Lost circulation is the loss of whole drilling fluid to natural fractures, caverns, permeable zones or fractures the mud pressure opens, and one event can cost up to a million dollars or more (Alsaba et al., AADE-14-FTCE-25, 2014).
Losses are found by volume. Every drilling fluid is meant to return up the annulus, so the volume that fails to come back gives the loss rate in bbl/hr (m3/h), and the rate sets the loss class and the treatment family.
Prevention keeps the mud pressure inside the operating window and carries background lost circulation material (LCM) in the active mud. Cures follow the class: LCM blends, sweeps and pills for seepage and partial losses, settable or squeeze pills for severe and total losses.
The four key figures of lost circulation are listed below.
- Seepage losses run at 1 to 10 bbl/hr (0.16 to 1.59 m3/h) and complete losses at 500 bbl/hr (79.5 m3/h) and above (Alsaba et al. 2014).
- LCM pills carry 40 to 60 lb/bbl (114 to 171 kg/m3) of pill, the typical field range.
- In 15 geothermal wells, 71.3 % of lost-circulation hours went to methods that failed.
- One event can cost up to a million dollars or more.
Standards and regulations are checked against their issuing bodies, product values against the named data sheet and figures against the dated source named in the sentence; see our methodology and fact-checking.
What Is Lost Circulation in Drilling?#
Lost circulation in drilling is the reduced or total absence of mud returns up the annulus while fluid is pumped down the drillstring, also called lost returns or loss circulation. Howard and Scott defined it in 1951 as the loss to formation voids of drilling fluids or cement slurries used in rotary drilling (JPT, 1951).
A loss lowers the mud column in the annulus and can bring stuck pipe, wellbore collapse, poor hole cleaning and well control issues (Scientific Reports, 2024). Each of these drilling fluid problems adds rig time to the cost of the lost mud and the treatment.
What Causes Lost Circulation?#
Lost circulation has four causes, each a path wide or weak enough to take whole mud (Alsaba et al., AADE-14-FTCE-25, 2014). The review sorts lost circulation mechanisms and solutions by the four loss zones below.
- Naturally fractured formations, whose open fractures take mud.
- Cavernous formations, whose voids take mud without returns.
- Highly permeable formations, such as gravels and permeable sands.
- Drilling-induced fractures, opened by the mud pressure itself.
Naturally fractured and cavernous formations#
Natural fractures and caverns take mud at any overbalance once their openings are wider than the mud solids can bridge. Small natural horizontal fractures produce partial losses, in the same group as gravels and induced vertical fractures (Alsaba et al. 2014).
Caverns and wide natural fractures produce severe and total losses, and the 2014 review assigns them to settable pills such as cement plugs and gunk rather than to more LCM in the mud. The width of the opening therefore decides the treatment family before any material is chosen.
Highly permeable, unconsolidated and depleted zones#
Gravels and highly permeable sands take whole mud as seepage or partial losses, because their pore throats are larger than the solids that build a filter cake. Bridging solids sized to those pore throats stop the loss where a filter cake alone cannot.
Depleted sands carry a lower fracture gradient than the shale around them, so the mud weight that holds the shale can fracture the sand. Wellbore strengthening is most likely to succeed in depleted sandstone and least likely in shale (SPE Distinguished Lecture, Wang).
Induced fractures: overbalance, circulating pressure and surges#
An induced fracture opens when the mud pressure at the zone exceeds the fracture gradient, from excess mud weight, high equivalent circulating density (ECD) or pressure surges. These vertical fractures take partial losses (Alsaba et al. 2014).
A narrow window shows the risk in numbers. In one published example, shale stability needs at least 17.52 lb/gal (2.10 sg; 2,100 kg/m3), while the depleted reservoir fractures near 14.77 lb/gal (1.77 sg; 1,770 kg/m3) (SPE Distinguished Lecture, Wang). Annular friction adds to the static mud weight in the equivalent circulating density, the pressure the zone sees while the pumps run. The gap of 2.75 lb/gal (0.33 sg; 330 kg/m3) means no single mud weight holds the shale and spares the sand.
