In the Volga-Ural basin, however, particularly after recognition of the enormous potential of the deeper Devonian strata, drilling targets were further below the Earth's surface. Moreover, the older, more lithified rock of the Volga-Ural basin was harder. This required higher drilling torque, which in turn demanded superior strength drill-string steel. The Soviet steel industry was basically unable to provide high-strength drill string in volumes necessary to develop the basin.

Engineers responded with turbo-drilling, which does not depend on rotating the drill string. Instead, immediately above the bit, they placed a turbo drilling motor, which itself did the work of turning the bit. This obviated the necessity of twisting the pipe and thereby reduced the required quality of steel.

Turbo drilling radically increased the productivity, Combined with the growing number of rigs available, the total number of feet of development drilling conducted per year nationwide jumped from 1.9 million feet in 1949 to 7.1 million feet by 1950 and 12.1 million feet by 1960."

Putting the motor at the bottom end of the drill string had a couple of other advantages. One is that it allows the hole to make angle, i.e. to turn in a tighter radius than if the whole pipe were rotating.

Motors can accomplish 13º-15º per 100 ft doglegs, while turn radii of rotary steerables are 10º or less.

Suddenly we did not have to have the complexity of joining 30-ft lengths of drill pipe together to deliver power to the end of the bit. This had always been constrained by the steel strength and joint limitations, now that could be designed out, and the power could be delivered to the bit hydraulically through the mud, since this could be used to drive the motor. Unfortunately because the drilled holes are small, the motors have to be small also, and this means that they started out being quite long. (A case history or two can be found on the web.)

Turbo motors work best at higher speeds, but to create the chips and achieve effective drilling with conventional tri-cones, rotation speeds had, historically been slow. And the problem remained of creating the high thrusts across the bit that were required for this type of drilling, when the motor turned faster.

One answer came in response to a second problem. As the rocks that have to be drilled become harder, so the forces used to cut through them also up, causing a materials problem. The materials used to make the drill bits were breaking under the higher forces. But until now we had tried to break the rock in compression by pushing the tooth into the rock (remember that old miner).

If, instead we dragged the bit across the rock without trying to chip it, in the same way as a metal-cutting bit on a lathe peels off a layer of metal, maybe we could lower the forces on the bit. And if we used a diamond tool to do this, then while each diamond insert would only remove a very small amount of rock, we could impregnate a whole bit face with small diamonds ( much cheaper than the single stone you buy for the intended, since they are much smaller, and more common). These diamonds can be dragged over the rock face and slice off very thin layers, but can do so when moved at a very fast speed. Putting the two together meant that a new drilling concept could be developed, and a new drilling bit.

Some pictures of the development of these tools have been given by Schlumberger.
They also show the next development. This came about with the development of larger polycrystalline diamond compacts (PDC's or PCD's depending on your level of technical correctness). By making these larger diamond coated discs and setting them on the drill bit it was easier to circulate the mud so that it kept the diamonds cool. This is important since, if you get the temperature of the inserts above about 3-400 degrees, the diamond starts to soften a bit and wears faster. In this regard the design of these bits is still not perfect, but it has become better. Some of the earlier work on this was helped along by Sandia Labs, but unfortunately most of the early work was in the days before they posted their work to the internet.

So now we have a tool that can make all sorts of bends and that can drill rock that is harder, and relatively quickly. DoE still considers this an area for further work and is continuing to fund further development. We'll talk about some of those new ideas more in future posts.

This is a series of highly informal posts that are aimed at giving some background to what goes into drilling and production from oilwells. Earlier posts in the series are:


the drill


using mud


the derrick


the casing


pressure control


completing the well


flow to the well


working with carbonates


spacing your well

directional drilling 1
As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.