钻井时泥浆漏失的原因及应对方式

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.

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