How Are Mud Losses Classified by Loss Rate?#
Mud losses while drilling are classed by loss rate in bbl/hr, and two published sets draw the boundaries differently: the AADE-14-FTCE-25 review (Alsaba et al., 2014) and geothermal practice (Cole et al., Stanford Geothermal Workshop, 2017).
The table below compares the two sets class by class, with the zones behind each class and its treatment family.
| Class | AADE-14-FTCE-25 (Alsaba et al. 2014) | Geothermal (Cole et al. 2017) | Zones that produce it | Treatment family (AADE-14-FTCE-25) |
|---|---|---|---|---|
| Seepage | 1 to 10 bbl/hr (0.16 to 1.59 m3/h) | Below 25 bbl/hr (below 3.97 m3/h) | Highly permeable sands and gravels | LCM blended into the mud |
| Partial | 10 to 500 bbl/hr (1.59 to 79.5 m3/h) | 25 to 100 bbl/hr (3.97 to 15.9 m3/h) | Gravels, small natural horizontal fractures, induced vertical fractures | LCM blended into the mud or pills |
| Severe | Within the partial class | Above 100 bbl/hr (above 15.9 m3/h) with some returns | Larger fractures | Settable pills |
| Complete or total | 500 bbl/hr (79.5 m3/h) and above | No returns (total) | Caverns, wide natural or induced fractures | Settable pills, squeeze pills, cement plugs |
Each set is one source's classification; neither is an API or ISO class. m3/h calculated with 1 bbl = 0.1589873 m3.
The AADE set rests on an earlier classification by Nayberg and Petty (IADC/SPE 14723). A loss that counts as seepage in the geothermal set can count as partial in the AADE set, so a daily report names the set it uses next to the rate.
What Are the Warning Signs of Lost Circulation?#
The first warning sign of lost circulation is returns smaller than the volume pumped, and the measured difference in bbl/hr places the loss in its class. No returns at all mark a total loss. Well-control procedure for a loss combined with a kick is outside this page.
In a narrow window the warning comes before any mud is lost, as an ECD close to the fracture gradient. An HPHT well study modeled 18.359 lb/gal (2.2 sg; 2,200 kg/m3) synthetic-based mud reaching an ECD of 19.177 lb/gal (2.298 sg; 2,298 kg/m3) against a fracture gradient of 19.194 lb/gal (2.30 sg; 2,300 kg/m3), a margin of 0.017 lb/gal (0.002 sg; 2 kg/m3) (AADE-26-FTCE-053, 2026). The authors recommended a lower mud density.
How Is Lost Circulation Prevented on the Fluid Side?#
Lost circulation is prevented on the fluid side in two ways: keeping the mud pressure at the zone below the fracture gradient, and carrying bridging solids in the mud before the loss zone is drilled (Alsaba et al. 2014). In lost circulation management, the review calls these steps preventive treatment and contrasts them with corrective treatment after losses start.
Keeping density and ECD inside the operating window#
Excessive drilling fluid density and high ECD cause induced losses, because both push the pressure at the zone toward the fracture gradient. ECD is the mud density plus the annular pressure drop above a given point.
The highest wellbore pressures come from breaking circulation and moving pipe during connections, tripping and running casing, with surge while running in and swab while pulling (AADE conference paper on ECD). Mud weight is the static share of that pressure, and the drilling fluid density also sets the kill-mud weight. Changes that improve hole cleaning can raise ECD, and in deepwater, cold seawater raises the mud rheology where fracture gradients are already lower.
A leak-off test carried to fracture extension can lower the maximum safe mud weight (Whitfill and Hemphill, Drilling Contractor 2004). Its result, written as an equivalent mud weight, sets the upper edge of the window.
