You can't squeeze blood from a stone...
But with 1000 bar of water pressure, you can shatter shale with the fists of Poseidon, and squeeze out precious, precious methane.
This is The Fracking Thread!
Fracking is an old technique, frequently used for increasing production from worked-out oil wells, but has found new application in unlocking natural gas from tightly-held geology such as shale.
Let's step through this apparently brutal, yet sophisticated process.
In short, you need to:
-Drill vertically to the shale, then horizontally along the shale layer.
-Case & cement hazard areas.
-Use a perforating gun to start the fracturing process.
-Pump fracturing fluid to extend fractures.
-Gas production.
We'll go over these in turn.
Firstly, what is shale gas?
Hydrocarbons are produced over millions of years from deep buried organic matter. Conventionally it can rise into a porous reservoir rock and become trapped under a cap layer.
Or it can form, reservoir *and* be trapped in impermeable shale.
For usable thermal shale gas to form it must be:
>2% organic matter.
Below 600m depth.
Exposed to suitably high pressure & temperature.
Be >20m thick for economic extraction.
Bedrock structure preventing gas from escaping: e.g mountain chain folding, fault movements & uplift.
How do you turn a vertical bore to a horizontal one? Many ways...
Whipstocks.
Directional jets.
Steerable mud motors.
Steerable PDC drill bits.
Downhole mud motor.
A lobed steel rotor rotates from hydraulic pressure inside an elastomer-lined stator with more lobes than the rotor. The number of lobes trade RPM for torque as shown.
The drill fluid also cools the bits and stabilise the shaft in turn.
Where is the drill bit?
Thousands of feet down, sometimes miles away.
Arrays of accelerometers and magnetometers measure inclination (up/ down) and azimuth (compass heading, following magnetic North surveying). Together with drill string length, this allows precise tracking.
What am I drilling?
Data is tracked while drilling, from the vitals:
-Torque, vibration, temperature, pressure.
To surveying options:
-Natural gamma ray sensors, electric resistivity, neutron porosity. These detect density, salt water, hydrocarbon presence etc
How do I transmit the data?
The most steampunk way of all: With a mud pulser!
'Mud' is a general term for drill fluid, used to cool the bit, carry away cuttings and pressurize the shaft.
A pulser transmits low bit-rate data through pressure pulses in the mud itself.
Wellbore casing.
A layered wall of cement & steel providing support for the wellbore, protecting against over & under-pressure, isolating the surface from high pressure zones and preventing contamination of geology or groundwater.
Not just a pipe.
Different casing stages isolate surface zones, production zones etc per engineering requirements. They start wide and get narrower with depth, and feature 'feet' that support cement filling between casing & wall to support & isolate zones.
The perforation gun.
Casing & cementing is well & good, but when we get to the extraction zone we need to be open to the reservoir to start fracturing!
A shaped charge cuts through casing & cement and drives fissures into the shale to start initial fracturing.
Hydraulic fracturing!
Fracking fluid is pumped into the reservoir fissures, over a mile below the surface, at 1,000 bar using huge 2,500hp reciprocating pumps. This places huge stress on the fractured rock, opening the fissures to 200ft-400ft in length.
It is done in stages...
1) Pre-flush stage.
A thin, low friction solution goes first, to open up fissures, lower frictional losses from subsequent stages and cool down the rock so that subterranean heat does not affect subsequent flow viscosity.
2) Pad stage.
High viscosity fluid is pumped in to enlarge the fractures further. This viscous fluid will hold sand, or proppant, in the third stage.
The extent of the fractures are monitored by micro-seismology to ensure they stay within the reservoir.
3) Proppant stage.
Proppant (usually sand) is mixed in with the viscous fluid and injected into the fissures to keep them open once the pressure recedes and prevent the fissures from collapsing, to allow gas extraction to occur.
4) Flush stage:
At the end of slurry placement, a volume of clean 'flush' is pumped in to clear tubulars of proppant. The pressure is then bled off to allow the fractures to close onto the injected proppant.
Chemicals.
Fracking fluid is almost entirely water, but it's carefully tailored to it's specific use, and some of the potential 'ingredients' can be hair raising...
So preventing groundwater contamination is crucial.
But a note of sanity: Fracking takes place over a mile below water tables, in impermeable shale. Waste water can be taken away for reprocessing.
There are risks, but it has the potential to be safe, *if* it is well regulated and kept away from water sources and communities.
It's getting more efficient.
Up to 140 wells have been fracked from a single site, up to 3 miles away.
Measurement & analysis is ever improving.
Waste water recycling is becoming more common.
And practice makes perfect...
The USA is fracking central, with over 1.7 million fracking wells completed. It's a vital link in the energy security of the entire world.
And when Russia turned off the taps in 2022, American LNG rode to the rescue of Europe.
So let's sing a hesitant hymn for this reviled monster, who toils in the gloom for us all. -That even if we do not appreciate it, we might understand it.
Here's to Fracking!
There's a downloadable lecture series for those who are interested, available on Researchgate.
I hope you enjoyed this!
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