lost circulation

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7Lost circulation

Contents

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-2

Formations in which circulation may be lost . . . . . . . . . . . . . . .. 7-3Cavernous/vugular formations . . . . . . . . . . . . . . . . . . . . . . . .. 7-3

Indication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-3Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-3

Fractured formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-4Indication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-4Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-4

Permeable formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-4Indication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-4Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-5

Corrective procedures and formulations . . . . . . . . . . . . . . . . . .. 7-5Gunk squeeze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-5Crosslinkable LCM pill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-6High-filtration squeeze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-8

Locating the loss zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-10

Lost circulation

7-2

Overview

Lost circulation or loss of returns describes thecomplete or partial loss of fluid to the formation as aresult of excessive hydrostatic and annular pressuredrop. Lost circulation is characterized by a reduction inthe rate of mud returns from the well compared to therate at which it is pumped downhole (flow out < flowin). This leads to a decrease in pit volumes. Loss ofcirculation may be detected by a sensor monitoringreturn flow rate or by pit volume indicators. Dependingon the severity of the rate of mud loss, drillingoperations may be significantly impaired. If the annulusof the well will not remain full even when circulation ofthe fluid has ceased, the hydrostatic pressure willreduce until the differential pressure between the mudcolumn and the loss zone is zero. This may induceformation fluids from other zones, previously controlledby the mud hydrostatic pressure, to flow into thewellbore resulting in a kick, blowout, or undergroundblowout. It may also cause previously stable formationsto collapse into the wellbore.

Lost circulation can occur in the following formations:

Type of formation Type of loss

Cavernous/vugular Complete or partial

Highly permeable or fractured Complete or partial

Permeable Seepage loss

Baroid fluids handbook

Revised August 1, 1997 7-3

Formations in whichcirculation may be lost

Cavernous/vugularformations

The circulation lost in a cavernous/vugular formation isthe most severe type of loss that can occur because themud loss is immediate and complete. Cavernousformations are associated with limestone reefs, dolomitebeds, or chalks. The loss occurs in actual caverns orcrevices in the formation.

Indication

This type of lost circulation is usually easy to diagnosebecause the bit can drop several inches or even feetwhen it breaks through the top of the cavern.

Treatment

The following methods are recommended forcombating lost circulation due to cavernous/vugularformations:

C Add 40 to 60 lb/bbl (114-171 kg/m) coarse-grade3products such as: BARACARB BARO-SEAL BAROFIBRE Coarse JELFLAKE MICATEX STEELSEAL WALL-NUT

C Spot a high filtration pill such as Diaseal M orZEOGEL.

C Spot a gunk diesel-oil-bentonite (DOB) or diesel-oil- bentonite cement (DOBC) squeeze pill.Synthetic based fluids may be substituted as acarrier.

C Drill blind (if possible) until the loss zone can becased off.

Lost circulation

7-4

Fracturedformations

Permeable or fractured formations can result in partialor complete loss of circulation. Formation fractures canbe natural or caused by excessive drilling fluid pressureon a structurally weak formation. Once a fracture hasbeen induced, the fracture will widen and take moremud at a lower pressure. To avoid inducing formationfractures:

C Maintain the minimum equivalent-circulatingdensity (ECD) and mud weight.

C Avoid pressure surges.

IndicationLost circulation of this type is indicated by a completeor partial loss of returns and a decrease in pit volume.

TreatmentIf a induced fracture is suspected, the hole can beallowed to heal by pulling into the casing and waiting 6to 12 hours. After the waiting period, stage back tobottom and check for full returns. If full returns havenot been established, treat the losses as if they werecavernous/vugular losses.

Permeableformations

Permeable and porous formations include:

C Loose, noncompacted gravel bedsC Shell bedsC Reef depositsC Depleted reservoirs

These types of formations cause seepage loss tocomplete loss of returns.

Indication

Seepage into permeable formations is indicated bypartial to full loss of returns and a decrease in pitvolume.



Treatment

The following methods are recommended forcombating this type of loss:

C Reduce mud weight as much as possible.C Treat the system with a combination of fine- to

medium-grade lost-circulation products such as: BARACARB BAROFIBRE BARO-SEAL HY-SEAL MICATEX STEELSEAL

Corrective procedures andformulations

Gunk squeeze When you are faced with a lost circulation problem andyou are using an oil/synthetic mud, mix the gunksqueeze with water and GELTONE instead ofoil/synthetic and bentonite. Formulations for water-based and oil/synthetic-based gunk slurries are listed inTables 7-1 and 7-2.

