lOS Note
~
4
OBSERVATIONS OF THE FORMATION OF
HYDROCARBON GAS HYDRATES
AT DEPTH IN 'SEAWATER
by
C.R. Topham
..
I
INSTITUTE OF OCEAN SCIENCES, PATRICIA BAY
Victoria, B.C.
IDS Note - 4
OBSERVATIONS OF THE FORMATION OF
HYDROCARBON GAS HYDRATES AT DEPTH IN SEAWATER
by
D.R. Topham
;
Institute dE Ocean Sciences, Patricia Bay
Sidney, B.C.
January 1978
This is a manuscript which has received only
limited circulation.
~
On citing this report in
bibliography, the title should be followed
by the words "UNPUBLISHED MANUSCRIPT" which is
in accordance with accepted bibliographic custom.
Observations of the Formation of
Hydrocarbon Gas Hydrates at Depth in Seawater
Introduction
The offshore explorations for oil and gas in Arctic waters has raised
the question as to the probable environmental effects of a well blowout.
A
near full scale experiment was carried out [lJ to investigate the gas/oil plume
arising from a blowout at depths to 200 m, those proposed for exploration in the
Beaufort Sea area of Canada.
The extension of exploration to offshore sites with water depths of up
to 1000 m requires that the conclusions reached for the shallower depths be reassessed in the light of the new conditions.
The most important factor to be
considered is the increase ;rr hydrostatic pressure at the sea bed which leads to
the possibility of the formation of gas hydrates solids.
It is conceivable that
a major portion of the jas released at depth v/ould form hydrate and thus the
strongly rising bubble plume of the shallow blowout would not exist, leaving the
oil free to rise under the action of its own buoyancy force.
This much less
v;gor9us plume of oil droplets could be carried considerable distances by currents
before reaching the surface.
Under conditions of thermodynamic equilibrium the existence of gas
hydrates is a function of gas composition, temperature and pressure and the
relevant phase diagrams can be determined.
Sze and Adams [2J (unpublished
manuscript) discuss hydrate formation and Figure 1 shows pressure-temperature
boundaries for various gases taken from their work.
Although the equilibrium
behaviour of gas hydrate is well understood, comparatively little is known of
the kinetics of formation. It appears that once formation conditions have been
attained some form of mixing is required to remove latent heat before significant
quantities of hydrate can form.
- 2 -
Undersea Gas Release Experiment
In the case of gas released at depth in the sea, the vertical motion of
the bubbles provides a vigorous mixing action and hydrate formation should be
rap; dly promoted.
To investi gate the pass ibil ity of hydrate forma t.i on under such
conditions a quantity of natural gas was released at sea depths of up to 650 m.
Observations of the subsequent bubble behaviour showed that hydrates formed at
the gas/water interfaces within a distance of 2 metres of the point of gas release.
These experiments are described in greater detail in the following paragraphs.
The gas was released from a nozzle suspended beneath the Pisces IV
submersible and the resulting bubble plume photographed through the observation
windows (Figure 2).
The cameras used were a pair of Star II Robot 35 mm cameras,
solenoid operated to fire simultaneously at a rate of 2 frames/sec.
Two sets of
tests were performed, one using ethane released at 175 m depth and the second
using a simulated natural gas mixture released at depths between 640 and 325 ro,
the local water temperlture being close to 7°C in all cases.
The bubbles ranged
in size from about one centimeter in diameter down to a few millimeters in
diameter and hydrates formed as a thin skin of small crystals at the gas/water
interface.
They were easily visible to the naked eye, giving the bubbles a
bright matt silver appearance as against the clear surface of a normal gas bubble.
The differences are not so obvious from photographs and those included here have
been chosen as the best examples of the differences.
Ethane
In the case of ethane the maximum depth for gas release was limited to
175 m by the available gas bottle pressure.
At 7°C temperature, this places the
release conditions just within the hydrate formation region of the phase diagram,
Figure 1.
- 3 -
Under these conditions only a small proportion of the bubbles were
coated with hydrates when observed at a distance of 2 metres above the release
point, and no residual hydrate crystals were left behind after the bubbles had
passed the observation pOint.
