Costa Rica Margin

The first LWD data recorded in hydrate-bearing sediments were acquired on the Costa Rica margin during ODP Leg 170. Several holes were drilled into the sedimentary wedge upslope of the Costa Rica trench, penetrating Plio-Pleistocene age sediments that are mostly derived from the Central American peninsula. The wedge was cored, and where significant amounts of material was recovered for sampling, disseminated hydrate was observed in fractures, cemented ashes, and diffuse pore space [Kimura, et al., 1997], Disseminated hydrate was recovered in these sediment cores, however, their signature was not clearly observed in the LWD resistivity data [Boissonnas, et al., 2000], Although LWD measurements are made within minutes of drilling and may best represent the undisturbed physical state of the in situ environment, the response of this tool could not distinguish the presence of hydrate in the formations on the Costa Rica margin. A likely explanation is that with only low hydrate concentration, large amounts of seawater and clay in the formation may be too conductive for the hydrate to have a significant effect on the measured resistivity using this electromagnetic device. If hydrate distribution is diffuse it may also rapidly dissociate during drilling and be easily obscured. Additional laboratory experiments and different LWD tools are needed to conclusively identify in situ hydrate occurrences in this environment.

3.5. North Slope of Alaska

The occurrence of gas hydrate on the North Slope of Alaska was confirmed with data from pressurized core samples, wireline logs, and the results of formation production testing in the Northwest Eileen State-2 well located in the northwest part of the Prudhoe Bay oil field. Collett [1993] calibrated these log responses to an additional 50 exploratory and production wells in the Alaskan permafrost region and inferred gas hydrate occurrences in six laterally continuous sandstone and conglomerate reservoirs, all of which are geographically restricted to the Kuparuk River-Prudhoe Bay area. The wireline logs from wells in the western part of the area also indicate free gas accumulations trapped below some of the inferred hydrate layers. The Northwest Eileen State-2 well drilled through five such hydrate-bearing layers.

Wireline logs (Fig. 5) from the Northwest Eileen State-2 well. Hydrate occurs in the upper three units and is characterized by high electrical resistivity (ranging from 20 to 70 ohm-m) and high compressional velocity (ranging from 2.0 to 2.8 km/s). Collett [1993] explain the increase in electrical resistivity and compressional velocity by the occurrence of gas hydrate.

GAMMA RAY BULK-DENSITY NEUTRON-POR. VELOCITY RESIST.

GAMMA RAY BULK-DENSITY NEUTRON-POR. VELOCITY RESIST.

Compressional Velocity Sanstone
Alaska.

The gamma ray, formation density, and neutron porosity logs in these layers are indicative of clean (to slightly shaley) sandstone reservoirs. Of particular note, the low neutron porosity and high density log values from about 654-656 m are likely due to the presence of a "hard", well cemented zone at the top of the sandstone reservoir. The zone characterized by low electrical resistivity, low density, and low velocity from about 751-775 m is interpreted to indicate water and gas-saturated sediment, without the presence of gas hydrate.

3.6. Mackenzie River Delta

The Mallik 2L-38 research hole was drilled to investigate the occurrence of gas hydrate in the permafrost region of the Mackenzie River Delta of Canada [Dallimore, et al., 1999]. A major component of the Mallik drilling program was to apply state-of-the-art wireline logging technology and refine its interpretation in naturally-occurring hydrate formations in a permafrost environment. As the Mallik hole was drilled explicitly for this purpose, the quality of the wireline logs in the hydrate-bearing interval of this formation is excellent. Figure 6 illustrates the logs acquired through a relatively thick hydrate-bearing interval from about 897 to 1,110 m depth. The logs show deep reading resistivity measurements ranging from 10-100 ohm-m and compressional velocities (Vp) ranging from 2.5-3.6 km/s. In addition, a shear-wave velocity log (Vs) was recorded, yielding values from 1.1-2.0 km/s, and Vp/Vs varies between 2.8-2.0 through the hydrate interval. The decrease in velocity at approximately 1120 m depth indicates the presence of a relatively thin free gas layer below the predicted base of the gas hydrate stability zone. High quality VSP data was recorded in the Mallik well and both VSP compressional- and shear-wave velocities are in good general agreement with Vp and Vs from the sonic logs [Walia, et al., 1999]. Collett, et al. [1999] estimate porosity from these logs in the range from 20-40%, which suggests that methane hydrate saturation within clean sandstone reservoirs may be especially high in this permafrost environment.

GAMMA NEUTRON BULK- RESISTIVITY COMPRES. SHEAR Vp/Vs RAY POROSITY DENSITY VELOCITY-Vp VELOCITY-Vs RATIO

GAMMA NEUTRON BULK- RESISTIVITY COMPRES. SHEAR Vp/Vs RAY POROSITY DENSITY VELOCITY-Vp VELOCITY-Vs RATIO

Pine Trees Pnw
Figure 6. Wireline log data from the Mallik 2L-38 well located on the Mackenzie River Delta of northern Canada.

4. SUMMARY

Because hydrate is difficult to sample and study in the laboratory, in situ detection methods in drill holes are critical in that they provide "ground truth" measurements for constraining surface geophysical data. However, the linkage between downhole and surface information is not necessarily straightforward and their correlation, in turn, must be evaluated in the context of the type and scale of the in situ measurement. Resistivity and velocity logging methods are used most often for this purpose. The dissociation of hydrate or the change in free gas concentration in the vicinity of a borehole due to the drilling process may significantly alter these measurements and the correction or quantitative evaluation of the results is usually required. To minimize such effects, LWD logging methods generally provide higher quality data, but the application of this new technology must be carefully interpreted for the particular host sediment in which gas hydrate may be present.

The measurement of in situ properties using logs is primarily useful for: (1) the identification of hydrate and hydrate-bearing sediment and their distribution with depth, (2) the estimation of porosity and methane saturation, and (3) the calibration on of drill hole information with surface seismic or other remote geophysical data. The table below generalizes the expected response of several common logging measurements in massive hydrate, hydrate-bearing marine sediment, and gas- and water-saturated marine sediments based on the field examples discussed above.

Massive Hydrate* Hydrate-bearing Water-saturated Gas-bearing Sediment Sediment Sediment

Vp (km/s)

3.2-3.6

1.7-3.5

1.5-2.0

1.4-1.6

Vs (km/s)

1.6-1.7

0.4-1.6

0.75-1.0

0.4-0.7

R (fl-m)

150-200

1.5-175

1.0-3.0

1.5-3.5

P (g/cm3)

1.04-1.06

1.7-2.0

1.7-2.0

1.1-1.5

O (%)

20-50

35-70

35-70

50-90

y (API)

10-30

30-70

50-80

30-80

Table 1. Common ranges of in situ log properties (*modified from Matthews, 1986).

Table 1. Common ranges of in situ log properties (*modified from Matthews, 1986).

In general, resistivity logs typically have high values relative to seawater-saturated sediment as hydrate forming in open pores replaces the more electrically conductive seawater. Both compressional and shear velocity logs typically increase because hydrate acts as a high-velocity pore filling material and as an intergranular cement, stiffening the bulk compressibility and rigidity of the sediment. Sonic log and VSP velocities tend to decrease in the presence of free gas. Natural gamma ray, neutron porosity, and bulk density logs show little or no apparent decrease in hydrate-bearing sediments. In the future new logging and LWD technologies measuring in situ resistivity, velocity and porosity, all of particular importance for detecting gas hydrate occurrences, will provide greater certainty for many of these key parameters.

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