Not finding what I was thinking of, so maybe I am confusing it with those exotic ice forms (and water existing in glass form)? Which gets me wondering, definitionally, can water ever be “rock”?
ETA thanks @TriPolar.
Crystalline minerals are not the only rock-forming components - other non-crystalline rock components include opal and amber
Yes. Glaciers and ice sheets would be monomineralic rock consisting of the mineral ice. And naturally-occurring ice is definitely a mineral.
Thanks. Interesting.
And a major structural component of rocky bodies in space past the snow line. This photo from Titan is ugly rocks of mostly water.
We are used to ice so close to its melting point that it is extremely easy to sinter.
There are a few issues with deep drilling, temperature is certainly one of them.
Firstly it should be noted that subsurface temperature gradients vary widely , if you go to Iceland you don’t have to drill far before things get zippy, however in the Gulf of Mexico you can be 30,000 feet true vertical depth ( with the first 10,000 feet being water) and still only be at 125C, them move 20 miles and be at the same depth but at 300C. So deep doesn’t always mean hot and vice versa, but if we are talking super deep then sure deep is hot.
If for a moment we ignore fracture and collapse gradient and pore pressures of the formations we are drilling through , which we will come back to , and could drill an 8 1/2 in diameter hole from surface as deep as we want to go , and we just wanted to go straight down so no directional work , what stops us?
Drilling uses a drill bit and behind it drill pipe that we are circulating fluids down through the bit and up the annulus. 5 in OD drill pipe typically weighs 19lb/ ft and using 135ksi steel has a tensile load of about 560 klbf.
However as you have to rotate the pipe you are applying torque at the top which drops the rating to 420klbf which gets you to 26,000 feet ( we have some buoyancy from the fluid) You could use thicker pipe , but then it just gets heavier faster and exceeds its tensile limits.
The way around that is to use skinnier pipe at the bottom and thicker pipe at the top , so maybe 4 in drill pipe at the bottom and 6 5/8 1 in wall at the top and optimize strength vs weight . However as you drill deeper you will have to keep pulling out and adjusting the amounts of the various pipes.
We could go for higher strength steels say 160 ksi, but they will get harder and more brittle , and expensive. As we get hotter the steels start to get softer and the tensile strength drops , needing further optimization of all the different weights and diameters of the drill pipe.
Lighter materials have been tried , aluminum drill pipe has all kinds off issues and has lower tensile strength so you need thicker pipe and really doesn’t help much , plus worse temperature issues . Titanium alloys have been tried, and other than really expensive also lose strength with temperature. For sure it is hot down at the bottom where the load will be lower, but the fluids we are circulating will heat everything else up up the drill string . Insulated drill pipe is a thing but rare.
Assuming we find a balance of pipe strength , weight and temperature , next stop is the bit being used to cut the rock , most cutting elements are polycrystalline diamond cutters (PDC) and those cutters start to degrade in wear performance over 400C , and as the rocks typically get abrasive that is another issue . Artificial diamonds impreg bits will go higher , but need higher rotary speeds to cut, which impacts the drill pipe wear. Diamonds don’t like iron either. There are some higher temperature rated artificial diamond like things, but the tend to have lower toughness and would struggle with compressive loads needed to break the rock. So yes we can drill hot stuff, it would just be a lot of trips in and out to keep changing the bit.
That’s all to say , yes even for the simplest concept of drilling straight down, and ignoring most actual real world issues ( pore pressure, collapse pressure, fluids needed to stop bad things happening , the casing needed to support the hole and cement , thermal stability of the fluids, any need to actually measure what is being drilled through or maintain the direction, the drilling rig needed and the cost of it all) we would need some significant improvements in just the metallurgy available for the pipe and cutter technology, and big bags of cash. Big big bags of cash.
Some work is being done on plasma drilling and millimeter wave drilling that could remove some of the constraints of needing the pipe to carry the energy to break the rock, but those are a way off.
Gosh, Tom Swift didn’t have all those problems when he drilled to the Earth’s core with his atomic earth-blaster and got gushers of molten iron in the Antarctic!
That’s because Tom looked at the problem of drilling deeply , in depth.
Fascinating! .So the liquid that is pumped down and then comes back up:..Shouldn’t that function as a coolant for the drill bit??
T
I believe it does, but it must have limits.
Yes it acts to cool the bit, and amongst other things it also carries the rock cuttings back to surface, cleans the bit , applies some hydraulic force to help cut the rock, provides lubrication between the pipe and the wall of the hole , it has to avoid chemically reacting with the rock itself, and probably most importantly , the hydrostatic pressure of the fluid prevents the hole from collapsing , or influxes of fluids contained in the pore space of the rock ( in oil and gas wells that would be a kick or a blow out) but care is needed if the fluid is too dense and the bottom hole pressure gets too high you end up fracturing the rock and things get messy. Balancing bottom hole pressure with rock strength is a difficult problem and that’s where strings of casing ( metal pipe ) have to be run in and the cemented in to isolate some formations. The well ends up looking like an extended telescope with the hole getting progressively smaller in diameter as you have to isolate the formations as you get deeper.
And yes it gets harder to keep the fluid cool as you get deeper . The hot fluid coming up the annulus heats the cool fluid going down the pipe bore, so the cooling gets progressively worse. I mention insulated drill pipe was something that was developed to help with that but it’s very unique and has its own challenges in strength etc