The physical process of grinding, breaking, and scraping away rock layers at the bottom of the borehole is carried out by the drill bit. The drill string, however, is what transmits the rotation provided by the rig’s rotational system from the surface to the bottom of the hole.
When the well is spudded, thick metal pipes called drill collars are screwed directly onto the drill bit as part of the bottom hole assembly (BHA). These drill collars provide the initial weight that will allow a force to be applied to the drill bit against the bottom of the hole. The BHA often include stabilizers, which make sure the lower BHA and bit remain centered in the hole. In addition, other pieces of equipment make up the BHA including mud motors, measurement-while-drilling (MWD) and logging-while-drilling (LWD), jars (used to impart a shock force in case the BHA is stuck), heavy-weight-drill-pipe (HWDP), and other specialized tools as the drilling operations require. After a specific length of BHA has been deployed as determined by the forces needed to effectively penetrate the rock, the BHA is connected back to the surface using drill pipe. These are far lighter weight drill string components allowing the rig to lift the drill string. A drill string consisting solely of drill collars would be inordinately heavy!
As the hole gets deeper, the weight of the overlying drill string begins to increase. Standard drill pipe weighs between 7 and 30 pounds per foot depending upon its outside diameter. That means that by the time the drill bit reaches 5,000 feet of depth, the drill string can weight up to already exerting between 110,000 to 175,000 lbf. Even for a hardened tungsten drill bit, this amount of weight is far too much for effective drilling. Besides, driller’s wouldn’t put that much weight on the bit as that would cause the drillstring to flex and bend too much, leading to catastrophic pipe breakage.
To keep the weight at the optimal level, drillers use the draw works to hold back most of the weight, only allowing the optimum force on the bit which is called the weight on bit (WOB). Instead of the entire weight of the drill string resting on the bit itself, most of the weight is held up by the drill line. Because having the correct WOB is so important for effective drilling, the string weight indicator and associated WOB dial is one of the most frequently referenced gauges on the driller’s instrument panel.
As the bit progresses through rock, the drawworks slowly let out the drilling line – if it weren’t, the drill string wouldn’t go down. The cable must be let out to apply enough weight so that the weight on bit remains in the desirable WOB range. When a connection is made, the drawworks reels the cable back in, and starts to lower the newly lengthened drill string once again.
Images: “Hands On” by Michael Black; “Three Bits” by Michael Black; “Drilling Tool: Stabilizer” by HHakim via iStock; “Draw Works” by Michael Black