Ghastli 21 Overview

GHASTLI combines a number of separate-pressure-and-temperature-control systems (Fig. 1) to simulate in situ conditions on a 71-mm diameter by 140-mm high right-circular sediment specimen (Fig. 2). The 25 MPa pressure (equivalent to the pressure exerted by a 2,500 m column of water) and temperature (25°C to about -3°C) capabilities of the system provide wide latitude in test configurations and procedures within which gas hydrate can be studied in reconstituted sediment or in field samples containing natural gas hydrate. Instrumentation packages and sensors placed within the different subsystems and in close proximity to the test specimen produce measurements that are logged and displayed by a computer running custom-designed software (Fig. 3). During testing, the sample resides within a silicone-oil filled, main pressure vessel (Figure 1). Separate internal sediment pore pressure and external confining pressure systems are used to adjust isotropic consolidation stress, ct'c to simulate in situ overburden pressure.

Figure 1: GHASTLI system showing the syringe pumps that control pressure and the main pressure vessel, located just to the right of center in the photo, that contains the sediment test specimen.

Figure 2: Close-up view of a test specimen about to be raised into the main pressure vessel showing the different pressure and coolant lines. The test specimen (Grey cylinder in the central part of rig) is 14 cm high. The sedimentn sample is vertically raised into the main pressure vessel, located at the very top of the photo, prior to testing.

Figure 2: Close-up view of a test specimen about to be raised into the main pressure vessel showing the different pressure and coolant lines. The test specimen (Grey cylinder in the central part of rig) is 14 cm high. The sedimentn sample is vertically raised into the main pressure vessel, located at the very top of the photo, prior to testing.

The test sample is surrounded by flexible membranes. Top and bottom end caps incorporate acoustic transducers and gas or water flow ports. The bottom end cap rests on an interchangeable internal load cell. The top end cap is contacted by a bath-controlled, variable-temperature heat exchanger which imparts a unidirectional cooling front downward through the specimen. To achieve better sample thermal equilibrium the perimeter of the main pressure vessel is also surrounded by cooling coils.

Five 500 ml capacity syringe pumps are used to maintain the confining pressure surrounding the specimen and the internal specimen pressures which include: pore pressure, back pressure, and methane-gas pressure. Flow rates of

Figure 3: Schematic of main pressure and data acquisition systems (modified from Booth et al., 1999).

0.001 ml/min to over 80 ml/min are controlled by the syringe pumps to a maximum pressure of 25 MPa. The back-pressure system contains a collector that is capable of separating and measuring water and gas volumes that are pushed out of the specimen at test pressures by the dissociation of gas hydrate. A separate syringe pump controls the movement of the load ram during shear tests and is used to determine the height of the specimen (which may change during a test) to increase the accuracy of acoustic-velocity measurements. Four thermocouples and four thermistors are placed against the outside perimeter of the specimen at different heights to measure temperature variations during cooling or warming stages and during gas-hydrate formation and dissociation events.

Because we can reduce specimen temperatures below freezing, ice formation is possible either alone or within the sediment pores. This is important because some physical properties of gas hydrate and ice are similar (Kuustraa et al., 1983; Miyairi et al., 1999) and gas hydrate is present in regions where permafrost is present (Dallimore and Collett, 1995; Collett, 1993).

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