Drilling with diamonds

Today I am going to talk about diamonds, and how re-making them can produce a much better drilling bit. Last tech talk I briefly reviewed how a tri-cone bit worked. These are the bits that are used in most oil wells, and while they crush the rock immediately under the bit teeth their most productive work comes in creating chips from the rock between two adjacent tooth indentations by a combination of wedging and uplift that breaks the rock under tension and shear.
Simplistic view of bit tooth penetrating rock.

As the bit is pushed into the rock it crushes the rock immediately under the tooth, and this crushed rock distributes the applied force around the edge of the zone as it plastically deforms. This gives the lateral and upward forces on the surrounding rock that causes it to crack and spall from the solid as a chip.

There is, however, a problem, in that some rock is too hard to be able to push the tooth into the rock and create a big enough crushed zone to do much good with conventional materials. So, obviously, the next step is to go to a harder bit material. And to make sure that we can cut into the hardest rock material, it is logical to want to use the hardest material, diamond, for the bit tooth. Historically, however, finding and being able to afford diamonds in the sort of half-inch size we need for the bit tooth was a bit difficult. The ones that were affordable were the very small diamonds known as industrial diamond, in the very small sizes. So the way in which we had to cut the rock was changed. Instead of having a few teeth that rotated over the surface of the rock, instead the face of the bit was coated with a thin layer of small diamonds set in what is called the matrix.

Diamond coring bit, the dark specks are individual diamonds (Source)

The very small diamonds scratch into the surface of the rock, in the same way that a diamond in a ring might cut into a glass plate. Individually the scratches are small, but if the core bit is pushed into the rock, they accumulate and remove rock, although quite slowly. A tri-cone bit might go through rock over 100 ft an hour, a conventional diamond bit which is taking much smaller bites, will go at only a few feet an hour. It also turns faster than a conventional bit, which is why they are often combined with a down-the-hole motor on the bottom of the drilling string to give this faster speed.

And if you push too hard on the bit, then you can push the diamonds down into the matrix, so that the matrix is rubbing against the rock, rather than the sharp diamond edge, and this slows things down. (This is particularly a problem if drilling through granite, which I once spent a summer doing). And so the question came as to how to make this sort of drilling faster. We would still need the diamonds, to cut into the harder rock, but couldn’t we find a better way of making an artificial diamond – after all we only need to have it on the surface of the bit, and perhaps just apply it as a coating to a bit.

Well it turns out that this was possible, and depending on which convention you adopt the Polycrystalline Diamond Compact (PDC), or the PolyCrystalline Diamond (PCD) was born. Simply put (and the technology is actually anything but) a thin layer of diamond power is put into a mold and a central core of tungsten carbide is then nested in the middle of the mold. The mold is then put into a special press where the assembled powder is subjected to extremely high pressure and temperature, using specially designed anvils. Temperatures are in the 2,000 oC range, and pressures around 60,000 bar (882,000 psi). The result is an element (they come in a variety of shapes) where the carbide bit is coated with a thin layer of a polycrystalline diamond, since all the diamond particles have fused together to form the surface layer.

(In reality the technology is a bit more complex, since a single thin layer of diamond is brittle in the way the shell of an egg is, without strong back support, and so there are graded layers to make this “diamond” shell tougher so that it does not shatter when it hits the rock hard).

The most typical shape that is used in oil and gas well drilling is a small cylindrical insert, with the bit made up of a number of these individual cutters.

Individual PDC cutters. The dark portions are the diamond coated segments (Source).

There was a considerable effort put into designing the best way of combining these cutters, back during the last Energy Crisis, with a lot of the work being done at Sandia Labs. The bits that have emerged are now much larger and more robust, and are quite widely used. Energy Tomorrow featured a picture of one back in May.
Large drill bit combining the buttons of a tri-cone (the silvery points) with polycrystalline diamond cutters (the dark circles). (Source)

Notice that the diamond cutters are along the edge of the rigid parts of the drill bit. This is because the diamond, although a very powerful cutter, is very sensitive to temperature. Since the cutter is being dragged over the surface plowing up and peeling off a slice of the rock, there is a lot of friction under the cutting point, and if the cutter heats up above about 300 degrees then it softens, which is not good. So by placing it on the face of the bit, and with an open passage to the face, cooling mud (of which I will write more next time) can now flow across the face of the cutter, keeping it cool, and thus sharp, and able to cut through all the rock in the way. (If you were to look at the full face there are more cutters in the center of the bit and along the edge to make sure that none of the cutters is asked to cut too much – remember that the whole bit is turning, so the cutters on the outside also move faster over the rock).

This type of cutter is now large enough that can now cut deeply enough into the rock that it can chip some of the rock out ahead of it and so the process also becomes a little more efficient. (But I will revisit that topic when I talk about the energy of different rock drilling methods in a later post).

One of the reasons that the support for the cutter is so long is that the edge of the cutter is pushed into the rock, and still has to crush the rock under it, to get enough purchase to be able to chip out the rock ahead of it. (Though one that starts less rock has to be crushed as the bit moves forward).
Schematic showing how dragging a PDC cutter across a rock surface will crush the rock under the bit, and then create a chip as the cutter gets under the rock ahead of it.

As usual with these tech talks, I have simplified the description in order to keep this short and to the point. If those knowing more wish to comment, please do so.