Deep drilling in the Earth's Crust

[Opengrave]

Vanilla’s post on oil drilling sort of set me thinking about drilling in the Earth. Specifically I’d like to know how deep current technology would allow man to drill into the Earth’s crust. I did some research and found that currently the deepest drilling has been done to a depth of about 6,421 meters (21,066 feet) http://www.schlumberger.com/ba.cfm?baid=1&prid=4681 but no real answer as to the limit of current technology.

My real curiosity is not about oil & gas but more of an Earth exploration nature. What things might we learn from drilling (core samples) to say, 20,000 meters (65,619 feet) and what technical limitations prevent us from doing so (lets ignore the cost factor and just stick to the technology).

If we could devise an automatic drilling device that would work on the ocean floor would we gain anything from starting at say, the bottom of the Marianas trench, or is that just a downward fold in the Earth’s crust that would still leave us having to drill through the same stuff we drill through on land?

[El_Kabong]

*Originally posted by Opengrave *
I did some research and found that currently the deepest drilling has been done to a depth of about 6,421 meters (21,066 feet)

Before we move on to your question, I’ll just mention that the referenced well in the OP is a world record for horizontal drilling, but not vertical drilling. In this case, the 6,421 figure represents the total measured depth of the borehole, including 184m of horizontal wellbore. The total vertical depth (TVD), measured from surface, was 6,062 meters (19,888ft). There are many wells with much longer horizontal sections, and numerous wells drilled for oil and gas have exceeded this vertical depth. I personally have worked on a well that was drilled to >23,000 feet TVD.

The deepest well of any kind drilled to date was the Russian Kola-1 geologic exploration well, which reached 12,263m (40,233) in 1990. A German consortium known as KTB executed a geologic exploration well in Bavaria in the early 90’s that was originally intended to exceed this depth, but IIRC, was abandoned just short of 12,000m vertical depth due to drilling problems.

Now, on to your questions. All deep wells have been drilled via adaptations of standard rotary drilling techniques used in the oil and gas industry. This involves the use of a drill bit attached to a hollow, jointed drill string through which a drilling fluid is circulated. The drilling fluid a) cools the drilling bit; b) balances fluid pressures in the formations being drilled; c) provides a way to clear cuttings from the bit face and bring them to surface. The drill bit can be rotated from surface by turning the entire drill string or via a fluid-operated motor near the bit.

20,000 meters of vertical depth could be attempted using rotary drilling technology, but probably only in certain locations where the surrounding rocks are relatively stable. Depth-limiting factors would include:

1. resistance of the drill string to rotary and bending forces;

2. bottomhole temperature; temps above about 300 deg. C would tend to break down the drilling fluid and place excessive thermal stress on the drill string

3. rock strength; loads produced by fluids in the pore spaces, or tectonic forces, would make the borehole wall unstable at some limiting depth, no matter what density drilling fluid is used.

Drilling in a deep ocean trench would pretty much be a non-starter, IMO, as the these are actually subduction zones that would feature very high temperatures and unstable rocks. The presence of the seawater column would present numerous engineering complications as well.

I know of no sedimentary basins that are estimated to reach 20,000m of vertical depth, so any scientific knowledge recovered would involve information about the structure of deeply buried igneous rocks (primarily from the cuttings), and such things as subsurface temperature gradients. While there has been some speculation about methane (natural gas) perhaps being brought up from deeper in the crust, IMO most experts in the oil industry do not endorse this view, and production of gas from igneous rocks (which are inherently non-porous) would be uneconomic unless extensive fracturing were present.

Hope this helps.

[El_Kabong]

whoops, forgot the (ft) after 40,233.

[El_Kabong]

whoops, forgot the (ft) after 40,233.

[Spritle]

Rocket, would that make the units squared feet?

sort of off topic…

If one could drill deep enough, would magma spurt out?
I’ve often envisioned a hole 20 feet wide (what? that’s something like 5,000 meters or something?) with a series of locks. This hole goes DEEP down into the ground (like billions of feet or whatever). The purpose is purely waste disposal. We could drop stuff down the hole in stages (thanks to the locks) until the heat of the earth’s core burns/melts it to teensy atoms. Then, perhaps, it could mix with magma and be swirled away. Somehow, the pressure issue must be addressed.

