The National Methane Hydrates R&D Program
The DOE/JIP Gulf of Mexico Hydrate Research Cruise
Special Report - Core Handling
From: Cruise Prospectus [PDF-827KB]
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Non-pressurized and Pressure Core Handling
Non-pressurized Core Handling (Fugro Hydraulic Piston Corer and Fugro Corer)
Core packed in ice bath
Cores that might contain gas hydrates should be recovered as quickly as possible. An ice bath may be used in some cases to slow the dissociation process. A core reception/preparation van will be on the deck of the Uncle John where individual cores (perhaps up to 9 m long) can be laid on ‘core hooks' and quickly drilled, labeled and sectioned. Infrared (IR) camera imaging will be done as soon as practical after core recovery. Both track-mounted and hand held IR cameras will be used to identify the position in cores of cold spots, indicating possible zones of hydrate occurrence.
If gas hydrates are present or if dissolved gas contents of sediments exceed about 10 mM, gas expansion voids will form in the core and pressure must be relieved to prevent extrusion of the core or possible explosive shattering of the core liner. Pressure is first removed by punching holes in the core liner at the position of the gas voids to sample gases by syringe attached by a stopcock to a core liner penetration tool. After or while gas samples are collected, holes can be drilled at regular (~50-cm) intervals along the core to permit degassing. Where cold spots indicate possible presence of gas hydrates, whole round sections of 10-15 cm length may be cut, capped, wrapped and immediately preserved in liquid nitrogen dewars. Cores with IR-temperature anomalies may also be subjected to X-ray CT scanning to provide high resolution, 3D images of sediment hydrate structures. Some gas hydrate containing cores may be decomposed under controlled conditions to measure gas yields and provide samples for gas analysis.
After cores have degassed (and if necessary restored to approximate original length) the cores will be measured and cut into nominal 1-meter sections. The whole-round core sample (10-20 cm in length) for interstitial water analysis should be cut from the top of the specified section (or shifted somewhat to provide optimal sample quality), and the 5-cm3 plug of sediment for headspace gas analysis taken from the base of the opposite section. Whole round samples (8-10 cm in length) for microbiology will be taken from specified cores and stored under refrigerator (4ºC) or freezer (-80ºC) conditions.
After the minimum sampling of specified intervals for ephemeral properties and microbiology, the whole-round core sections should be logged using the multi sensor core logger (MSCL). Properties to be measured by MSCL include gamma ray density, P-wave velocity, magnetic susceptibility, and electrical resistivity.
General non-pressurized core flow plan
Pressure Core Handling (Fugro Pressure Corer and HYACE Rotary Corer)
It is crucial that autoclaves potentially containing gas (dissolved or free) and/or gas hydrate are kept close to in situ conditions
Pressure core transfer chamber
(temperature and pressure) to prevent gas hydrate dissociating, gas exsolving and pressures rising in the pressure vessels. Consequently, the coring tools and autoclaves will be immersed in an ice bath as soon as possible after arriving on the rig floor. Once temperature and pressure are stable (around in situ conditions) the autoclave will be removed and connected to a cold Shear Transfer Chamber (STC) in a cold van and the core transferred into a HYACINTH Storage Chamber (SC). Once inside the SC the core can be either logged in the X-ray CT or the MSCL-V, or connected to the cold MSCL-P (Multi Sensor Core Logger – Pressure) for high-pressure measurement of physical properties.
The X-ray CT can provide very rapid 2D and 3D information on the sediment texture while the core is under pressure in the aluminum SC's and can reveal the presence of gas, water and massive gas hydrate layers or nodules. The MSCL-V will provide an accurate 1D density profile of the core through either the steel SC's or the aluminum SC's and can reveal the presence of gas, water and massive gas hydrate layers or nodules. The MSCL-P can be used with either of the 2 CMCs. One CMC was specially developed to enable measurements of Vp, Vs, electrical resistivity, and strength/resistance of the core through holes in the core liner. The other CMC was developed to log Vp automatically and rapidly through the core liner.
While the X-ray CT logging provides valuable information on the contents of pressure cores, there is some concern that multiple transfers could compromise the MSCL-P measurements. Alternate trials on initial cores will help determine the optimal procedures for shipboard logging and experiments with pressurized samples.
Apart from rapidly assessing the overall nature of the contents of any individual pressure core inside an aluminum SC, the X ray CT may determine the spatial distribution of hydrates contained within the core. This might be assessed at either in situ pressures (if the hydrate is relatively massive) or as a result of sequential imaging during dissociation experiments (if the hydrate is more dispersed in nature). The X-ray CT logging system complements the MSCL-V and will be fitted into the same cold container.
One decision in processing pressure cores is to decide whether the quantity, nature, distribution and physical properties of hydrates inside the core are to be assessed by controlled measurements and dissociation inside pressure vessels. Alternatively the general nature and distribution of hydrates could be determined by rapid depressurization and visual observation with subsequent decomposition and sampling of the gas and water. It is possible to depressurize cold cores rapidly to get immediate access to a relative undisrupted gas hydrate core. The major problem is not the dissociation of gas hydrates (as this is a relatively slow process) but with the more rapid process of exsolution of gas dissolved in the pore water. Experience from Leg 204, where pressure was rapidly released around a frozen core, indicates this is feasible and can be done safely.
With pressurized cores, close to in situ conditions, any gas hydrates inside are stable and hence initial measurements from all logging instruments can be made enabling informed decisions about how each core is to be subsequently processed. Undoubtedly the best science and understanding will come from a combination of techniques and procedures.
General pressurized core flow plan