Here's mud in your eye

Today I thought I would again take a little time off from which country is producing what, and talk again about drilling oilwells. Last time, I wrote about physically breaking the rock. In most cases, this is carried by pressing a series of small teeth set in rings around the perimeter of three cones that rotate around the bottom of the borehole, and that are attached to the bottom of the drill string. (picture here and as a timeline development picture here).

The cones are mounted with their largest diameter running around the outside of the hole, and thus each cone will have the greatest number of teeth along that edge. We call the outside edge of the hole the gage, and the three cones, bearings and mounts combine to form the drill bit.

Now if we just turned the bit round and round in the hole, it would start to drill into the rock, but after a short while the chips and crushed rock would fill up all the space between the bit and the solid rock, and the bit could go no further. We have to get the crushed rock out of the way, and preferably before it is crushed by the following bit tooth since that would waste energy.

When the old miner was hand steeling, he could either blow the chips out of the hole with his breath, or wash them out with a squirt of water. On a more sophisticated level this still holes true, but with some differences. In very short holes, or special circumstances, compressed air can be used to blow the chips that have been cut from the rock (hence the name cuttings) up out of the hole. However as the hole gets deeper this becomes less practical and some form of liquid must be used. Again it could be water, but there are several reasons why this is not usually the case.

Firstly the rock is much denser than water, and so if the water flow back up the hole (usually between the drill pipe and the rock wall – a gap often called the annulus) is not moving fast enough, then the cuttings will settle back down the hole, blocking the gap and sticking the drill pipe in the ground. For this reason the water is usually mixed with very small particles of different types of material that will increase the density of the fluid so that it will help lift the cuttings all the way from the bit to the surface. This can be as far as 3 miles up or more (15,000 ft or so) and so it's important to choose the right density. Usually the particles that are added to the water are made from finely ground clays, and this makes a mud, and the fluid has thus now, regardless of what is in it, become known as a drilling mud.

The mud has to be thick enough to help carry the individual chips to the surface, but, as we will talk about in the next post on this topic, it also has to keep them held in suspension when the flow stops while another length of drill pipe is being added to the string.

The mud, however, has a few additional things that it must do as it is pumped down to the bit, through the inside of the drill pipe, and then circulates back up the outside carrying the cuttings to the surface. This circulation is sometimes reversed (i.e. reverse circulation) so that the mud flows back up the inside of the drill pipe, but this is not common.

The first thing that it must do is keep the bit cool. As the bit rotates, and the cones turn on their axes there is a lot of friction generated under the thrust being used to push the bit into the rock. Some of this friction will generate heat (in the same way as happens if you press your palms hard together and rub them back and forth) . Because the bit is in a confined hole, without the mud there is nowhere for that heat to go, and so it would otherwise build up, until it got hot enough that the bearings failed, and the bit fell apart at the bottom of the hole. (This is not a good thing to happen since how can you drill through the parts of a broken bit with a new one?) So the mud flow also serves to keep the bit cool enough to keep working.

But there is also a lot of heat that comes from the rock. This is because of something called the geothermal gradient. This is one of the last, almost untapped, sources of energy that we have. While it varies around the world, the numbers where I come from are these: At 500 ft below the ground the rock temperature is 60 degrees F. For every 60 ft you go further down, the rock temperature goes up a degree. So that if you are 1100 ft down, then the temperature is 1100 – 500 = 600 (extra depth)/60 = 10 extra degrees + 60 = 70 degrees F. And at 8,000 ft (a deep gold mine) it will be ((8000-500)/60) +60 = 185 degrees. Which is why they refrigerate the air down there. And this rock temperature in deeper wells can also damage the bit. So the mud flow has to also take account of the depth and the rock temperature.

