On a North Sea major gas producer, high pressure has been observed in the outermost annulus of several wells and anomalous pressures were observed below the gravity platform base (inside skirt compartments). Gas was suspected to migrate along well-paths, into well annuli and, in the worst case, past the conductor shoe and continue to the surface and platform foundation.
A detailed study was initiated to investigate the gas source, gas migration mechanism and the pressure building component in a system that was believed to be shallow gas driven. The gas migration as part of natural gas flux in the seabed and the relation to formation of pockmarks in the Norwegian trench was investigated as part of this study. The work has provided a good understanding of shallow gas migration below a production platform and has identified the pressure building mechanisms observed in wells and foundation. They are all explained by natural seabed processes and no concern exists for the long term safety of the platform.
The Troll A Platform is a huge concrete gravity-based structure (GBS) located in the Norwegian trench at a water depth of 305m (see Figure 1 and Hansen et al.1). The platform was installed in 1995 and, in the following two years, 40 wells, 39 gas production wells and one monitoring well were installed. A typical well design is shown in Figure 2. The platform is the North Sea's largest gas producer with an average yearly production in the order of 26 GSm3 (923 billion ft3).
After a few years of production it was realised that pressure bleed-off activity from the outer annuli was high, for some wells 10?20 bleed-offs every month. It is standard practice to set safe threshold values for pressure in well annuli and, if this value is reached, pressures are reduced by bleeding off gas. The threshold for the outer annuli was 6 bar, and this was raised to 15 bar after an evaluation of wellhead seals in 2001. This action reduced the problem of very high bleed-off activity to more normal conditions, typically to once a month for an average well.
High annulus pressure is frequently observed in oil and gas production wells both onshore and offshore. The pressures are mostly associated with small amounts of gas seeping along the well-path and entering into the annulus. Cement between the casing and the formation or between two casings doesn't stop this migration process2.
In parallel to the problem of high well annuli pressures, high pressures in the foundation skirts had been observed (Figure 3). High pressures had been expected here, since the Troll A structure is heavy and sits on soft soils. Also the foundation solution, 36m-long cylindrical concrete skirts that displaced the soil during installation as they were pushed into the seabed, created high pore pressures in the soil and skirt compartments. The combined pressures from platform weight and soil displacement by concrete skirts are a result of foundation consolidation causing increasing pressure at the foundation base