Rising gas occurs naturally and as a result of drilling, injection and construction activities, such as air jetting, fracturing, jet grouting and tunnelling. The effect of rising gas is difficult to quantify or model, but is known to reduce the bearing capacity of both shallow and deep foundations. Under gravity based structures (GBS) it can cause problems of flotation and subsequent base sliding and internal over pressure. Storm waves can liquefy shallow gaseous deposits. Gas venting facilities are essential but not always provided on offshore structures. The paper discusses rising gas in terms of sources, pathways, flow calculations, gas experiments trials, and calculations, settlement effects and precautions, as well as its effect on GBS, on bearing capacity (especially during storms) and on shallow and deep foundations.
The most obvious sources of gases are hydrocarbons rising naturally or as a result of drilling operations. Additionally gases, such as nitrogen, are injected as liquid into strata to increase well production, while other gases are injected for storage or disposal. Compressed air is used in tunnels, caissons, jet grouting and jetting. There are also volcanic gases, hydrates and organic deposits releasing methane.
Pathways for rising gas are mainly dependent on the lamination and dip of the strata and local anomalies such as faulting, joints or weaknesses. Layers of gas can be trapped below individual laminations or cyclotherms (due to seasonal depositional changes in particle size). Seismic surveys show numerous strong ‘ghost’ reflections, which correspond to the pattern of lamination but cannot usually be distinguished in recovered core. A likely explanation is that trapped gas provides the ghost reflection. The air lifts the cuttings, but can also escape laterally and appear in adjacent boreholes. Shot hole blasts can show a similar behaviour.