To keep the bit cool and carry away the cuttings the mud flow has to be quite fast. But there is a small problem that arises (and is not always fixed). As the hole gets deeper the weight of the mud in the hole will press down on the chips that are being formed by the bit. It will try and press them back down against the rock, (it is called chip hold-down). To stop that happening the mud has to be formed into a stream of fluid that can be pointed at the rock and (just as you move dirt with the pressure from a garden hose) will push under the chips and lift them into the circulating mud flow, as it then flows back up the annulus. To do this the mud feeds from the center of the bit out through a set of jet nozzles that point the streams down onto the rock. The deeper the bit is drilling, the closer the nozzles have to be to the rock, in order to have enough power to lift the chips and get them moving.

The third thing that the mud has to do is to act as a seal where the bit drills through the different rock layers. As the hole goes down some of the rocks it will go through are very permeable. In other words the mud can flow into them, or water can come out of them. We don't want that to happen. If the mud flows into the rock, then it is lost, and the circulation of mud stops (its called lost circulation). Without the mud the cuttings settle back down to the bottom of the hole, and the bit is stuck again. So to prevent that the clay particles are made large enough, so that if the mud starts to flow into the rock, the particles cannot enter the very small gaps in the rock. Instead they are filtered out and are left on the edge of the hole. As the water flows into the rock, leaving the clay behind, the clay builds up and forms a layer of clay along the rock wall. This seals the rock from the mud in the well, and stops the fluid from leaking out of the well.

To do all these things, with different temperatures in the well, different chemistries in the different rock being drilled through, and different fluids flowing into the hole, requires that the mud be very carefully selected for each job. And sometimes that chemistry may be such that simple water cannot be used as the carrier fluid. There is a shale in Texas, for example, called the Gumbo shale, that turns from a hard rock to "gumbo" when it gets wet. So if you are drilling through it, either the water has to be treated with chemicals so that it stops being able to wet things, or you use a different base fluid, and change to perhaps an oil. (Googling "Gumbo shale" will give a number of papers on this). So making mud, and using it properly can be an ongoing job on an oil rig.

Again, if there are questions, or comments, let me know.

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This is great stuff. It's very interesting, and somehow confidence building, to hear some of the practical detail.

Good job!!

But if the average well drilled in the Ark-La-Tex is 10,000' or deeper (which is true right now), think about this:

(12000-500)/60+60 = 252

(13000-500)/60+60 = 268

(15000-500)/60+60 = 302

These temperatures assume normal thermal conditions. Around salt domes or around upthrust areas (the Sabine Uplift, for example), these temperatures are typically 50-100 degrees higher.

Now, when you analyze the expenses associated with geothermal wells as power sources, the single largest expense is DRILLING THE WELL.

When a well is finished producing oil or gas, the big expense is PLUGGING and ABANDONING the well, roughly around $50,000 per well.

Both types of wells utilize the same hardware (casing, wellheads, etc.), and from a practical syandpoint, they are identical schematically. But the standard thinking in geothermal is to inject into a series of fractures via one well and recover superheated steam in an adjacent well.

Read the following DOE/NREL article, paying particular attention to the temperatures required for BINARY power system generators...

Finiahed? Ok, then oil companies are PAYING to plug old wells that are ALREADY deep enough for BINARY power generation.

Plug the producing part of the well (seal the perforations and squeeze cement into the formation - all we want is the deep heat). Circulate cold fluid down the annulus to pick up the heat and return it through the production tubing to surface and feed it into the binary power generator, using a very low boiling point working fluid.

Hmmm....why hasn't somebody snapped to this?

I sent numerous letters to the DOE and NREL. Never received a response. I worked out the frictional losses and heat issues, and it is doable using solar pumping, energy stored in pressure vessels for night operation, and earth geothermal to minimize losses of the returning fluid. There is a company who makes small trunkey generators that will fit this and turn out enough to take care of 4-5 homes. With well spacing at 20 acres in northeast Texas and other areas, this would work very well.

I never got a single response from either agency. Not one.

If it isn't BIG and HUGE, there is no interest.

But I already own a few of these wells, and that is in my plan....

