Thursday, 2 November 2017


Bottom hole Assembly components types mechanism

The drilling bottom hole assembly (BHA) is the part of the Drilling String which is below the Drill Pipes. 

The most common components of the BHA are Drill Collars, Stabilizers, Shock Absorbs, Jars, Reamers, Heavy Weight Drill Pipe and The Drilling Bit. 

The purpose of the Bottom Hole Assembly BHA is to drill a usable hole economically. This objective is achieved with the proper Drill Bits Selection and Drill Collars selection which are required to provide high WOB bit weights to Improve ROP The Penetration Rate. Stabilizers are needed to minimize the rate of hole angle change and prevent the formation of doglegs and key seats. Shock absorber play an important part to prolong the service life of the bit and drill pipe. Down hole drilling jars may be needed to unstick the BHA.

The BHA design is based upon the requirements of having enough weight transfer to the bit (WOB) to be able to drill and achieve a sufficient The Penetration Rate (ROP), giving the Driller or Directional Driller directional control to drill as per the planned trajectory and to also include whatever Logging While Drilling (LWD) / Measurement While Drilling (MWD) tools for formation evaluation. As such Bottom Hole Assembly BHA design can vary greatly from simple vertical wells with little or now LWD requirements to complex directional wells which must run multi-combo LWD suites.

Prior to running a BHA most oilfield service providers have software to model the BHA behaviour such as the maximum WOB achievable, the directional tendencies & capabilities and even the natural harmonics of the assembly as to avoid vibration brought about by exciting natural frequencies.

Main Types Of BHA Used In Oilfield

  1. Rotary Assemply: Rotary assemblies are commonly used where formations are predictable and the rig economics are an issue. In such an assembly the weight of the drill collars gives the BHA the tendency to sag or flex to the low side of the hole, collar stiffness length and stabilizer diameter and placement are engineered as a means of controlling the flex of the BHA. this will bring about the desired hold, build or drop tendency.The ability to vary the directional tendency of the assembly comes from varying the weight on the bit. A fixed assembly has only one directional tendency. The weight on bit allows you to tune that tendency
  2. Directional BHA: which may be using mud motor with bent sub or using rotary steerable systems

BHA Components:

  1. Stabilizers
  2. Reamers
  3. Shock Absorbers
  4. Jarring Devices
  5. Drill Collar
  6. Drilling Bit

How Drilling BHA Act / BHA Behaviour

Rotary assemblies are designed to build, drop, or hold angle. The behavior of any rotary assembly is governed by the size and placement of stabilizers within the first 120 ft from the bit. Additional stabilizers run higher on the drillstring will have limited effect on the assembly’s performance.

Rotary assemblies are not “steerable”; first, the azimuth behavior (right/left turn) of a rotary assembly is nearly uncontrollable. Second, each rotary assembly has its own unique build/drop tendency that cannot be adjusted from the surface. Thus, tripping for the assembly change is required to correct the wellbore course.

Rotary BHA Types

Main Rotary BHA Types are : Slick assembly, fulcrum and Pendulum Assembly


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