Friday, 13 January 2012

Shale gas and BGS blogs....

A quick post before the weekend. The BGS (British Geological Survey) are getting into shale gas, which is definitely a good thing - the more scientists around the better. However, this blog post by the Executive Director has me all wound up (probably more than it's worth):

http://britgeosurvey.blogspot.com/2011/12/british-geological-survey-shale-gas.html

Specifically, the line 'the gas is tightly bound to silt and sand grains and needs to be pushed out by injection in a process that is called fracking' has got me annoyed, because it appears that the BGS Executive Director doesn't know what fracking is.

Firstly, the gas is not tightly bound to the silt - that implies some sort of chemical bonding between the gas and the shale. While this might happen to some extent, the majority of the gas exists as a free phase within the pores of the shale. Despite a low permeability, shale can easily have a porosity of 10%. This means that the gas is there as a free phase, trapped in the spaces in the rock, but the low permeability stops it going anywhere.

This is where fracking comes in - by creating fractures in the rock, the gas can escape from its shale cage and flow down the fractures to the well, where it is produced. Yes, fracking requires injection, because the fractures are created by pumping in water at high pressure until the tensile strength of the rock is exceeded. However, this all takes place over an hour or so. After that, with the fractures created the gas flows out naturally along the fractures due to the pressure differential between the formation and the well (just like conventional gas and oil), and continues to do so for several years without further stimulation or fracking. The gas does not need to be 'pushed out by injection'.

The BGS blog implies that continuous injection and activity is needed to squeeze the gas out from the rock. This sort of impression will make shale gas seem far less attractive to the general public - continuous activity and injection for years of production, and some sort of weird chemical interaction. Rather than what actually happens: frack once (taking maybe a week or so to complete all the stages in a horizontal well), then leave the well to produce naturally for years.

However, in their defense, the web resource which the blog was advertising - the BGS's new shale gas web page, does look like a useful store of shale gas information.

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