Flushing out of well – circulation (continuous or periodical) of wash medium (gas, foam, water, drilling mud) during drilling to remove drilling cuttings (sludge) from the bottomhole and transport them to the surface or to sludge traps, to transfer energy to the bottomhole motors, to cool and lubricate drill bits and diamond tool. Jet rock breaking in the bottomhole is also achieved through wash medium effect (at liquid efflux rate of 200–250 meters per second) during hydraulic rotary drilling in soft and intermediate rock.

Circulation scheme may be of the following types: general natural circulation, counter-current circulation, bottomhole (local) circulation and combined circulation.

Wash medium loss may be observed during well flushing due to partial or full completion (loss) of liquid in the reservoir. Sometimes during unexpected penetration into a major cavity or cavern catastrophic loss of drilling mud may be observed, usually resulting with a crash. Well flushing process may be disrupted with intensive inflow of produced water and with gas outburst.

Well flushing process under given well geometry and geotechnical conditions is controlled by changing wash medium properties and circulation mode. Normally the controlled parameters are the following: wash medium density and flow properties; pump delivery rate and discharge defining the ascending flow velocity; hydraulic horsepower expended across bit and mud motor; pressure differential at the bottomhole, etc. These parameters determine the equipment choice.