There are two key technologies that have enabled gas to be extracted economically from shales. Hydraulic fracturing is the technique that has garnered all of the attention. However, of equal importance is extended reach lateral drilling: turning the drill bit horizontally to drill sideways for several kilometers.
This technology was developed in the 1980/1990s - this graph shows the number of horizontal wells drilled in the US through the late 1980s to mid 1990s.
The number of horizontal wells has subsequently boomed, as the shale revolution has spread across the US - as the next figure shows for the Barnett:
Typical shale wells extend between 2 - 4 km in length. However, it is possible to go much further than this. Some of the longest "extended reach" laterals were drilled in the UK in the 1990s, allowing the operators of the Wytch Farm oil field to access reservoir compartments under Poole Harbour and the English Channel from their onshore drilling site.
The scale of these extended reach wells can be difficult to contextualise, but I'm going to try, using an image taken from Dr Ian West's incredibly comprehensive website (warning - for real geo-geeks only).
Dominating this cross-section is a profile of a Wytch Farm extended reach well. Staying topical (as ever), I will use the recent drilling at Barton Moss as a comparison. The extended reach well is over 10km long. If this well were being drilled at Barton Moss, outside of Manchester, then the toe of the well could reach Albert Square in the city centre. To represent this, I have added images of the Barton Moss drilling site and Albert Square to Ian West's image.
The well targets layers over 1.5km deep. This is equivalent to over 5 Shards, stood end on end. So I've also added a Shard, to scale, to the image.
Ian's image also compares the depths of various shale wells, including the rather shallow layer targeted at Balcombe, and deeper wells targeting the Bowland shale in the UK, and the Haynesville in the US. The Barton Moss well will also extend to 3km.
I don't have a particular point to make with this post, I just came across Ian's image while preparing an exam for Bristol's undergrads, and wanted to share how spectacular it is, when you consider the scales involved. Most shale wells in the UK will not extend to 10km. However, the Wytch Farm extended reach wells show what can be done, should the need arise in particular, difficult to reach areas.