hydrate are very similar. This is maybe as expected since the dominant crystalline structures of the two are similar.

  • Compressional and shear velocity versus time. The figure shows the response to a warming of the sample from 5 to 20°C followed by a cooling back to 5°C again. Confining pressure: 9,000 psi. Source: Helgerud (2001) Compressional and shear wave velocities versus confining pressure for methane hydrate. The horizontal scale is from 4,000 to 9,000 psi. Source: Helgerud (2001)

  • Compressional and shear velocity versus time. The figure shows the response to a warming of the sample from 5 to 20°C followed by a cooling back to 5°C again. Confining pressure: 9,000 psi. Source: Helgerud (2001) Compressional and shear wave velocities versus confining pressure for methane hydrate. The horizontal scale is from 4,000 to 9,000 psi. Source: Helgerud (2001)

  • Compressional and shear velocity versus porosity for ice. Source: Helgerud (2001) P-wave velocity versus porosity assuming that the methane hydrate is a part of the rock frame (black line), of the pore fluid (green line). The red line shows a rock which is 100% water saturated (no hydrate), the blue solid line represents 1% patch gas saturation, and the dark blue line represents 1% homogenous gas saturation. Source: Helgerud (2001)

  • Compressional and shear velocity versus porosity for ice. Source: Helgerud (2001) P-wave velocity versus porosity assuming that the methane hydrate is a part of the rock frame (black line), of the pore fluid (green line). The red line shows a rock which is 100% water saturated (no hydrate), the blue solid line represents 1% patch gas saturation, and the dark blue line represents 1% homogenous gas saturation. Source: Helgerud (2001)

Fluid, Frame or Cement?

Is hydrate a part of the fluid, the frame or is it cement?

We have seen that pure hydrate and ice are very similar with respect to traditional seismic parameters, such as velocities and densities. However, in nature, the hydrate is found as a part of a rock, making the rock physics relations more complex. What happens when methane hydrate enters into a sedimentary rock? Hydrate is found both in clay rich sediments and in sands. Several models have been proposed for this, varying from regarding the hydrate as a part of pore fluid fill, via being a part of the rock frame or acting as cement between sand grains. Helgerud found that the P-wave velocity is slightly higher when assuming that the hydrate is a part of the rock frame.

There are two ways of addressing this problem: either to perform measurements in wells drilled into hydrate-bearing rocks, or to inject hydrate in a controlled way into a rock in the laboratory. We will discuss both methods in the next issue of GEO ExPro.

This article is Part II of a four part series:



References:

  • Dewar, J., 2001. Rock Physics For The Rest Of Us – An Informal Discussion: CSEG Recorder.
  • Gassmann, F., 1951. Uber die elastizitat poroser medien. Vierteljahrsschrift der Naturforchenden Gesellschaft in Zurich 96, 1-23.
  • Helgerud, M., 2001. Wave speeds in gas hydrate and sediments containing gas hydrate: A laboratory and modeling study, PhD thesis, Stanford University, USA.
  • Wang, Z., 2000. The Gassmann equation revisited: Comparing laboratory data with Gassmann’s predictions, Seismic and Acoustic Velocities in Reservoir Rocks, Vol. 3, Recent Developments, SEG Reprint Series, pp.1–23.


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