5,.Drill String/BHA Components and Design

01. Describe the purpose of the BHA.

Ans. BHA or Bottom hole Assembly is the lower portion of the drillstring. It consists of all the special instruments used while drilling.  These components include, but are not limited to: The Bit, HWDP, NMDC, MWD and LWD tools, DD tools, stabilizers, jars, and subs.

 

02. Describe the basic components that make up the BHA and drill string, including:

Bits: Used to crush or cut the rock. It’s located at the bottom of the drillstring and must be changed when it becomes excessively dull or stops making progress.

Stabilizers – (including spiral, integral, straight blade, clamp on):

It’s an important part of the BHA as they stabilize the bit and the drill collars in the hole. When the bottom hole assemble is properly stabilized, optimum

drilling weight can be applied to the bit. They have variety of designs.

Stabilizers are specifically used to:

q Control hole deviation

q Reduce the risk of differential sticking

q Ream out doglegs and key-seats

 

Spiral Stabilizers are provided with spiral blades, they may be either open or tight spiral.

Integral Stabilizers are made from one piece of material rolled and machined to provide the blades; they can have three or four blades. I.B. Stabilizers normally have tungsten carbide Inserts (TCI’s).

Straight Blades Stabilizers are those with straight blades, which might be either straight or offset.

Clamp on Stabilizers Allow more flexibility in the BHA design, they can be positioned on the NMDC’s, MWD, PDM’s, etc. at the required spacing to maintain direction control, non-magnetic clamp on stabilizers are also available.

 

 

 

 

 

 

 

 

 

 

 

Drill Collars:

The drill collar is a component of a drill-string that provides weight on bit for drilling. Drill collars are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of non-magnetic nickel-copper alloy or other non-magnetic premium alloys. That is to allow the LWD and MWD tools to properly operate. Last, threaded connections, male on one end and female on the other, are cut so multiple collars can be screwed together along with other downhole tools to make a bottom hole assembly (BHA).

 

Crossovers: Used to connect different pin and box sizes and types.

 

Float subs: Float subs have a float valve to prevent the fluid from traveling up inside the drill pipe.

 

Bent subs: A small section of drill collar used in directional drilling.  The pin end of the sub is slightly offset so that when a connection is made, a bent joint is created.

 

Orienting subs (UBHO): A sub with a sleeve located inside that can direct survey equipment in the direction of the BHA bend.

 

Junk Baskets: Used in retrieving junk from downhole and to prevent the junk from settling down to the bottom when circulation is stop.

 

Jars: Transfers the potential energy stored in the stretched drill pipe to kinetic energy in the BHA above the jars in an attempt to free a stuck drill string.

 

Reamer: Used in drilling to smooth the wall of a well, enlarge the hole to the specified size, help to stabilize the bit and drill directionally.

 

Hole openers/under reamer: It is normally used to enlarge the hole when problems have been encountered and casing has been placed below the planned depth.

 

Drillpipe - Tubular steel conduit fitted with special threaded ends called tool joints. The drill pipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit. The different grades include G, S, E and the thread for the 5” is 4 ½ IF.

 

Heavy Weight Drill Pipe (HWDP): Similar to drill pipe but thicker walled.  Located above the drill collars, to allow a smooth transition between the drill collar and the drill pipe.

 

Steerable PowerPak motor: Mud motor with variable bend section.

 

Baffle plate (TOTCO ring): Landing plate for surveying tools.

 

Rotary Steerable systems: Directional-drilling systems that are designed to allow steering while the drillstring is rotating – such as the PowerDrive system.

 

 

 

 

03. Define and identify classic examples of the following BHAs:

 

Build: In build BHA, we position the BHA so that we exert positive side forces on the formation through the bit - Fulcrum effect.  This has the effect of pushing the bit up.

 

Drop: Generally opposite to build BHA’s.  By not placing a stabilizer at the bit, you create a Pendulum force which makes the bit push towards the low side of the wellbore and drop in inclination.  The stabilizers should be placed at a greater distance away from the bit in order to increase the drop.

 

Hold: Lock up BHA’s. In order to keep the hole inclination constant.

 

Steerable: Some of the same principles as for rotary BHA’s.  Objective is to kick off the well and build inclination to maximum angle along the desired hole direction

 

 

 

04. Explain why stabilizer placement and stabilizer size is critical to drill string design in both vertical and directional wells.

Ans: The size, position & no. of the stabilizers are critical.

v If a full gauge stabilizer is placed near to the bit, it will create a build-up assembly due by the force of the unstabilized collar.

v If an additional stabilizer is placed around 30 meters from the bit, the build-up rate will be reduce.

v The build-up effect could be completely avoided by using a third stabilizer at 40 meters from the bit, which would keep the hole straight.

 

05. Explain the differences between a rotary assembly, a motor assembly and steerable assembly.

Ans: The differences are listed below:

The Rotary assembly does not have downhole motor; the bit is turned just with the use of the rotary table or top drive.

Assembly with a downhole motor is called Motor assembly but without bend.

The Steerable assembly involves the use of a downhole motor that has an adjustable bend.  

 

06. Explain some important aspects of BHA manipulation

a) Importance of minimizing the handling of BHA.

Ans: Over-handling puts undue stress and strain on equipment, especially at the connections.

 

b) Potential problems caused by always breaking the same tool joint connections.

Ans: Breaking the drill-string in the same place every POOH could create fatigue on the same thread which could weaken it, wash it out, or break down-hole. For that reason it is always important to remove one pipe of any of the stands when POOH. Then the disconnection and connection point for a stand will always change.