Page 626 - Economia Azul - versão: inglês

Gas hydrates occur in permafrost re-
gions (permanently frozen soils of polar
regions), and as a result of the diagene-
sis of sediments from lakes and deep seas
(> 500 m water-blade), when appropriate
conditions of pressure, temperature and
saturation of gas are found. The physi-
cal-chemical equilibrium of these struc-
tures can occur in the water column, at
the water-sediment interface and in the
sediment pores, characterizing the gas
hydrate stability zone (GHSZ – Figure X2).
The catastrophic release of methane by
the destabilization of such deposits due to
the global climate changes that have been
observed can present negative feedback in
these processes, accelerating such chang-
es, as the instantaneous release of these
gases increases the so-called greenhouse
effect. Additionally, the destabilization
of these deposits poses a geological risk,
due to their potential to cause landslides,
earthquakes and tsunamis.
The origin of hydrocarbons for the for-
mation of hydrates can be biogenic, result-
ing from the action of methanogenic mi-
croorganisms in sediments, thermogenic,
associated with the migration of gas from
deep hydrocarbon fields, or mixed. The
most common gases that form hydrates are
methane, butane, propane, and carbon di-
oxide (MILLER et al., 2015).
Even though it is a little-known sub-
stance, methane hydrates are abundant in
the world’s oceans. Conservative estimates
indicate that the energy resources associ-
ated with gas hydrates can exceed all oth-
er fossil fuels added together (COLLET et
al., 2009). Based on this enormous poten-
tial, it is expected that gas hydrates will
624 BLUE ECONOMY
progressively replace natural gas (GRAULS, estimates based on these seismic data have
Figure X2: BSR (Bottom- 2001), which could foster a new ener- sparked interest in the area (estimated at
Simulating Reflector) – reflection gy revolution. According to Kvenvolden 780 tcf) (SAD et al. 1998).
approximately parallel to seafloor (1993), 1 m³ of methane gas hydrate con- The confirmation of the existence of
reflection, with reversed polarity, tains 164 m³ of methane and 0.8 m³ of gas hydrates in Brazilian territory (Pelotas
presumably caused by the water when brought to the surface (STP). Basin) only occurred with the develop-
contrast between overlapping This number he calls the hydrate expansion ment of the “CONEGAS Project: Origin,
sediments containing gas hydrate factor. On the other hand, Sloan (2003) occurrence and characterization of gas
and underlying sediments states that if each cavity is filled with a gas hydrate deposits in the Rio Grande Cone,
saturated with free gas, due
to the difference in densities. molecule, 1 m³ of hydrate contains 180 m³ Pelotas Basin”, (MILLER et al., 2015). Lat-
Indicates the lower limit of the (Surface-Treating Pressure, STP) of meth- er, in 2015, an oceanic mission to the Foz
gas hydrate stability zone, the ane. The conclusion is that hydrate-bearing do Amazonas basin confirmed the occur-
GHSZ, where hydrocarbons are reservoirs contain more methane per m³ rence of this resource in the area (KETZER
trapped in a solid structure. when compared to the volume of free gas et al, 2018). Such projects were devel-
in the same space (KVENVOLDEN, 1993). oped by the team from the Institute of
In Brazil, detailed studies on gas hy- Petroleum and Natural Resources at PU-
drates are still incipient, despite their enor- CRS over a decade. These discoveries in
mous potential given the large area of Bra- Brazilian territory have raised the need
zilian ocean basins. The existence of gas for additional research to promote the
hydrate deposits in the country was, until understanding of its forms of occurrence
the beginning of this century, only based and the future use of this resource in oth-
on evidence or indirect markers, such as er sedimentary basins, in addition to the
the presence of the so-called BSR (Bottom known ones, the Pelotas and Foz do Am-
Simulating Reflector) in seismic sections azonas basins (Figure X3).
(FONTANA, 1989; FONTANA; MUSSUME- In addition to their importance as an en-
CI, 1994; SAD et al., 1997 and 1998). This ergy resource, studies of gas hydrates also
marker consists of a seismic reflector par- provide important information about the
allel to the seafloor (Figure X2), normally Earth’s carbon cycle and climate change.
between 200 and 700 m depth, caused Small changes in ocean water temperature
by the abrupt change in acoustic imped- conditions can, for example, cause the dis-
ance at the base of the gas hydrate stabil- sociation and release of large amounts of
ity zone (GHSZ) in the sedimentary mass, methane and carbon dioxide into the at-
depending on the existence of a zone with mosphere. The massive destabilization of
gas hydrates above and a zone with free the gas hydrate system in a given region
gas below this reflector in the sediments can also trigger large mass wasting events
(MACKAY et al., 1994). In the Rio Grande with potential impact on subsea instal-
Cone (RGC), in a thick sedimentary se- lations, such as oil platforms and subsea
quence deposited during the Neogene, the communication cables, or even major ca-
Source: Authors elaboration, (2022) presence of BSR was continuously identi- tastrophes in coastal areas such as of tsu-
fied in seismic surveys. Significant volume namis (MIENERT et al., 2010).
State of the Art of Gas Hydrate Occurrences 625