Saturday, 24 September 2011

Why shale gas reservoirs need to be fracked

Seeing as I'm on a roll today (it's a cloudy Saturday, and I'm a little hungover with no current desire to do anything but lie on the sofa and mess about on t'internet) I thought I'd add one last post that might help explain why we need fracking to extract gas from shale formations (this will also give me a chance to test out how blogger handles images). This comes from a simulation I use to teach undergraduates at Bristol about fracking and shale gas.

In the first example, we try to extract gas from a shale reservoir with 5 vertical wells, and no fracturing. The low permeability means that gas is only extracted from the regions immediately adjacent to the wells. The resulting gas production rates are plotted - a paltry 3000m3/day.

The second example shows what happens if the wells have been fracked. A high permeability zone (in green) now extends from each well. This means that we can now access the gas from a much higher proportion of the reservoir, as it flows into the fractures and then into the wells. Now, the gas production rate hits a peak of 500,000m3/day during the first year, before tailing off to a rate of 50 - 100,000 m3/day. Much more gas, much more money. All hail fracking!

Or so it would seem. However, this simulation also highlights a possible issue with shale gas and fracking. After the first year or so, production often tails off dramatically, meaning that new wells must be drilled, and new fracking operations carried out, almost constantly to keep extracting the gas. It's not like some conventional field where you drill a well, and then it keeps producing for the next 20 years. Good news for drilling engineers and geologists who'll be kept in a job, but it makes extraction more expensive, and the more you frack, the greater the potential for things to go wrong.

Note of acknowledgement - this reservoir simulation was creating using 'Tempest', which is developed by Roxar Ltd.