Mud Related Drilling ProblemsLost Circulation

Lost circulation or loss of returns describes the complete or partial loss of fluid to the formation as a result of excessive hydrostatic and annular pressure drop. •Lost circulation is characterized by a reduction in the rate of mud returns from the well compared to the rate at which it is pumped downhole (flow out <> •If the annulus of the well will not remain full even when circulation of the fluid has ceased, the hydrostatic pressure will reduce until the differential pressure between the mud column and the loss zone is zero. –This may induce formation fluids from other zones, previously controlled by the mud hydrostatic pressure, to flow into the wellbore resulting in a kick, blowout, or underground blowout. –It may also cause previously stable formations to collapse into the wellbore. •Permeable or fractured formations can result in partial or complete loss of circulation. •Formation fractures can be natural or caused by excessive drilling fluid pressure on a structurally weak formation. •Once a fracture has been induced, the fracture will widen and take more mud at a lower pressure. •To avoid inducing formation fractures: –Maintain the minimum equivalent-circulating density (ECD) and mud weight. –Avoid pressure surges.

Lost Circulation –Fractured

•Permeable or fractured formations can result in partial or complete loss of circulation. •Formation fractures can be natural or caused by excessive drilling fluid pressure on a structurally weak formation. •Once a fracture has been induced, the fracture will widen and take more mud at a lower pressure. •To avoid inducing formation fractures: –Maintain the minimum equivalent-circulating density (ECD) and mud weight. –Avoid pressure surges. •Indication: –Lost circulation of this type is indicated by a complete or partial loss of returns and a decrease in pit volume. •Treatment –If a induced fracture is suspected, the hole can be allowed to heal by pulling into the casing and waiting 6 to 12 hours. –After the waiting period, stage back to bottom and check for full returns. –If full returns have not been established, treat the losses as if they were cavernous/vugularlosses.

Lost Circulation –Permeable

•Permeable and porous formations include: –Loose, noncompactedgravel beds –Shell beds –Reef deposits –Depleted reservoirs •These types of formations cause seepage loss to complete loss of returns. •Indication –Seepage into permeable formations is indicated by partial to full loss of returns and a decrease in pit volume. •Treatment –Reduce mud weight as much as possible. –Treat the system with a combination of fine-to medium-grade lost-circulation products

Lost Circulation –Corrective Measures

•Conventional Lost Circulation pill –Consider using a combination of LCM material with varying sizes to provide for an optimum bridging agent with this type of pill. –Small amounts of Lime may be used to slightly flocculate the Bentonite, to increase the viscosity preventing the LCM material from settling out and plugging the bit. –It is cheaper to obtain the viscosity using small amounts of Lime. –The Lime addition will also provide a higher fluid loss than the Gel slurry thereby increasing the sealing rate. •The actual concentration of LCM in the pill may vary; the formulation listed below assumes no jet or very large nozzles in the bit. •Once the approximate point of loss is established, a 15 -30 m3(100-300 bbl) pill should be mixed •Fresh Water15-50 m3(94 –310 bbl) •Soda Ash0.50-0.75 kg/m3(0.15-0.25ppb) •Caustic Soda0.50-0.75 kg/m3(0.15-0.25ppb) •Bentonite70-75 kg/m3(25-26 ppb) •Sawdust15 kg/m3(5 ppb) •FibreSeal15 kg/m3(5 ppb) •WalnutShells/Mica 15 kg/m3(5 ppb) •Lime1.0-1.5 kg/m3(0.35 –0.5 ppb) •Once the pill has been mixed, spot just above the loss zone by pumping slowly; 160-320 litres/min. (1-2 bbl./min.) until the hole is full and circulation is regained. •If the hole remains full, close the hydriland squeeze the annulus with 300-500 kPa(50-75 psi) for 30 minutes. •If this procedure fails, repeat once. •A second failure may indicate that another technique may be in order. Effective control of lost circulation into a permeable zone may require a broad range of particles •Gunk Squeeze –When you are faced with a lost circulation problem and you are using an oil/synthetic mud, mix the gunk squeeze with water and Organofilic Clay instead of oil/synthetic and bentonite. •To mix a gunk squeeze, follow these steps: –Drain and clean the mixing tank thoroughly. •Prepare a gunk slurry –Pump the following in this order: •Spacer to cover approx. 500' of drillstring •Squeeze to cover approx. 2 times open hole volume •Spacer to cover approx. 500' of drillstring –The spacer fluid should have the same base fluid as the squeeze. –Displace the squeeze to the bit. –Close blowout preventers (BOPs). –Pump down the drillpipe and annulus in equal volumes until the squeeze and spacer are displaced from the drillpipe. –Maintain equal pressure on drillpipe and casing.