Sorry if this is too stupid for consideration, my degree is in Chemistry, not Earth Geology or whatever this is.

[toadspittle]

“What Ever Happened to Project Mohole?”

http://web2.airmail.net/myself/secular/writing/mohole.htm

Discusses the failure of a project in the 1960s designed to drill through the ocean floor to a place where the Moho (a boundary between between the crust and the mantle) is near the surface, enabling scientists to retrieve samples of the mantle.

The failure seems to have been an administrative one, not a practical one. It should still be possible to do it, if anyone wants to. (Volunteers?)

I’ve seen other sites that mention scientists have been considering trying again in the Indian Ocean, but nothing seems to have actually been done.

[don_willard]

There was a Doctor Who episode in which a mad scientist was drilling down deep and the Doctor said something like, “You don’t know what tremendous forces this will unleash!”
I think this was the story in which on the way down the drill penetrated an area of green slime which eventually gave rise to four-foot larvae that electrocuted people as they writhed around. Or else that was a garbage disposal story…???
I also know that (in real life) there is a Mohorovicic Discontinuity down there that may have been reached.
In any case that was a great response above from the one who used to do the actual drilling.

[El_Kabong]

Dang, don’t know why that correction showed up twice…

*Originally posted by spritle *
If one could drill deep enough, would magma spurt out?

Possibly, although the small diameter of a deep wellbore (usually 6-8 1/2in.) means that it might flow slowly enough to solidify before reaching surface.

I’ve often envisioned a hole 20 feet wide (what? that’s something like 5,000 meters or something?) with a series of locks. This hole goes DEEP down into the ground (like billions of feet or whatever). The purpose is purely waste disposal. We could drop stuff down the hole in stages (thanks to the locks) until the heat of the earth’s core burns/melts it to teensy atoms. Then, perhaps, it could mix with magma and be swirled away. Somehow, the pressure issue must be addressed.

Drilling a hole 20ft in diameter to any significant vertical depth is impractical, although 6ft holes have been drilled to about 2000ft with conventional rigs (these were for underground nuclear testing).

For what you’re proposing, just bundle your trash into containers that are heat resistant and impervious to salt water, then start chucking 'em into an ocean trench that is a known subduction zone. A few centuries later, and voila!, they’re gone…

[Opengrave]

*Originally posted by Rocket88 *
The deepest well of any kind drilled to date was the Russian Kola-1 geologic exploration well, which reached 12,263m (40,233) in 1990.

Thanks for the clarification - I saw a page about the Russian hole but wasn’t reading either site closely enough to catch the vertical vs. horizontal aspect.

2. bottomhole temperature; temps above about 300 deg. C would tend to break down the drilling fluid and place excessive thermal stress on the drill string.

Is the bottomhole temprature increase a function of depth (greater depths mean more pressure on the drill head due to the weight of the drill string) - surely there is no geothermal heat at such a shallow depth or am I way off here?

Fortunately I was born in east Texas have a basic working knowlege of many things oil & gas… that is why this issue sort of puzzled me - thanks for your information. Wait, did I just use the term ‘bottomhole temprature’ in a serious post???

**
[/QUOTE]

[CC]

Yep - it’s finally happened - I’ve waited many years to see the Mohorovicic Discontinuity written someplace. I’d heard of it years ago, maybe in some Urpie Classroom Films movie I showed my students, and I’ve been aware of it since then. I’ve tried to drop it into casual conversation, but it’s really hard to do this. Then, I see this thread and think, could it be here? Could it? YES! This is really a wonderful way to waste time. The SDMB. I love you, man!

[ScoobyTX]

[QUOTE]
*Originally posted by Rocket88 *

*Originally posted by spritle *

I’ve often envisioned a hole 20 feet wide (what? that’s something like 5,000 meters or something?) with a series of locks. This hole goes DEEP down into the ground (like billions of feet or whatever). The purpose is purely waste disposal. We could drop stuff down the hole in stages (thanks to the locks) until the heat of the earth’s core burns/melts it to teensy atoms. Then, perhaps, it could mix with magma and be swirled away. Somehow, the pressure issue must be addressed.

