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doi:10.2204/iodp.proc.311.103.2006

Pressure coring

The main objectives of pressure coring during Expedition 311 were to quantify natural gas composition and concentration in sediments and to determine the nature and distribution of gas hydrate and free gas within the sediment matrix. To achieve these objectives, we

• Measured the quantity and composition of gases released during controlled degassing experiments,
• Conducted nondestructive measurements (X-ray imaging, P-wave velocity, and gamma ray density) at in situ pressure and during degassing, and
• Preserved gas hydrate–bearing sediments at in situ pressure for more comprehensive shore-based investigations.

The nondestructive measurements not only provide a direct indication of the existence of gas hydrate, but the resulting data (acoustic impedance) can be used to help interpret regional seismic data. At Site U1325, drilled at the first slope basin, the BSR is a relatively weak reflector with an estimated depth of ~230 mbsf. LWD data from Hole U1325A showed alternating high and low resistivities from 122 to 260 mbsf (logging Unit 2; see "Logging units") that are especially well defined from 190 to 220 mbsf. Specific objectives at Site U1325 were to confirm and quantify the presence of gas hydrate above the BSR, with special attention to the layers of alternating resistivities and the occurrence of free gas below the BSR.

Operation of pressure coring systems

Pressure coring tools were deployed seven times at Site U1325 (Table T16). Two PCS cores, two HRC cores, and one FPC core were taken in Hole U1325B, including two cores within the layer of alternating high and low resistivities at 190–220 mbsf. In Hole U1325C, an FPC core was deployed within the alternating resistivity zone, and a PCS core was recovered from well below the estimated depth of the BSR. Figures F46 and F47 show the pressure and temperature history of the cores during deployment, coring, recovery, and chilling in the ice shuck. Only the pressure cores from Hole U1325C were retrieved under pressure, and only the deepest (Core 311-U1325C-10P) contained sediment under pressure.

Pressure coring was extremely difficult at Site U1325. The PCS became stuck in the BHA during the deployment that recovered Core 311-U1325B-28P because the PCS outer core barrel had deformed, possibly as a result of loss of circulation, and a pipe trip was required to free the tool. The HRC deployments both failed because of incomplete penetration, which resulted in collapsed/deformed liners that prevented the lower flapper valve from operating correctly. The FPC deployment that returned an autoclave under near–in situ pressure had an inverted core catcher, which indicated that the stiffness of the sediments was very high and probably exceeded the capabilities of this tool. These problems resulted from attempting to recover pressure cores in sandy lithologies in which even the XCB system had recovery problems.

Degassing experiments

At Site U1325, Core 311-U1325C-10P, which was the only core recovered successfully under pressure, was investigated by controlled shipboard degassing experiments (Table T17). This core was taken at 256.1 mbsf, which is ~25 m deeper than the estimated depth of the BSR.

The degassing experiment included the following steps. First, the volume and density of sediment inside the inner core barrel of the PCS was monitored by X-ray analysis. Next, the PCS was slowly degassed in a temperature-controlled van (7°C), and the volume and composition of released gas and water, the pressure inside the core, and the ambient air pressure and temperature were monitored (Table T18) as described in "Pressure coring" in the "Methods" chapter. During the degassing procedure, the vertical density distribution in the PCS core was repeatedly determined by GRA scans to examine the evolution of gas voids within the sediment. After degassing was completed, we X-rayed the PCS core again, collected the water remaining in the outer core barrel for mass balance considerations, curated the sediment that was extruded from the core, and subsampled it for IW chemistry, dissolved gases, and physical property analyses.

Degassing of Core 311-U1325C-10P yielded 3.4 L of gas. The composition of the released gas did not change significantly in the course of degassing. Methane was the major gas component, accounting on average for 89.0% ± 3% of the emitted gas (Table T18). Nitrogen was the second most abundant gas, contributing 8.6% ± 3% to the gas released. Carbon dioxide, ethane, and higher hydrocarbon concentrations were below the detection limit of the Agilent gas chromatograph used for continuous gas analysis during the degassing experiment. Mass balance calculations yielded a pore space methane concentration of 175 mM, indicating <0.3% of free gas in the pore space (Tables T18, T19; Fig. F48). The pressure inside the PCS core dropped below the predicted gas hydrate stability conditions when the port valve of the PCS was first opened and water expanded from the outer core barrel into the manifold system. Therefore, no pressure plateaus or rebounds from dissociation of gas hydrate could be expected and a steady decrease of core pressure versus removed gas volume was observed (Fig. F49).

X-ray images and density profiles showed limited gas evolution during depressurization (Fig. F50). A low-density zone from 5 to 15 cm core depth seen in the X-ray scan taken before degassing remained in the X-ray scan taken after degassing, probably indicating an interval of disturbed core.

Gas hydrate concentration, nature, and distribution

Mass balance calculations were only performed for Core 311-U1325C-10P, which may have contained a very small amount of gas hydrate (0.4%) or free gas (0.3%) (Table T20; Fig. F48), depending on the placement of the base of gas hydrate stability. This core was located below the estimated depth of the BSR (230 mbsf), but possibly above the base of the GHSZ (see "In situ temperature profile") and at the base of the zone of alternating high and low resistivities, where core recovery increased. The quantity of excess methane was too small to determine the phase (gas hydrate or free gas) from the nondestructive measurements available for PCS cores (X-rays and gamma ray density).

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