Drilling advances

	Vol. 227 No. 5
LES SKINNER, PE CONTRIBUTING EDITOR

Steering the bit. The drilling industry is steeped in traditions most of which change slowly over time, if they ever change at all. For generations, drillers endeavored to drill straight, vertical holes. The tradition came from cable tool drilling, which required vertical holes for the system to work. Then, as technologies improved and the need for “slant” holes became apparent, drillers decided that they could drill directionally.

Thanks to Mr. Hughes, the rotary bit replaced cable tools, but holes were still drilled vertically. The inclination angle was measured with a “single shot” inclination tool that used gravity to mark a tin or brass sheet using a stylus. Unfortunately, Mr. Hughes’ original bit was a two-cone bit, which we now know is an excellent bit for building deviation angle. Upshot: some of the new rotary holes began drifting off vertical. To the surprise of many, drilling continued in these early crooked holes. The problem is, nobody knew precisely where they were going, only that they were not vertical.

All this changed in 1929, when John Eastman invented and patented the multi-shot survey tool. That instrument, along with the open hole whipstock resulted in wells drilled to offshore targets from onshore sites in Huntington Beach, California.

The whipstock and similar devices were used for the next decades in “push-the-bit” directional drilling. Pendulum bottomhole assemblies work much the same way with drill collar weight providing the lever and stabilizers placed strategically in the string acting as fulcrums to push the bit and build hole angle. Later, bent subs and bent-housing motors accomplished the same purpose.

Slide drilling with a bent motor builds deviation angle and it creates a smooth wellbore curve as long as sliding continues. Slide drilling is accompanied by rotary drilling with the toolface of the bit changing continuously. The result is a section of straight hole below the curved hole made while slide drilling. If extreme angles are required, a segmented hole configuration in the angle-build portion of the hole results as slide and rotary drilling are alternated.

In the past, a segmented, curved-straight-curved hole shape was acceptable. Now, there is a need for longer horizontals and long, extended reach wells. The segmented hole shape creates too much torque and drag to drill these long-reach wells. So, a new concept was required, something that would “point-the-bit.”

In this new type of drilling, the bit toolface is continuously steered at the proper angle and azimuth to create a smooth curve. Rotary-steerable systems along with MWD and LWD capability obviate the use of slide drilling to build angle, while alleviating the greatest weakness of slide drilling – keeping the toolface stationary by not rotating the drill string.

Hole cleaning is always a problem in directional wells. In near-vertical drilling, cuttings carried by the mud are evenly distributed in the mud column around the drill string. In horizontal hole sections, cuttings migrate to the bottom of the hole. Fortunately, that’s also where the drill string resides. Each time the string moves, cuttings are stirred and re-suspended in the mud (assuming cuttings have not trapped the drill string). Usually, slide drilling in horizontal wells is limited to minor directional changes, so the drill string is stationary only for short time periods, not so in the angle-build section.

When the hole deviation is between about 40° to 55°, cuttings migrate to the “low” side of the hole. Often, the drill string is held in tension on the “high” side of the hole. As drilling continues, an unusual effect takes place. The collected cuttings begin to tumble down the slope, avalanching. The longer the drill string is stationary, the more avalanching continues creating thick cuttings beds in the annular space below and around the drill string. When pipe is picked up for a connection, these cuttings beds are dragged or “plowed” upwards wedging above drillpipe connections. This results in stuck pipe and bridging.

In rotary steerable drilling, the drill string rotates continuously. Most systems use a stationary, non-rotating sleeve equipped with pads or “blades” that rest against the inside of the hole. These pads can extend or retract using hydraulic cylinders to raise or lower the pads thereby changing the fulcrum to produce the correct angle. They can also be used to exert side-thrust to point the bit at the proper azimuthal angle. Best of all, the tool is programmable, so the hole shape matches the desired well path. Even better, the tool can be re-programmed.

Downhole measurements are taken closely behind the bit, so minor deviations can be detected early and corrections are made almost instantaneously by the computer controlling the sleeve’s pads. The result is a beautifully smooth wellbore that promotes long extensions with reduced friction, torque and drag. This is produced not only from the smooth profile, but also from improved borehole cleaning by constant rotation of the entire drill string. 

Most of these systems use mud pulse technology for both data transmission from downhole devices and rotary steerable course corrections. Adaptation of electromagnetic telemetry should be possible. Thus, when a course correction is needed, it should be possible to send an EM signal through the earth or an electronic signal down wired drillpipe to re-program the system.

Years ago, I recall standing on a rig floor joking about technology advances with a group of crusty old drillers. I predicted that with the advent of robots in the workplace along with improved measurement and data transmission capabilities, while drilling directional wells, we would be able some day to go to a telephone booth, drop a coin in the slot, call the drill bit and find out what’s going on downhole. I told them I expected to hear a computer-generated voice (with a British accent, of course) that would greet me politely and provide me with the depth, angle, azimuth, pressures, temperatures, rate of penetration and all other information about the drilling operation directly from the “mouth” of the bit itself. If I didn’t like what I heard, I’d just tell the bit to drill in some other direction for a while.

We all laughed then at the notion of a talking bit and of at-will directional changes through a telephone line. As it turns out, that potential may not be too far from reality.