Simulated Natural Gas
A mixture of gases was used which after consultation with Imperial Oil
of Calgary Ltd. was considered typical of that which might occur in an oil well
flow.
The chemical analysis is detailed in Table 1 and the corresponding hydrate
formation curve is shown on Figure 1.
Tests were commenced at a depth of 650 m
(950 psi ambient pressure) and the first observations made at the point of injection, and although no hydrates were observed on the bubbles within 1.0 metre of
the nozzle (this distance being limited by the field of view), small quantities
emerged from
th~
pi pe ex it after the gas f1 0\'1 was shut off.
On lowering the injection point below the observation window, all the
bubbles had become coated with hydrate at the 2 metre position, representing an
elapsed time of about 2.5 seconds.
The leading bubbles were mushroom shaped and
about 5 cm in diameter and the hydrate formed at the trailing edges, as sketched
in Figure 3.
After the main body of bubbles had passed, a small quantity of
hydrate crystals about 1 mm in length remained in suspension and slowly rose
under the action of their own buoyancy, the hydrates having a specific gravity
lying between 0.92 and 0.96.
To more closely approach conditions of an actual well blowout a small
quantity of Normal viells crude Oil was injected into the gas outlet pipe, in an
effort to produce a thin coating of oil at the bubble surface.
that such a coating might inhibit the formation of hydrates.
It was thought
There was no
significant difference between the appearance of the bubbles with and without
oil injection, there being a coating of hydrate crystals in both cases.
In the
- 4 -
case-of oil injection, oil droplets were observed amongst the bubbles, but it
was not possible to determine whether or not the bubbles themselves contained oil.
Additional observations of bubble behaviour Here made at depths between
650 and 325 metres, and the proportion of bubbles coated with hydrate decreased
as the ambient pressure decreased.
Figure 4_ shows two typical photographs of
natural gas bubbles at 650 m-and 325 m respectively.
In the latter case the
reflective surface of the uncoated bubbles contrasts v/ith the matt appearance of
the hydrate coating at the deeper depth.
.
-
The limit for hydrate formation appeared
to be at about 300 m.
Conclusions
Gas hydrates were observed to form read ily at the surface of bubbles
rising in seawater, provided that conditions of temperature and ambient pressure
for formation were satisfied. -The formation time for the hydrates was-of the
order of seconds from the time the gas first came into contact with the Hater.
The introduction of a small quantity of crude oil into the bubble plume had no
significant inhibiting action on the-hydrate formation, although there was no
means of determining whether in fact oil had coated the bubble surface.
REFERENCES
1.
Topham, D.R. Hydrodynamics of an oilwell blowout.
Technical Report 33.
2.
Sze,
Beaufort Sea Project
Y.K. and Adams, W.A. The formation of clathrate hydrates of natural
gases under blowout conditions in the Beaufort Sea. Unpublished
manuscript, Glaciology Division, Inland Waters Directorate, Environment
Canada.
,
TABLE 1
Composition of Simulated Natural Gas
,
Methane
81.5 %
Ethane
12.2 %
Propane
4.4 %
I-Butane
0.63%
N-Butane
0.88%
I-Pentane
0.25%
N-Pentane
0.20%
N-Hexane
0.40%
25.
I
I
I
I
I
T
I
I
I
1
I.<u
o
CJ.)
s...
~
rc:l
s...
;'
2l.
simulates natural
gas mi xture'
CJ.)
0..
E
CJ.)
~
I-
Eth( .ne
""
./
-",'"
15.
,
I
V
10.00
l-
/
4.4~V
1- - - - - -1. 1
100
Figure 1.
~
/
V
- - - - -~- - -
V
V
V
V
/
[7
V
-_L
~
I
V
V
/
'"
Methane",'" '"
V
/'
V"
'"
'"
'"
-
'" '"
V
-'
V
V
V
-
/
-,-
-1- r-
-- ,-
-
r
1000
Hydrate formation from hydrocarbon gases in seawater (after Sze and Adams)
I
Pressure psia
I
I
I
6000
Figure 2.
Schematic of gas release points on Pisces IV
coating
o
Figure 3.
p
a
D
Hydrate formation on large rising bubbles
(a)
Figure 4.
(b)
Natural gas bubbles in seawater; (a) at 650 m depth, (b) at 325 m depth