Gas Hydrate at the Reservoir and Accumulation Scale

Estimates of the amount of gas hydrate and associated gas within a given gas hydrate accumulation can vary considerably (Table 2). For example, recent estimates of the volume of gas that may be contained in the gas hydrate and free-gas beneath gas hydrate on the Blake Ridge range from about 70 trillion cubic meters of gas over an area of 26,000 km^ (Dickens et al., 1997) to about 80 trillion cubic meters of gas for an area of 100,000 km^ (Holbrook et al., 1996). The difference between these two estimates has been attributed to the observation that the amount of free-gas directly measured within pressure-core samples (Dickens et al., 1997) from beneath the gas hydrate accumulation is significantly larger than that estimated from borehole vertical seismic profile data (Holbrook et al., 1996).

Cubic meters

References

Blake Ridge, USA

18 X1012

Dillon and Pauli (1983)

80X 1012

Holbrook et al. (1996)*

70X 1012

Dickens et al. (1997)*

38X1012

Collett and Ladd (2000)

57 X1012

Collett and Ladd (2000)

Nankai Trough, Japan

50X 1012

MITI/JNOC (1998)**

Andaman Sea, India

6x 1012

GAIL/ONGC/DGH (1997)**

Prudhoe Bay Alaska, USA

1.2 X 1012

Collett (1993)

Mackenzie Delta (Richards Is.), Canada

0.2 X 1012

Collett et al. (1999)

Table 2. Gas Hydrate Accumulations In-Place Natural Gas Resources Estimates of the amount of gas within known and inferred gas hydrate accumulations. * Includes associated free-gas. ** Unpublished industry reports

Table 2. Gas Hydrate Accumulations In-Place Natural Gas Resources Estimates of the amount of gas within known and inferred gas hydrate accumulations. * Includes associated free-gas. ** Unpublished industry reports

Recent analysis of downhole electrical resistivity log data from ODP Leg 164 coreholes (Collett and Ladd, 2000) reveal that the gas hydrate and underlying free-gas accumulations on the Blake Ridge contains about 57 trillion cubic meters of gas (assuming a gas hydrate areal distribution of

26,000 km2). Other published studies indicate that the gas hydrate at the crest of the Blake Ridge alone (area of about 3,000 km2) may contain more than 18 trillion cubic meters of gas (Dillon and Paull, 1983). The broad range of these estimates demonstrates the need for high-resolution measurements of the gas hydrate and associated free-gas volumes within any gas hydrate accumulation of interest.

It has been suggested that the volume of gas that may be contained in a gas hydrate accumulation depends on five primary "reservoir" parameters (modified from Collett, 1993): (1) areal extent of the gas-hydrate occurrence, (2) "reservoir" thickness, (3) sediment porosity, (4) degree of gas-hydrate saturation, and (5) the hydrate gas yield volumetric parameter which defines how much free-gas (at STP) is stored within a gas hydrate (also known as the hydrate number). In the following section, the five "reservoir" parameters (Table 3) needed to calculate the volume of gas associated with the gas hydrate on the Blake Ridge (ODP Sites 994, 995, and 997; Shipboard Scientific Party, 1996), along the Cascadia continental margin (ODP Site 889; Shipboard Scientific Party, 1994), on the North Slope of Alaska (Northwest Eileen State-2 well; Collett, 1993), and in the Mackenzie River Delta of Canada (Mallik 2L-38 well; Dallimore et al., 1999) are reviewed.

