Boreholes circulating muds

During drilling operations sometimes large volumes of drilling mud are lost into a formation. In this case normal mud circulation is no longer possible and the fluid level inside the borehole will drop, creating a potentially dangerous situation as described below. The formations in which lost circulation can be a problem are ... 

If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

A mud rotary rig cuts a borehole by rotating a bit into the formation materials and removing the cuttings by continuous circulation of a drilling fluid. In a conventional rotary system, mud is pumped down through the drill pipe and out through nozzles in the bit. The mud flows upward in the annular space around the drill pipe to the surface, where it is channeled into a settling pit and a... 

The simplest borehole flow invasion problem can be posed using the radial flow model of Chapter 6, and, in particular, applying pressure-pressure boundary conditions at the well and farfield boundaries. Because our mud is assumed to be lossy, we can ignore the presence of cake buildup many shallow wells are, in fact, circulated with water or brine as the drilling fluid. 

Drilling fluids arc commonly called drilling mud or slurry. Drilling mud is mixed on the surface and pumped down the drill string. The mud comes out at (he drill bit and is either left in (he annulus of the borehole or circulated back to the surface. Drilling mud is a mixture of wata, pranium bentonite, and if needed, small amoimts of polymer. Bentonite is a non-hazardous material. 

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