Type Bentonite, lb (kg) Cement, lb (kg)

Additions per barrel of diesel

Diesel-oil/synthetic 400 (181) 0 (0)bentonite

Diesel-oil/synthetic 200 (91) 200 (91)bentonite cement

Table 7-1: Gunk formulation for water-based muds. This example uses a diesel-oil/synthetic bentonite or diesel-oil/synthetic bentonite cement mixture. If PETROFREE/PETROFREE LE/XP-07 is being used, substitute appropriate base fluid for diesel oil.

Lost circulation

7-6

Material (1.26 sg) (1.56 sg) (1.92 sg)

Mud weight

10.5 lb/gal 13 lb/gal 16 lb/gal

Water, bbl (m ) 0.660 0.628 0.5823

Q-BROXIN, lb (kg) 3.5 (10) 3.5 (10) 3.5 (10)

Caustic soda, lb (kg) 1.5 (4) 1.5 (4) 1.5 (4)

*GELTONE, lb (kg) 220 (627) 150 (428) 100 (285)

BAROID, lb (kg) 175 (499) 370 (1,055)

Table 7-2: Water-GELTONE gunk squeeze formulation. This formulation is foroil/synthetic-based muds. * Use the GELTONE recommended for the area.

To mix a gunk squeeze, follow these steps:

1. Drain and clean the mixing tank thoroughly.2. Prepare a gunk slurry (diesel oil/synthetic bentonite

cement, diesel-oil bentonite, or water-GELTONE;see Table 7-2).

3. Pump the following in this order:C Spacer to cover approx. 500' of drillstringC Squeeze to cover approx. 2 times open hole

volumeC Spacer to cover approx. 500' of drillstring

Note: The spacer fluid should have the same base fluid as the squeeze.

4. Displace the squeeze to the bit.5. Close blowout preventers (BOPs).6. Pump down the drillpipe and annulus in equal

volumes until the squeeze and spacer are displacedfrom the drillpipe.

7. Maintain equal pressure on drillpipe and casing.

CrosslinkableLCM pill

N-SQUEEZE may be used as an essentially non-damaging crosslinkable LCM pill. This pill is designedto control whole fluid losses or seepage losses. It may



be either pumped as a sweep or crosslinked and spottedacross a loss zone.

If required, the pill may be weighted with calciumcarbonate or barite. Cleanup can be accomplished byflow back, acidizing, or biodegradation.

Note: The highest degree of formation damageprotection is provided when N-SQUEEZE is used aloneor with calcium carbonate as a weighting agent.

The N-SQUEEZE pill may be mixed in freshwater, KClor NaCl brines. It reaches its highest yield in water witha lower salinity value.

Formulations for water-based N-SQUEEZE pills arelisted in Table 7-3.

Materials (1.26 sg) (1.56 sg) (1.92 sg)

Mud weight

10.5 lb/gal 13.0 lb/gal 16.0 lb/gal

Fresh water, bbl (m ) 0.919 0.826 0.7133

N-SQUEEZE, lb (kg) 10 (29) 10 (29) 10 (29)

BAROID, lb (kg) 120 ( 343) 257 (735) 422 (1206)

* N-SQUEEZE, lb (kg) 10 (29) 10 (29) 5 (14)

Table 7-3: N-SQUEEZE formulations. This type of squeeze can be pumped as a sweepor crosslinked and spotted across a loss zone.

* Add second amount of N-SQUEEZE after allother products have been mixed and blended for20-30 minutes.

Notes:

C N-SQUEEZE LCM pill can be pumped as a sweepor crosslinked.

C To crosslink add 5 gallons of N-PLEX per 10barrels of N-SQUEEZE slurry pumped.

C Water-based N-SQUEEZE can also be used inoil/synthetic fluids

Lost circulation

7-8

%

C Stable to 180F

Caution : Do not add N-PLEX directly to the N-SQUEEZE in the mixing tank. This could cause theslurry to set up and be to thick to pump. The N-PLEX (crosslinker) should be added to thesuction line at the time the N-SQUEEZE slurry isbeing pumped downhole.

