Hydrates

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Hydrates are typically methane gas molecules trapped in ice-like crystals of water. The low temperature(<15°C) and high pressure(>5 MPa) conditions of stability for naturally occurring hydrates commonly exists in deep water just beneath the sea floor. The bottom-simulating reflector (BSR) is a reflection event that is closely associated with identifying hydrates in seismic cross-section. Identifying and analyzing hydrates is important. Drilling through hydrates can be challenging, and can cause drilling to be hazardous and cost more. Hydrates are also a potential energy source with 200 years worth of energy from just 15% of the world’s reserves. Gas hydrates are also of interest environmentally due to the possible release of greenhouse gas into the atmosphere where the gas hydrates might dissociate from the subsurface.[1][2]

A Brief History of Hydrates

Naturally-occuring gas hydrates were first discovered when anomalous high pressures restricted flow in natural gas pipelines.[3][4] A BSR (Bottom Simulating Reflector) is the first event from geophysical data that has been associated with identifying hydrates. This BSR exists on the Blake Outer Ridge along the eastern United States margin.[5] Discoveries with hydrate identification in the Blake Outer Ridge followed. Drilling into the BSR proved the existence of high methane contents, and further verified the association between BSR’s and hydrates.[6] Seismic velocities within the upper 600 meters were 2.0 to 2.2 km/s, and Stoll et al. (1971) suggested these anomalously high velocities to be a result of increased rigidity when methane is present in the sediment.[7] Seismic data also returned high amplitude responses there. It was later suggested that the high amplitude responses of the Blake Outer Ridge were caused by an accumulation of gas occurring at the base of the hydrated zone.[8] Tucholke et al. (1977) showed a strong correlation between the BSR and the pressure and temperature of the phase boundary in the methane + 3% NaCl + water system.[9] Shipley et al. (1979) then applied these studies to other continental slopes and rises around the globe to further support the identification of hydrates through geometric relations, reflection coefficients, reflection polarity, and pressure-temperature relations.[10]

Pressure and temperature conditions for Hydrate Stability.[11]

Hydrate Stability

Naturally occurring gas hydrates are stable for certain temperatures and pressures. The hydrate stability zone (HSZ) tends to occur in deep water just beneath the sea floor. This zone exists from the seafloor down to the base of gas hydrate stability, where temperatures and pressures are not ideal for hydrates. The base of gas hydrate stability (BGHS) depth depends on water depth, the seafloor temperature, and the geothermal gradient. The formation of hydrates requires high pressure (>5 MPa) and low temperature (<15°C). The figure to the left represents three stability curves. The methane + 3% MaCl + H2O stability curve most likely resembles that of naturally-occurring gas hydrates. It is evident that at increasing pressures, hydrates maintain stability with increasing temperatures.[12] This behavior directly correlates with the BGHS. Fortunately, the BGHS increases with increasing water depth. The increase in hydrostatic pressure and the decrease in water temperature allow the BGHS to go deeper to higher temperatures with higher pressure. A change of 100 m in water depth would increase the temperature of stability by 3°C and deepen the stability. For a thermal gradient of 4°C/m (Blake Outer Ridge), that would increase the depth of the BGHS by 5 m.[12] If the temperature or pressure changes, it could cause a dissociation and release a substantial amount of methane into the atmosphere. [13]

The bottom simulating reflector (BSR) is pointed out at the base of the hydrates. CREDIT: (Shipley et al. 1979)[12]

BSR

The bottom-simulating reflector(BSR) is the most common attribute used for gas hydrates. The BSR is an indication of a physical boundary between the upper gas-hydrate bearing sediments and the lower non-bearing sediments.[1] the BSR mimics the ocean floor at a depth to which the hydrates maintain stability. The depth of stability depends on the temperature and the pressure. It is because of the relationship between stability and temperature and pressure that we see a general decrease in BSR depth landward. One behavior of the BSR is that it cuts through strata, which makes it more easily identifiable for seismic interpreters. Once a BSR candidate is found, it can become a stronger candidate if the depth decreases landward. Further investigation could involve a quantitative analysis of the depth of stability; if it correlates well to the temperature and pressure.[1]

The BSR will have a polarity opposite to that of the ocean bottom reflector, and a hard reflection is likely to exist due to the sharp change in acoustic impedance. The sharp change in the acoustic impedance is from a dense concentration of hydrates at the base and fast velocities of about 2.0 to 2.2 km/s within the hydrate sediments with free gas beneath the BSR causing slow velocities. This hard reflection can also cause a polarity reversal. With a calculation of the reflection coefficient, one could determine the magnitude of the polarity reversal.[12][14]

Unfortunately, there is uncertainty associated with the BSR. There have been BSR's found with no hydrates present after drilling, and there have been hydrates present with no BSR's in seismic cross section. Attributes beyond the scope of the BSR can be used in identifying hydrates.[1]

Reflectance attribute used to identify free-gas and hydrates.[15]

Other Indications of Hydrates

Other than BSRs, there are other seismic attributes that correlate well with hydrates.