For what you’re proposing, just bundle your trash into containers that are heat resistant and impervious to salt water, then start chucking 'em into an ocean trench that is a known subduction zone. A few centuries later, and voila!, they’re gone…**

I have wondered why this isn’t considered for high-level nuke waste disposal. By the time it circulated around & spurted out of a volcano, it should be decayed to a safe element (100k/millions of years?)

[El_Kabong]

Is the bottomhole temprature increase a function of depth (greater depths mean more pressure on the drill head due to the weight of the drill string) - surely there is no geothermal heat at such a shallow depth or am I way off here?

Actually, there are two separate issues in the passage quoted above.

Temperature: from my experience, the so-called geothermal gradient varies from place to place but averages about 3deg. C/100m of depth, starting from whatever the surface temperature happens to be. Now, the temperature of a formation at, say, 3000m can already be higher than the boiling point of water, but a given volume of the circulating drilling fluid is adjacent to this zone for only a few moments, therefore only a fraction of the heat is transferred to the fluid and drill string. It is possible through downhole telemetry to measure the temperature of the drilling fluid at the bottom of the hole while circulating, and I don’t recall seeing any figures higher than about 170deg C, even on deep wells, and a lot of that was due to frictional effects related to pumping the fluid, rather than heat transfer from the formation. As I’ve tried to point out, however, the actual formation temperature was undoubtedly much higher.

Pressure: it is a general misconception that weight is applied from above to provide the impetus for the bit to drill, and to resist formation pressures. In fact, nearly all the drill string is kept in tension at all times; if this were not done, the drill string would buckle, putting unacceptable stresses on the jointed connections. Furthermore, applying the full weight of a typical drill string would crush the bit to powder within seconds. The weight necessary to allow the bit to drill efficiently is applied from a few lengths of heavy-walled drill pipe (known as drill collars) installed just above the bit. This ensures that the bulk of the string can remain in tension, as required.

Finally, the pressure necessary to maintain hole integrity and prevent invasion of formation fluids comes more or less entirely from the pressure of the column of drilling fluid in the hole. Finely-divided solids are added as needed to the drilling fluid to increase its density (and thus the pressure exerted by the fluid column). The drill string is only used as a fluid conduit and a way get the bit to bottom.

OK, I’ll stop rattling on now.

[Fillet]

don willard, no one’s reached the Moho yet. A good place to try would be near one of the mid-ocean ridges, where the ocean crust is already thin. Haven’t heard of any plans to do that, though.

Opengrave, I see on preview that Rocket88 has once again fielded your questions perfectly. (Don’t stop now, Rocket88! ) On an interesting related note - the South Africans are considering developing special automated equipment to continue mining below depths of a couple of miles, because it’s just too hot for human workers to cope. http://deepmine.csir.co.za/

TXLonghorn, there are environmental concerns related to dumping radioactive waste on the sea floor, trench or no trench. Some folks are reluctant to see what might happen if marine life is exposed to mutation-inducing levels of radiation. Godzilla, anyone?

[don_willard]

For excellent cocktail conversation (stopper) there’s almost nothing like the Mohorovicic Discontinuity, although
a reference to the Erpi or Urpi Classroom Films excells it! I thought I was the only one left who remembered these grim teachings and the globe of the world turning around and around wrapped with something and “Erpie Classroom Films”
written on it or below it or someplace. High schools used to employ a trustworthy student to bring the projector and run these entertaining movies. One was about HOSTILITY, and showed these people being hostile and gave the attributes of it so everybody would think they had this mental problem.
Remember the glass cases they would bring in every month and hang on the wall and they would have a stuffed raccoon and some artificial branches in them, or a set of squirrels and some acorns? These were very educational. I understand now they can’t put anything like that in classrooms because the pupils would destroy them, along with the plumbing in the bathrooms.
There also used to be special visitationsby authorities such as THE SCHOOL COUNSELOR, THE MUSIC LADY, and
THE DISTRICT SUPERINTENDENT OF XXXXX. The music lady would sit at the piano and play in a big print dress and you felt like you were actually doing something singing, “Just a song at twilight, when the lights are low, and the flickring shadows softly come and gooooooo…” She was more dressed up than the teacher and I was impressed.