Modelling gas and vapour migration

The extent and rate of gas migration out of the ground or along in ground pathways depends on three key factors  

In order for sustained migration to occur, the gas must be replenished at source. As such significant gas generation is usually required for large volumes of gas 

Gas concentration within a source is a poor indicator of migration or surface emission risk. The methane concentration could be measured at similar levels in both balloons but the risk of migration is obviously higher in the left-hand one. Even where migration can occur via diffusion there needs to be a constant source of gas to replenish the gas that migrates, in order to maintain a concentration gradient. 

Stone columns or piles provide migration pattiway through 

The geological conditions in and around a site have a major influence on the risk of gas migration. A commonly used value of 250 m is often incorrectly perceived as being the limit to which gas migration can occur. Migration has been observed to occur for distances up to 400 m along open features (e.g. open faults or other similar features). Conversely there have been sites where no migration has been observed as close as 10 m to sources that are surrounded by impermeable clay (e.g. London clay). 

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