Reflection Strength

Sediments that underly the hydrate stability zone (HSZ) typically contain free gas. The free gas lowers the seismic velocities and have high reflection strength due to variations in gas saturation. Only a few percent of free-gas can cause high changes in impedance (Bright Spot).[14][15]

Instantaneous Frequency

Presence of free gas is also indicated by shadows in the instantaneous frequency plot. High reflectivity and low frequency 'shadows' over a large time window indicate a thick sequence of strata altering between gas-rich and gas-poor beds. The low frequency response is due to high attenuation from the free-gas within the pores.[15]

Seismic 'blanking'

Within the HSZ, gas hydrates cement the layers and decrease the seismic amplitudes. The seismic amplitudes are decreased because the layers are homogenized to reduce the acoustic impedance contrast between layers. Seismic 'blanking' can be observed through the reflectance attribute. Low reflectance resembles 'blanking'. The reflectance attribute is a method for analyzing hydrates and quantifying the thickness of the HSZ.[15]

External Links

References

  1. ↑ 1.0 1.1 1.2 1.3 Ojha, M., & Sain, K. 2009. Seismic attributes for identifying gas-hydrates and free-gas zones: application to the Makran accretionary prism. 32, 264-270.
  2. ↑ Kroeger, K. F., Crutchley, G. J., Hill, M. G., Pecher, I. A., 2017. Potential for gas hydrate formation at the northwest New Zealand shelf margin – New insights from seismic reflection data and petroleum systems modeling. Marine and Petroleum Geology, 83, 215-230. 
  3. ↑ Deaton, W. M., and E. M.Frost, Jr., 1946, Gas hydrates and their relation to the operation of natural gas pipe lines: U.S. Bur. Mines Mon. 8, 101
  4. ↑ Hammerschmidt, E. G., 1940, Elimination of hydrate troubles: Oil and Gas Journal, 39, 61-68.
  5. ↑ Markl, R. G., G. M. Bryan, and J. I. Ewing, 1970, Structure of the Blake-Bahama Outer Ridge: Jour. Geophys. Research, 75, 4539-4555.    
  6. ↑ Hollister, C. D., et al, 1972, Initial reports of the Deep Sea Drilling Project, v. II: Washington, D.C., U.S. Govt. Printing Office, 1077
  7. ↑ Stoll, R. D., 1974, Effects of gas hydrates in sediments, in Natural gases in marine sediments: Marine Sci., 3, 235-248.
  8. ↑ Bryan, G. M., 1974, In situ indications of gas hydrate, in Natural gases in marine sediments: Marine Sci., 3, 299-308. 
  9. ↑ Tucholke, B. E., G. M. Bryan, and J. I. Ewing, 1977, Gas-hydrate horizons detected in seismic-profiler data from the western North Atlantic: AAPG Bull., 61, 698-707. 
  10. ↑ Shipley, T. H., M. H. Houston, R. T. Buffler, F. J. Shaub, K. J. McMillen, J. W. Ladd, & J. L. Worzel, 1979. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG bulletin, 12, 2204-2213.
  11. ↑ Shipley, T. H., M. H. Houston, R. T. Buffler, F. J. Shaub, K. J. McMillen, J. W. Ladd, & J. L. Worzel, 1979. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG bulletin, 12, 2204-2213.
  12. ↑ 12.0 12.1 12.2 12.3 Shipley, T. H., Houston, M. H., Buffler, R. T., Shaub, F. J., McMillen, K. J., Ladd, J. W., & Worzel, J. L., 1979. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG bulletin, 12, 2204-2213.
  13. ↑ Tinivella, U., & Giustiniani, M. 2013. Variations in BSR depth due to gas hydrate stability versus pore pressure. Global and Planetary Change, 100, 119-128.
  14. ↑ 14.0 14.1 Taylor, M.H., Dillon, W.P., and Petcher, I.A., 2000, Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data. Marine Geology, 164, 79-89.
  15. ↑ 15.0 15.1 15.2 15.3 Ojha, M., & Sain, K. 2009. Seismic attributes for identifying gas-hydrates and free-gas zones: application to the Makran accretionary prism. 32, 264-270.
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