Parts of BHA
- Drill Bit: Drill bits are the cutting tools, the shape and size of the bit are designed as per the type of rock that is to be drilled. 3-cone, 4-cone, PDC (Polycrystalline diamond cutter), and roller bits are some of the vast variety of bits available. Some of the data required for correct bit selection are proposed lithology column with a detailed description of each formation, details of the Drilling fluid, and lastly well profile.
- Drill Collar: They provide weight to the BHA, and thus it helps the drill bits. They are thick-walled, tubular pieces, usually made from plain carbon steel or nonmagnetic nickel-copper alloy or other nonmagnetic premium alloys. In addition to providing weight to bit, they also act as a shock absorber.
- Stabilizers: Stabilizers serves the purpose of mechanically stabilizing the bottom hole assembly. It consists of a hollow cylindrical body and stabilizing blades, both made of high-strength steel. The blades of the stabilizer can either be spiral or straight and are used according to their requirements.
The bottom hole assembly is the lower part of the drill string and refers to an assembly of the bit, drill collars, stabilizers and other accessories. It is often seen that the assembly includes a mud motor, directional drilling measuring equipment or even measurements while drilling tools. Some of the basic BHA configurations are:
- Pendulum assembly
- Packed bottom hole assembly
- Rotary build assembly
- Rotary drop assembly
- Steerable assembly
- Mud motor and bent sub-assembly
There are three main ways of using the BHA in the process to control the direction of the well, they are, as follows:
- Pendulum Assembly: This principle is used for very high angle wells where it is relatively easier to drop an angle. Simply put, this kind of assembly heavily relies on the principle of gravity. The force of gravity is used to deflect the hole back to the vertical position. This force will be dependent on the length of the drill collars between the drill bit and the first pint of the tangency. This length is often coined as the active length of drill collars and is resolved into two perpendicular forces- the one acting at 90 degrees to the axis of the wellbore (also called the side force), and one that acts along the hole. Increasing this length causes the side force to increase at a high rate when compared to its component along the hole. The side force is the force that brings about the deflection of the hole back to the vertical. It should be noted that if a higher value of WOB (Weight on bit) is used in this assembly, then it can cause the hole angle to build instead of dropping! Also, the pendulum assemblies often tend to move to right depending on the type of bit used and since they are mostly flexible, they are expected to follow the natural movements of the drill bit and along with that.
- Fulcrum Principle: The primary objective of this assembly is to build angle ( or we can say to increase hole inclination). It is achieved by utilizing a near bit stabilizer, it is made to act like a pivot or the fulcrum of a lever. The length of the lever is from drill collars to their point of contact with the low side of the hole. The angle is made by pressing the drillbit to the high side of the hole, this is continued as the drilling progresses. It is seen that the drill collars tend to bend more as the WOB is increased. Hence, the rate of angle build is directly proportional to the weight on bit applied. Furthermore, the build rate also increases as the distance from near bit stabilizer to the first stabilizer in the bottom hole assembly increase. Any increase in the hole angle also increases the build rate. Lastly, a reduction in RPM(Rotation per minute) increases the build rate as well.
Packed Hole Stabilization Principle: It is used to maintain the hole inclination and direction of the drilling. This type of assembly is typically employed to drill the tangent section of the well. The packed BHA mostly relies on the principle that two points will contact and always follow a sharp curve, while three curves will lead to a straight line. A packed BHA will have several full gauge stabilizers, mainly in the lower section of BHA. This makes the BHA stiff and expectedly, it tends to maintain hole and direction.