High-filtrationsqueeze

A high-filtration squeeze requires tightly packeddehydrated solids to seal the loss zone.

Formulations for water-based and oil/synthetic basedsqueezes are listed in Tables 7-4 and 7-5.

Materials (1.20 sg) (1.68 sg) (2.16 sg)

Mud weight

10.0 lb/gal 14.0 lb/gal 18.0 lb/gal

Fresh water, bbl (m ) 0.93 0.78 0.6323

Lime, lb (kg) 0.5 (1.4) 0.5 (1.4) 0.5 (1.4)

ZEOGEL, lb (kg) 12 (34) 10 (29) 8 (23)

BAROID, lb (kg) 82 (234) 304 (866) 525 (1,496)

LCM, lb (kg) 20-60 (57-171) 20-60 (57-171) 20-40 (57-114)

Table 7-4: High-filtration water-based squeeze formulation. This type of squeeze plugsthe loss zone to prevent additional losses.

Notes:

C Sepiolite clay can be substituted for ZEOGEL.C Other weighting agents such as BARACARB or

BARODENSE can be substituted for BAROID.

A high-filtration squeeze can also be used with oil/synthetics as the continuous phase for situations whereoil/synthetics is the base fluid.



Materials Amounts

Base fluid, bbl (m ) 0.63

SUSPENTONE, lb (kg) 3 (9)

TRIMULSO, lb (kg) 1 (3)

LCM, lb (kg) 10-15 (29-43)

BAROID, lb (kg) 575 (1,639)

Table 7-5: High-filtration oil/synthetic based squeeze formulation. This mixture resultsin an oil/synthetic based squeeze weighing 18 lb/gal (2.16 sg).

Note: BARACARB can be substituted for BAROID inthis formulation. The maximum pumpable density whenusing BARACARB is 14.0 lb/gal (1.68 sg).

High-filtration squeeze procedure:

1. Spot the slurry into the loss zone.2. Close blowout preventers (BOPs).3. Apply pressure for several hours.

The formulation for a Diaseal M slurry with oil is listedin Table 7-6. Because of variations in the densities ofoil and barite, pilot tests should be conducted todetermine exact formulations. If the slurry becomes toothick, add up to 1 lb/bbl (3 kg/m) EZ MUL or3

DRILTREAT oil-wetting agent.

Lost circulation

7-10

Density, lb/gal (sg) Diaseal M, lb (kg) Barite, lb (kg) Oil, bbl (m )3

8.0 (0.96) 4,400 (1,996) 3,800 (1,724) 88.0 (14)

9.0 (1.08) 4,100 (1,860) 8,800 (3,992) 85.5 (13.6)

10.0 (1.20) 3,800 (1,724) 13,800 (6,260) 83.0 (13.2)

11.0 (1.32) 3,500 (1,588) 18,800 (8,528) 80.5 (12.8)

12.0 (1.44) 3,250 (1,474) 23,800 (10,796) 77.0 (12.2)

13.0 (1.56) 3,000 (1,361) 29,000 (13,154) 74.5 (11.8)

14.0 (1.68) 2,700 (1,225) 34,300 (15,558) 72.0 (11.4)

15.0 (1.80) 2,400 (1,089) 39,700 (18,008) 69.5 (11.0)

16.0 (1.92) 2,150 (975) 45,200 (20,503) 67.0 (10.6)

17.0 (2.04) 1,900 (862) 50,800 (23,043) 64.5 (10.2)

18.0 (2.16) 1,650 (748) 56,500 (25,628) 61.0 (9.7)

Table 7-6: Diaseal M oil slurry formulation. This formulation is for 100 bbl of slurry.

Note: Lost circulation materials may be added to theDiaseal M squeeze. If absorbent lost-circulationmaterials are added, the slurry viscosity will increase.This slurry is effective without conventional lost-circulation materials.

Locating the loss zone

The best source of information for determining losszones is your knowledge of the formations and thecharacteristics of a given region.



If there Then the loss zone is probably at the

Is an indication of formation change Bit

Has been an increase in density Weakest point in the hole (i.e., below the lastcasing shoe)

More specific methods for locating the loss zoneinclude:

C Measurement-while-drilling (MWD) tools such asSperry Sun's Formation-Evaluation-While-Drilling(FEWD)

C Radioactive tracersC Temperature surveysC Hot-wire surveys

lost circulation

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