14. Pendulum Effect of Drill Collar Towards Hole Deviation

Here I am going to discuss about the Pendulum Effect, its mechanism and the forces acting, thus causing the drill bit along with active string to tend towards the lower side of wellbore.

Pendulum effect is the effect which tends to bring the drill bit along with the active string to the lower side of hole.

Consider the figure below:

Here,

W₁ = Weight of bit along axis of drill collar

W₂ = Component of W₁ along axis of hole

W₃ = Component of W₁ along normal to hole axis

W₄ = Vertical force of active drill collars assembly

W₅ = Component of W₄ acting along axis of assembly

W₆ = Component normal to W₅ but opposite to W₃ 

The effect of gravity on mass of drill collars above the bit is vertically downward (W₄).

A part of this force is transmitted to the bit along the axis of assembly W₅.

The complimentary component of this part W₆ acts towards perpendicular to the axis of the assembly. It is supported by formation at the wall contact point at the bit. The influence of this lateral force at bit is always to reduce inclination and it's magnitude, automatically increases with inclination. If desired we can enhance the effect by increasing the active drillstring length the natural hole straightening tendency can usually be employed to avoid excessive inclination in vertical holes.

Formation dips also influences the hole deviation. For laminated formation when dips are less than 45 degrees the bit tries to drill perpendicular to bedding plane, and if it is in excess of 45 degrees bit tends to drill parallel to bedding plane. Likewise formation attitudes also have effect on directional tendencies. If proposed direction is due to up dip it follows the natural bit tendencies and drift angle can be readily built If proposed direction is due to up dip, it follows the natural bit tendencies and drift angle can be readily built. If the proposed direction is in left of up dip the bit will try to turn to right and if proposed direction is in right of up dip the bit will deviate to left. 

Bit drilling up dip where formations are dipping at angles less than 45 Degrees

Bit drilling up dip where formations are dipping at angles greater than 60 degrees