8. drill assembly
The drill pipe is the hub connecting the surface and the underground. In rotary drilling, the drill string is used to transfer the energy needed to break rock, apply weight on bit to the bottom of the well, and transport drilling fluid in the well. When drilling with downhole power drilling tools, it withstands the reverse torque of downhole power drilling tools and delivers liquid energy to downhole power drilling tools at the same time.
Drilling tools are composed of different components, and their composition varies with the drilling conditions and methods.
The basic components are: kelly pipe, drill pipe, drill collar, drill pipe stabilizer and converter joint. With the continuous development of excellent drilling technology, the structure and performance of drill string are more and more demanding, and reliable drill string must be selected.

Common BHA:

1. packed hole assembly
The so-called full hole assembly refers to a section of the drill string at the top of the bit and the hole diameter is equal to or has a small clearance of the drill string assembly. There are usually two kinds of full hole drill string combinations:

· 1) Full hole drilling tool composed of drilling tool stabilizer
A full hole drilling tool is usually composed of two or three drilling tool stabilizers. The relationship between the gap between the drilling tool stabilizer and the wellbore and the use effect of the full hole assembly is very important, should be strictly controlled, generally the smaller the gap, the better, especially the near-bit drilling tool stabilizer and the middle drilling tool stabilizer, and the actual gap between the wellbore is too large often lead to the effect of the full hole assembly.

· 2) Tower anti-inclined drilling tool
The so-called tower drilling tool is a full hole anti-inclination drilling tool composed of round drill collars of different sizes. The lower part of this drill tool uses a larger size drill collar, and its size gradually decreases from the bottom to the top, shaped like a pagoda, so it is called tower drill tool

2. pendulum drill assembly

In the inclined well section, the reducing force generated by the weight of the drill string below the tangent point has the tendency to promote the drill bit to drill in the straight direction, much like the movement of the pendulum, the drill tool composed of this principle becomes the pendulum drill, the pendulum drill has the following three forms:

· 1) slick assembly pendulum drill assembly
· 2) Single tool stabilizer pendulum drill assembly
· 3) Multi-stabilizer pendulum drill assembly

3. pendulum drill-packed hole assembly

The full hole drilling tool is connected to the pendulum drilling tool, that is, the pendulum - full hole drilling tool combination method.
Lunnan well structure assembly of Tarim Oilfield:

· packed hole assembly during first spud-in

12 1/4" bit +12 1/4" bottom hole centralizer +8" spiral drill collar +12 1/4" string type centralizer +8" spiral drill collar +8" while drilling +8" spiral drill collar +NC male *NC50 female connector +5" weight drill pipe +5" drill pipe "+NC50 male *NC50 female protection connector +NC50 male *520 cock +5 1/4" kelly (bottom-up)

pendulun drill assembly during second spud-in

8" bit +430*4A10 connector +6 1/4" spiral drill collar +8 1/2" centralizer +6 1/4" spiral drill collar +6 1/4" Spiral drill collar +6 1/4" while drilling +6 1/4" Spiral drill collar +5" weighted drill pipe +5" drill pipe +5 1/4" kelly pipe (bottom-up)
Drilling assembly of Shanqian well in Tarim oilfield
· packed hole assembly during first spud-in

26" bit +26" centralizer +9" spiral drill collar +16" centralizer +9" spiral drill collar +16" centralizer + sub +8" spiral drill collar +8" while drilling +8" spiral drill collar + sub +5" weighted drill pipe +5" drill pipe + sub + Lower cock +5 1/4" kelly pipe

· packed hole assembly during second spud-in

16" bit +16" centralizer +9" spiral drill collar +16" centralizer +9" spiral drill collar +16" centralizer + sub +8" spiral drill collar +8" while drilling +8" spiral drill collar + sub +5" weighted drill pipe +5" drill pipe + sub + Lower cock +5 1/4" kelly pipe

· packed hole assembly during third spud-in
· pendulum tool assembly during fourth spud-in
· pendulum tool assembly during fifth spud-in

9. Drill string working status
In the drilling process, the drill string works alternately in the two steps of tripping and normal drilling. When tripping, the drill string is in tension state. While in the drilling state is more complex, in tension, pressure, torsion and other states. In the rotary drilling, the working state and force of the drill string are especially complicated, and the drill string is like a slender rotating shaft. Under the axial pressure caused by part of the dead weight, the lower drill string is unstable and bending. Due to the limitation of borehole, it can bend several times. Because of the centrifugal force produced by rotation, the upper drill string does not keep a straight state, and coupled with the action of torsion, the whole drill string takes on a complex rotating motion in the form of an approximate spiral curve.