8/19/2019 Factors Affecting Lost Circulation1

1/1

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 lostcirculation.

 Going into Hole Too Fast (Surging) - When the drillstring is lowered rapidly

into the well, it creates a pressure surge that may induce fractures andsubseuent 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 uite

large in small holes with low annular clearances. This high bottom holepressure can induce fractures and lost circulation. This problem can

become acute when trying to break circulation with high gel fluids. Sloughing or Balled-U Tools - !artial 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 uicklycause a formation breakdown if pumping continues. "nnular plugging is

most common around the larger drillstring components such as stabili#ersor balled up bottom hole assembly.