The production of hydrocarbons from shale has dramatically increased over the last decade. For example, the figure below shows the increase in production of gas from 2004 to 2014 in the Bakken. This shale boom has been driven by the advent of two relatively new technologies, directional drilling and hydraulic fracturing. These technologies, combined with high oil prices, have allowed for the economical production of oil and gas from shales.
What’s different about shales
In a typical conventional reservoir a source rock, a rock containing a lot of organic material is baked beneath the earth releasing hydrocarbons which rise upwards through a reservoir rock, a rock with high permeability, before being trapped beneath a caprock. To access the hydrocarbons an oil company will drill through the caprock and pump the hydrocarbons which are trapped beneath it. This is very different than shale reservoirs. In these systems the source rock and the reservoir rock are the same and they have very low permeability. In a shale play the way to access the hydrocarbons is to drill horizontally into the shale, then frack to increase the permeability and release the hydrocarbons, which then flow into the wellbore. So why are shales so much less permeable than conventional reservoirs?
The answer lies in the structure of shales. Shale is a fine grained sedimentary rock composed of mud that may include clay minerals and organic material called kerogen. Within this structure are tiny pores, known as nanopores. These pores are so small that over 2,000 nanopores can fit within the diameter of an average human hair!
The diameter of human hair can fit over 2,000 nanopores. The diameter of hair is about 50 to 70 microns which is equal to 50,000 to 70,000 nanometers.
In a shale reservoir, these tiny pores are filled with oil and gas. Unfortunately, due to the small size of these pores, the permeability of shale is about 9 orders of magnitude less than that of a conventional sandstone reservoir. Using directional drilling and hydraulic fracturing we are able to increase this permeability and economically produce from these reservoirs.
Pores and organic matter at the nano-scale (Ruppel and Loucks (2008))
Image of a nanopore in shale. The width of this nanopore is just over 200 nm although they can be as small as 10 nm and possibly smaller. To give you an idea of how small this is, 1 inch is equal to 25,400,000 nanometers.
When producing shale gas there is a huge amount of gas that is retrieved at first then production slows down, this is because of how the gas is stored in the shale. Companies have to continuously drill wells in these shales to maintain production. To dive a little deeper we will have to look at what happens in the shale when you frack.
What goes on inside shale
There are three main processes which produce gas from a shale play.
- When a well is fracked the gas that is released is free flowing gas found in the nanopores. The fracking operation connects pathways from the nanopores to the wellbore allowing the gas molecules to move to the well in a process called ‘advection’.
- After the gas in the pores has been removed the next process that produces gas is known as ‘desorption’. This is gas that is bound to the walls of the pores that can now be released because the pores have emptied.
- Finally, when the gas adhering to walls has been released, gas within the kerogen moves through a process called ‘diffusion’ to the walls where it can then move through desorption into the pores and then flow via advection to the well bore.
Each of these processes occurs on slower time scales which causes gas production to quickly drop in a new well but continue at slower rates for an extended time.
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