Lost circulation Page 1 of 1
LOST CIRCULATION
Factors affecting lost circulation
• Strength of the weakest formation in the open hole
• Flow properties of drilling fluid, such as viscosity and gel strength.
• The speed at which drillstring is run in the well and the acceleration of
drillstring.
• The annular clearance between the hole and drillstring.

Causes of Lost circulation
• High density of drilling fluid - If the pressure exerted by column of drilling
fluid with high density exceeds the fracture pressure of the weakest
exposed formation, the weakest formation will breakdown resulting in lost
circulation.
• Going into Hole Too Fast (Surging) - When the drillstring is lowered rapidly
into the well, it creates a pressure surge that may induce fractures and
subsequent lost circulation. The problem of surging is compounded if the
drill string has a float in it.
• Pressure Due to Annular Circulating Friction - The pressure applied to the
bottom hole due to the friction pressure loss in the annulus can be quite
large in small holes with low annular clearances. This high bottom hole
pressure can induce fractures and lost circulation. This problem can
become acute when trying to break circulation with high gel fluids.
• Sloughing or Balled-Up Tools - Partial plugging of the annulus by sloughing
shale can restrict the flow of fluids in the annulus. This imposes a
backpressure on the formations below while circulating and can quickly
cause a formation breakdown if pumping continues. Annular plugging is
most common around the larger drillstring components such as stabilizers
or balled up bottom hole assembly.

Warning sign of lost circulation
• Decrease in pump pressure
• Decrease in flow returns
• Loss of surface mud volume
• Increase in string weight
• Increase in pump pressure due to restriction in the annulus. This increase in
pressure may lead to lost circulation.