Drill String Design Drill String Components • A drill string design includes several components. Drill strings are unique. • The BHA can include the drill bit, drill collars, stabilizers, reamers, and Heavy Weight drill pipe. • The remainder of a drill string is drill pipe. • A correctly designed drill string can: • Produce a high quality hole • Maximize performance of components • Minimize drilling and production problems Drill String Designs Drill Bit • At the bottom of all BHAs is a drill bit. • The bit design will vary depending on the formation. • Its primary function is creating the hole by digging into the earth. • The correct bit will provide a good rate of penetration (ROP), last a reasonable number of hours, and drill holes the same size as the bit. • There are essentially 2 types of bits. • Roller Cone Bit. • This bit gets its name because the bit teeth roll over the bottom of the hole. Most • Roller Cone Bits have three cones, although they may have four or two cones. • The Roller Cone Bit is sometimes referred to as the “jetted bit” because its high pressure jets spray mud. Drill Bit • The Fixed-Head Bit (right) doesn’t have moving parts like the Roller Cone Bit. • It penetrates a formation by the weight and rotation of the drill string. • The cutters are made from natural, synthetic or hybrid diamonds. Stabilizers • Stabilizers are included in multiple places in a drill string, usually before and after one or more drill collars. • Depending on the stabilizer type its function can vary. • A stabilizer can help maintain hole direction when used throughout the BHA design. • A stabilizer will help increase the stiffness of a BHA when used with larger or smaller OD drill collars. Reamer • A reamer has several drill string purposes including smoothing the wall of the wellbore, maintains hole gauge and helps stabilize the bit. • Reamers are generally required during hard rock formation drilling. • The reamer is placed directly above the bit to prolong the bit life and prevent sticking. Drill Collars • The Drill Collar is located just above the drill bit to put weight on the bit (WOB). The WOB will affect the rate of penetration. • The Drill Collar performs additional functions including preventing the drill string from buckling, bit support and stabilization, and maintaining a vertical or straight hole. Jars • A Jar frees stuck drill stem components during drilling or workover operations. The driller controls the impact force of “jarring” both up and down. • It can be placed almost anywhere in the BHA for optimal performance. Heavy Weight Drill Pipe (HWDP) • The HWDP is typically located above the drill collars, and technically it is part of the BHA. • Sometimes referred to as transition pipe, it provides a graduated change in stiffness between the limber drill pipe above and the BHA below. • The graduated change in stiffness reduces the likelihood of drill pipe fatigue failures. • The HWDP has thicker walls than standard drill pipe, causing it to weigh twice as much. • The HWDP has a center upset which reduces the pipe wear and aids in preventing critical buckling. Drill Pipe • Drill Pipe is located at the very top of the drill string. • It makes up the distance between the Kelly and the remainder of the drill string downhole. • The drill pipe turns the drill string and provides a conduit for the drilling mud.
