As part of the Troll Shallow Gas Project, small scale model tests were conducted to develop a better understanding of the mechanisms for gas migration through soft seabed sediments. Soft seabed sediments were simulated by a synthetic transparent material called Laponite. Air was introduced into the material through several small nozzles, and the migration of the gas was followed by both digital camera and video. The results have shown that the main mechanism for gas migration in soft seabed sediments is by fracturing of the soil. However, under certain circumstances it was observed that voids of gas would expand and the migration could be described by an expanding cavity theory. During the experiments it also became evident that fractures filled with air were able to migrate to the surface even without being supplied by air from the nozzle.
The pressure under each of the 19 domes of the concrete base of the gravity-based structure (GBS) of the Troll A platform is monitored and logged, and the pressure is routinely controlled and bled down when reaching predefined threshold levels. In 2002, rapid pressure fluctuations occurred under one of the domes, and samples from the bleed off system showed that there was free gas present1. Following this observation, the Troll Shallow Gas Project was initiated by Statoil. One of the purposes of this project was to gain a better understanding of the migration of free gas through soft seabed sediments, as well as the many pockmarks found at the seabed around the Troll platform2. Therefore small scale model tests were carried out in 2005 as collaboration between Statoil, the Norwegian University of Science and Technology (NTNU) and SINTEF Building and Infrastructure.
Comparable experiments have previously been conducted, amongst others, by Takada3, Johnson et al.4 and Boudreau et al.5, who studied the migration of air in gelatine. All these experiments showed that air bubbles would migrate through the material either by fracturing the soil or by reopening existing fractures. In soft, cohesive soils, gas bubbles were often found to be highly eccentric oblate spheroids (coin-shaped). The same migration mechanism and shape of fracture was also observed in the studies carried out by Gylland et al6
Nakashima7 and Nunn8 both suggested that fluid can migrate through geo-pressured sediments in isolated fractures, where the soil or rock is cracked at the top of the fracture and a collapse of the fracture occurs at the bottom. In the experiments carried out by Takada3, it was observed that fluid-filled fractures isolated from the needle and propagated to the surface due to buoyancy.
Figure 1; (a) Principal sketch and (b) photograph of the model (available in full paper)
The model tests were carried out in a glass box of 70 x 10 50cm3 (length x t x height) filled with a synthetic, transparent clay-like material called Laponite. A principal sketch and a photograph of the model are shown in Figure 1. Laponite is a synthetic material that closely resembles natural clay minerals in both structure and composition.