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doi:10.2204/iodp.proc.311.104.2006

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, gamma ray density) at in situ pressure and during degassing, and
• Preserved hydrate-bearing sediments at in situ pressure for more comprehensive shore-based investigations.

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. Site U1326, situated on the first uplifted ridge, had a strong BSR. The LWD/MWD data from Hole U1326A displayed the highest resistivity values seen during Expedition 311, with gas hydrate saturations calculated from Archie's equation as high as 60% (see "Gas hydrate and free gas occurrence"). There were also high resistivities below the estimated depth of the BSR (234 mbsf), potentially indicating free gas (see "Gas hydrate and free gas occurrence"). Specific objectives at Site U1326 were to confirm and quantify the presence of gas hydrate above the BSR, targeting the very high resistivity layer near 90 mbsf, and free gas below the BSR.

Operation of pressure coring systems

Pressure coring tools were deployed only three times at Site U1326 (Table T17): one FPC core (Core 311-U1326C-11Y), one PCS core (Core 311-U1326C-12P), and one HRC core (Core 311-U1326C-13E). The pressure coring tools were deployed in succession in a narrow interval between 82.7 and 86.7 mbsf, where the Hole U1326A LWD data had shown very high resistivity values. Figure F42 shows the pressure history of the cores during deployment, coring, recovery, and chilling in the ice shuck. Figure F43 displays the same data plotted as a pressure/temperature trajectory, showing the stark contrast between a deployment where the core remains clearly inside the gas hydrate stability field (Core 311-U1326C-11Y) and one that ends up in the laboratory well outside the gas hydrate stability field (Core 311-U1326C-12P).

The FPC deployment that recovered Core 311-U1326C-11Y was unexceptional until the sand line tension, when lifting the bit from the BHA exceeded the expected load by 2 tons. It was later concluded that a significant amount of sand had entered the BHA and had hindered the recovery. Scouring the outer barrel of the FPC also indicated clean sand had been penetrated. The autoclave was retrieved under nearly full pressure (17.5 MPa, compared with an in situ pressure of 19.2 MPa). X-ray images showed that this core was very short (15 cm) and situated in the center of the core barrel, implying that the core below it had washed out of the core liner. The liner and recovered portion of Core 311-U1326C-11Y were transferred from the autoclave, analyzed in the Pressure Multisensor Core Logger (MSCL-P; see "Measurements on HYACINTH cores"), and transferred to a storage chamber for further shore-based studies.

The PCS deployment (Core 311-U1326C-12P) recovered a partial core at 3.4 MPa. While the core was in the X-ray system, we observed a gas bubble trapped in the inner core barrel. After degassing was completed, the short core was extruded and some very coarse sand and rocky material was found in the outer core barrel.

During the final deployment with the HRC (Core 311-U1326C-13E), the lower part of the autoclave and bit assembly were lost, ending operations in Hole U1326C. The failure was the result of two joints becoming unscrewed during coring. Fine sand found near the top of the tool, 11 m above the bottom, led us to conclude that sand and fluid must have flowed back up the BHA and into the tool during the deployment, jamming the corer in a way that allowed the left-hand threads to come unscrewed.

Degassing experiments

At Site U1326, Core 311-U1326C-12P, which was the only PCS core recovered successfully under pressure, was investigated by controlled shipboard degassing experiments (Table T18). This core was taken at a depth of 83.7 mbsf.

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 laboratory (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 T19) as described in "Pressure coring" in the "Methods" chapter. During the degassing procedure, the vertical density distribution of the PCS cores 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.

The degassing of Core 311-U1326C-12P yielded 21.0 L of gas. The composition of the released gas did not change significantly in the course of degassing (Table T19). Methane was the major component, accounting on average for 91.7% ± 2% of the emitted gas. Nitrogen was the second most abundant gas, contributing 6.1% ± 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 (Table T19). A subset of gas samples was analyzed using methods described in "Organic geochemistry" in the "Methods" chapter and yielded an average ethane value of 54 ppmv and C₁/C₂ ratios >16,000. Recovery was particularly low for Core 311-U1326C-12P; the recovered core length was estimated at 35–40 cm from the gamma ray density profiles, though only 27 cm of sediment was extruded from the 1 m long PCS core barrel. Assuming that all methane was released from 40 cm of recovered sediment, mass balance calculations yield a pore space methane concentration of 2965 mM. This concentration equates to 35% of free gas or 40% of gas hydrate in the pore space (Tables T20, T21; Fig. F44). 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. F45).

The X-ray image taken before the core was degassed (Fig. F46) showed 28 cm of sediment at the top of the barrel, along with a gas bubble in the inner core barrel below this sediment. The water below the sediment also appeared to contain some suspended sediment. Repeated gamma ray density profiles (Fig. F46) showed that the sediment at the top of the core remained stationary throughout the degassing experiment, and that a small plug of sediment (~10 cm long) near 60 cm core depth also stayed in place during degassing. However, by the time the final X-ray image was taken, the lower plug of sediment had been homogenized and distributed along the core. Chlorinity measurements in the upper portion of the core show pore water freshening.

Measurements on HYACINTH cores

Simultaneous and automated gamma ray density, P-wave velocity, and X-ray measurements were made in the MSCL-P system on Core 311-U1326C-11Y. All measurements took place at 18 MPa (near recovery pressure; 95% of in situ pressure). The velocity and density profiles are shown alongside the X-ray image in Figure F47. The sediments have high velocities compared to density (Fig. F48) indicating there may be some gas hydrate in this short sample, especially when compared to other pressure cores retrieved during Expedition 311 (see "Pressure coring" in the "Site U1327" chapter). This core was transported to the Pacific Geoscience Center, Sidney, B.C., directly after Expedition 311 and was rapidly depressurized. When the core was removed from the storage chamber, some bubbling and "fizzing" was observed in the hard silty material. The sample was considered to be too small and disturbed to warrant being stored under pressure and was curated in a plastic bag.

Gas hydrate concentration, nature, and distribution

Mass balance calculations were only performed for Core 311-U1326C-12P, recovered from a depth of 83.7 mbsf. The gas hydrate content is estimated at 40%, placing it in the GHSZ (Fig. F44) regardless of the choice of thermal gradient (see "In situ temperature profile"). This pressure core was targeted at a zone of extremely high resistivity seen in the LWD data from Hole U1326A, and we can confirm that this high-resistivity zone contains gas hydrate.

IR images (see "Infrared images") and visual observations in the chemistry laboratory show that most of the gas hydrate is associated with sandy layers at Site U1326. Core 311-U1326C-11Y captured some extremely coarse sands and indurated material that was observed to fizz upon depressurization and warming and most likely contained gas hydrate. The core material missing from the base of this core, the sediment/water/gas mixture in the PCS, and the spectacular, spontaneous disassembly of the HRC highlight the difficulty in recovering gas hydrate–bearing sands.

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