The following "resource" assessment (modified from Collett, 1998) has been conducted on a site-by-site basis; that is, for each site examined the volume of gas hydrate and associated gas within a one square kilometer area surrounding each drill-site have been individually calculated (Table 3). For this "resource" assessment, the thickness of the gas-hydrate-bearing sedimentary section at both the marine and permafrost drill sites was defined to be the total thickness of the downhole log inferred gas-hydrate accumulation (Table 3). Average core derived sediment porosities for the gas-hydrate-bearing reservoirs at Sites 994, 995, 997 (Blake Ridge) and 889 (Cascadia continental margin) range from about 52 to 58 percent (Table 3). The "corrected" density log derived sedimentary porosities for three gas-hydrate-bearing units identified in the Northwest Eileen State-2 in northern Alaska (Units C, D, and E) range from an average value of about 36 to 39 percent (Table 3). The density log derived sediment porosities for the gas-hydrate-bearing interval in the Mallik 2L-38 well in northern Canada averages about 31% (Table 3). Gas-hydrate saturations at the marine drillsites (Sites 994, 995, 997 and 889), calculated from available downhole logs, range from an average value of about 3 to 6 percent (Table 3). Gas-hydrate saturations in all three gas-hydrate-bearing units (Units C, D, and E) in the Northwest Eileen State-2 well, calculated from available downhole log

Site/Wei 1 ID

Log Depth of inferred gas hydrates

Hydrate zone thickness

Sediment Porosity (%)

Gas hydrate saturation

Gas Volume within hydrate

1 Z / 3x per km (m )

Ocean Drilling Program Drill Sites:

Site 994

212.0-428.8

216.8

57.0

3.3

669,970,673

Site 995

193.0-450.0

257.0

58.0

5.2

1,267,941,673

Site 997

186.4-450.9

264.5

58.1

5.8

1,449,746,073

Site 889

127.6-228.4

100.8

51.8

5.4

466,635,705

Northwest Eileen State 2 Drill Site:

Unit C

651.5-680.5

29.0

35.6

60.9

1,030,904,796

Unit D

602.7-609.4

6.7

35.8

33.9

133,382,462

UnitE

564.0-580.8

16.8

38.6

32.6

346,928,811

Total for Northwest Eileen State 2

1,511,216,069

Mallik 2L 38 Drill Site:

HY Unit

888.8-1101.1

212.3

31.0

44.0

4,749,066,080

Table 3. Volume of gas within the downhole log inferred gas hydrate occurrences at ODP Sites 994, 995, 997 (Blake Ridge), and 889 (Cascadia contiental margin), and in the Northwest Eileen State-2 (northern Alaska) and Mallik 2L-38 (northern Canada) wells

Table 3. Volume of gas within the downhole log inferred gas hydrate occurrences at ODP Sites 994, 995, 997 (Blake Ridge), and 889 (Cascadia contiental margin), and in the Northwest Eileen State-2 (northern Alaska) and Mallik 2L-38 (northern Canada) wells data, range from an average value of about 33 to 61 percent (Table 3). The resistivity well log derived gas-hydrate saturations in the Mallik 2L-38 well average about 44% (Table 3). In this assessment, a hydrate number of 6.325 (90% gas filled clathrate) was assumed, which corresponds to a gas yield of

164 m^ of methane (at STP) for each cubic meter of gas hydrate. The log inferred gas hydrate at Sites 994, 995, and 997 on the Blake Ridge contain between 670,000,000 and 1,450,000,000 cubic meters of gas per square kilometer (Table 3). The volume of gas within the log inferred gas hydrate at Site 889 on the Cascadia continental margin is about 467,000,000 cubic meters of gas per square kilometer (Table 3). Cumulatively, all three log inferred gas-hydrate-bearing stratigraphic units (Units C, D, and E) drilled and cored in the Northwest Eileen State-2 well may contain about 1,511,000,000 cubic meters of gas in the one square kilometer area surrounding this drill-site (Table 3). It was also determined that the log inferred gas-hydrate-bearing stratigraphic interval drilled in the Mallik 2L-38 well contains about 4,750,000,000 cubic meters of gas in the one square kilometer area surrounding the Mallik drill-site (Table 3).

A close examination of the gas hydrate saturations in Table 1 reveals a potential problem associated with production of gas from marine gas hydrate. Even though vast portions of the world's continental shelves appear to be underlain by gas hydrate, the concentration of hydrate within most marine accumulations appears to be very low. Low gas hydrate concentrations may significantly affect the economic production potential of marine gas hydrate (gas hydrate production is discussed in more detail later in this paper).

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