Fracking Nukes: Counting the kilotons

The first serious attempts to extract gas from impermeable rock in the late 1960s, using the sort of extreme fracturing now routinely used for shale gas extraction, involved the use of nuclear bombs (part of the ill conceived Operation Plowshare), to fracture the rock and release the gas. In 1967 Project Gasbuggy detonated a 29 kiloton nuclear bomb 4000 ft below the Carson National Forest, New Mexico. Two further tests (Project Rulison in 1969 and Project Rio Blanco in 1973) attempted to fracture low permeability sandstone reservoirs in a similar way. Unsurprisingly, the gas that was obtained from these tests was too radioactive to be used. The recent development of massive slickwater hydraulic fracturing to achieve the same ends, along with directional drilling, is what is enabling the exploitation shale gas in the US and around the world. What few people realise is that the scale of these modern hydraulic fracturing operations are very similar to these previous tests using nuclear weapons. See If fracking has been happening since 1947 what is there to worry about? for more details.

We were recently asked for more details on how to calculate the energy of a particular hydraulic fracturing operation in order to compare it in scale to a nuclear bomb, and thought that it would be best to share the information with everyone so you can all quote the size in kilotons of your favourite frack job. So here we present a quick tutorial on how to calculate the energy associated with a frack job. The calculation is necessarily approximate. There are many different components that could be considered including the work done opening up the cracks in the shale and frictional loses involved in pumping the fracking fluid through the pipes, and ideally the calculation would take into account all these factors. However since you are unlikely to have the detailed information about the frack job necessary for a more detailed calculation, here we limit ourselves to calculating the energy embodied in the fracking fluid itself while under pressure. This is easy to calculate with limited information but will provide only a lower limit on the size of the frack job since a number of factors are not being considered. On the plus side though only two quantities are needed to do the calculation and if necessary they are relatively easy to estimate if you do not know precise figures.

The calculation is based on the fact that the pressure of a fluid is in fact its energy density, and so the fluid pressure multiplied its volume is the energy contained within a fluid. It is really a very simple calculation the main issues are just converting all the values to a consistent set of units. The relevant conversions to and from S.I. units, from the units usually used by the industry, are:

1 psi = 6895 Pascals (Pressure)
1 US gallon = 0.003785 cubic metres (Volume)
1 Ton of TNT = 4.184 billion Joules (Energy)

Therefore the equation for the energy in the fluid is:

which can be simplified to:

So the taking example of a frack pad with:

16 wells per pad
7 million US gallons of fracking fluid per well
10,000 psi fluid pressure

you would used 16 x 7 = 112 million for the fluid volume and 10,000 for the pressure which results in a total energy is equivalent to 7 kilotons of TNT.

Note that if you do not know any of these numbers for the case you are interested in then you can usually make educated guesses that will not be too far off. It should be born in mind though that as a result the process of extreme energy, fracking is not a stable technique, but one that is constantly evolving as it is adapted to new areas and situations. When making estimates of these properties you should always make sure that you use the most appropriate examples that are as close as possible in time and space to the case in question. For instance the average volume of fracking fluid used per well has been steadily increasing over time as the length of the laterals drilled has increased. The number of wells per pad has also been increasing in recent years. In general the pressure used will be affected by the depth and thickness of the shale formation being fractured. Understanding these trends will allow you to make more accurate estimates of these quantities.

Example calculations of energy in kilotons of TNT for a variety of hydraulic fracturing operations

Examples based on some scenarios for various fracking operations can be found in the table above. It can be seen that modern large frack pads involve energies comparable with those released by a nuclear bomb. In comparison the bomb dropped on Hiroshima had a yield of around 16 kilotons and the one dropped on Nagasaki had a yield of 21 kilotons. Of course the results of these calculations are only approximate but they clearly demonstrate the scale of massive slickwater hydraulic fracturing. Note that while a nuclear bomb releases its energy in a fraction of a second, a frack job releases the same sort of energy over a period of days or weeks. In some ways fracking can be seen as similar to an underground nuclear detonation in slow motion. While it may take longer to occur the amount of subsurface damage done is similar.