Bottom hole assembly BHA The BHA is made up of a drill bit which is used to

Bottom hole assembly bha the bha is made up of a

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Bottom hole assembly (BHA) The BHA is made up of: a drill bit , which is used to break up the rock formations ; drill collars , which are heavy, thick-walled tubes used to apply weight to the drill bit; and drilling stabilizers , which keep the assembly centered in the hole. The BHA may also contain other components such as a downhole motor and rotary steerable system , measurement while drilling (MWD), and logging while drilling (LWD) tools. The components are joined together using rugged threaded connections. Short "subs" are used to connect items with dissimilar threads. Transition pipe Heavyweight drill pipe (HWDP) may be used to make the transition between the drill collars and drill pipe. The function of the HWDP is to provide a flexible transition between the drill collars and the drill pipe. This helps to reduce the number of fatigue failures seen directly above the BHA. A secondary use of HWDP is to add additional weight to the drill bit. HWDP is most often used as weight on bit in deviated wells. The HWDP may be directly above the collars in the angled section of the well, or the HWDP may be found before the kick off point in a shallower section of the well.
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Drill pipe Drill pipe makes up the majority of the drill string back up to the surface. Each drill pipe comprises a long tubular section with a specified outside diameter (e.g. 3 1/2 inch, 4 inch, 5 inch, 5 1/2 inch, 5 7/8 inch, 6 5/8 inch). At each end of the drill pipe tubular, larger-diameter portions called the tool joints are located. One end of the drill pipe has a male ("pin") connection whilst the other has a female ("box") connection. The tool joint connections are threaded which allows for the make of each drill pipe segment to the next segment. Running a drill string Most components in a drill string are manufactured in 31 foot lengths (range 2) although they can also be manufactured in 46 foot lengths (range 3). Each 31 foot component is referred to as a joint. Typically 2, 3 or 4 joints are joined together to make a stand. Modern onshore rigs are capable of handling ~90 ft stands (often referred to as a triple). Pulling the drill string out of or running the drill string into the hole is referred to as tripping . Drill pipe, HWDP and collars are typically racked back in stands in to the monkeyboard which is a component of the derrick if they are to be run back into the hole again after, say, changing the bit. The disconnect point ("break") is varied each subsequent round trip so that after three trips every connection has been broken apart and later made up again with fresh pipe dope applied. Stuck drill string A stuck drill string can be caused by many situations. Packing-off due to cuttings settling back into the wellbore when circulation is stopped. Differentially when there is a large difference between formation pressure and wellbore pressure. The drill string is pushed against one side of the well bore. The force required to pull the string along the wellbore in this occurrence is a function of the total contact surface area, the pressure difference and the friction factor.
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