It has been a fair while since I did anything with geothermal (though I still have most of the reports). One of the big problems that I seem to remember was that they wanted to hydrofrac the first well, and then drill into the crack from a second, so that they could do the circulation. Bear in mind that this was the late 70's and back then I don't think they had a lot of success with hitting the hydrofrac. A lot of this work was done at Sandia under what I think they called the "Hot Dry Rock" program. Obviously that scheme was on a much bigger scale than what you propose.

Nice piece. I got to hang around some wildcat rigs as a little shaver when my dad occassionally worked as a derrick hand in the '50's. With the safety and liability restrictions a kid couldn't do that these days. I remember those mud pumps as being driven by some pretty big diesels and that the drill stem moved down very slowly when they got very deep. And the trips to change the bit took a long time and they had to be very careful. More details on these tings would help people who haven't been around a rig understand why drilling is an expensive and slow proposition.

Here's a couple of websites of drill bit makers: (Loads of pictures and specs)

(Loads of images)

(Loads of images, specs and even some animations)

My dad was in the oil business his whole life. He retired in Midland, Texas, where there's an excellent petroleum museum that shows among other things the evolution of exploration and recovery technology over the years. You can see and touch the different kinds of drill bits that have been used. Definitely worth a visit if you ever find yourself in the Midland/Odessa region, not that there's a lot else to do there.

When I was in graduate school at UT-Austin almost 30 years ago, I did a bunch of the systems analysis for a project looking at producing energy from geopressured brines. One of the most unusual faculty members of the petroleum engineering group at that time was an old guy with, if I recall correctly, a high-school education and 40 years of experience in the oil fields. He taught an "applied mud" class.

Anyway, the project started as an attempt to tap geothermal energy. But it turns out that there is simply a staggering amount of methane in the geopressured brines under Texas and Louisiana -- some estimates are that there is more than the proven conventional natural gas reserves of the world. Producing the stuff was/is the problem. The brines are also saturated with a variety of nasty salts (toxic and very corrosive). Disposing of such fluids when there are billions of barrels involved is a hard problem. The energy budgets worked out so that it was feasible to put two wells into a reservoir, withdraw the fluid through one, extract the methane and much of the heat energy, and use the heat energy to pump the fluid back down the second well. However, the stuff is so corrosive that the pumps and turbines were going to have a rather short usable lifetime.

I know DOE ran a few test wells around 1980, but they disposed of the fluid using shallow wells -- probably not adequate if attempting it on a large scale. If you can work out and patent a way to produce the methane while leaving the brine in the reservoir, there are boatloads of money to be made.

Stainless steel casing and equipment would mean a hell of a well cost. But the issue of the pumps can be addressed with off-the-shelf stuff.

Do you have any other leads about this? Some names maybe?

I had heard about this as well, but thought the economics wouldn't work. At $90/bbl, lots of stuff works!

And BTW the oil company which my father worked for his entire working life, starting in the lab and working his way up to a VP position, was Unocal. I'd appreciate it if you would not refer to it as a "second rate" oil company. "Second tier" would be a more objective description.

And the old ODECO Ocean Star is a museum piece in Galveston these days for those who want to see an offshore jackup. While it is one of the smallest, it gets the point across in the presentation materials in a very good way about drilling and general operations.

And the experts in this field are of course called mudmen. One guy had a pilot's licence so he could fly out to remote sites frequently. He was called the "Flying Mudman".

Actually, we like to call them mud engineers. It is chemistry "on-the-fly" while we drill through unknown can get very tricky to manage all the necessary properties, I assure you. And there are many guys with pilots licenses that work the barges in the marsh and fly helos in the jungles. It is very common in this business. Most of the big players run float planes just for delivering drill bits in some areas.

Good job explaining drilling technology in layman's terms. Just add a bit on stringing pipe and the dangers of hitting an unexpected gas layer and you'll have covered a lot of the bases.

It's not safety-smart but bet it was cool to see a gusher come in during the old days!