Background LCM and preventive bridging solids#
Background LCM is a preventive load of 20 to 25 lb/bbl (57 to 71 kg/m3) of bridging solids carried in the active mud before the loss zone is drilled (Whitfill and Hemphill, Drilling Contractor 2004). The blend is 5 to 10 lb/bbl (14 to 29 kg/m3) resilient graphitic carbon with 10 to 15 lb/bbl (29 to 43 kg/m3) sized calcium carbonate. In the same joint-industry tests, fibers alone did not raise the fracture reopening pressure.
Geothermal practice carries about 3 % by volume of micronized cellulose and calcium carbonate in the active system and pumps 10 to 15 lb/bbl sweeps before casing (Tuttle, Stanford Geothermal Workshop 2025). Two models explain how wellbore strengthening with sized solids raises the pressure a fracture holds: the stress cage (Alberty and McLean, SPE 90493, 2004) and fracture closure stress (Dupriest, SPE/IADC 92192, 2005). Both models underpin LCM design for wellbore strengthening.
Which Treatments Cure Each Loss Class?#
Each loss class has its treatment family: seepage and partial losses take LCM blended into the mud, severe losses take settable pills, and depleted or highly permeable zones take squeeze pills (Alsaba et al. 2014).
The decision table below sets each loss situation against its fluid-side response and dose, with the basis of every dose.
| Loss situation | Fluid-side response | Dose (basis) | Source |
|---|---|---|---|
| Seepage or partial losses, any mud type | LCM pill or sweep pumped to the zone | Pill 40 to 60 lb/bbl (114 to 171 kg/m3) of pill, typical field range; sweep nominal 50 lb/bbl (143 kg/m3) | Whitfill and Hemphill, Drilling Contractor 2004; Tuttle, Stanford Geothermal Workshop 2025 |
| Pill pumped through a downhole motor | Graphite and sized calcium carbonate blend | Up to 80 lb/bbl (228 kg/m3); graphite alone above 100 lb/bbl (285 kg/m3) | Whitfill and Hemphill, Drilling Contractor 2004 |
| Severe or total losses in fractures or caverns | Settable pill: gunk, cement plug or deformable viscous cohesive plug | Set per zone | Alsaba et al., AADE-14-FTCE-25, 2014 |
| Depleted or highly permeable zone | High-fluid-loss, high-solids squeeze pill | Product-specific | Alsaba et al., AADE-14-FTCE-25, 2014 |
| Induced fractures in a narrow window | Lower density or ECD; strengthening solids in the mud | Pressure control, no pill | Alsaba et al., AADE-14-FTCE-25, 2014 |
Doses are lb of LCM per bbl of the pill or mud named, with kg/m3 at 1 lb/bbl = 2.853 kg/m3.
Corrective blends are tested in a permeability plugging apparatus (PPA) or an HPHT cell with slotted or ceramic discs and judged by the 30-min fluid loss and the time to seal, while no preventive test is standard (Alsaba et al. 2014). Sizing starts from the Abrams rule, which sets the median particle size (D50) at no less than one-third of the median pore size or fracture width (Abrams, JPT 1977).
Seepage and partial losses: LCM blended into the mud#
Seepage and partial losses are cured inside the circulating system, with background LCM already in the mud or with an LCM pill or sweep pumped to the zone. In geothermal wells, the field pill held about 40 lb/bbl (114 kg/m3), the low end of the laboratory range that plugged a 1/4-in (6.35 mm) screen (Tuttle, Stanford Geothermal Workshop 2025).
Lost circulation materials differ in density, so blends are sized by volume, not by weight. Resilient graphitic carbon has an SG of 2.1, sized calcium carbonate 2.7, walnut shell 1.2 and fiber 0.5 (Whitfill and Hemphill, Drilling Contractor 2004). One pound of fiber fills about 5.4 times the volume of one pound of sized calcium carbonate, which shapes how lost circulation materials (LCM) combine in a pill.
Downhole motors set a coarse limit on pill strength, and the motor row of the decision table is a field record of concentrations that passed through motors, not a rating of any tool.
Severe and total losses: squeeze pills and settable plugs#
Severe and total losses need a pill that sets or dehydrates in the zone, not more LCM in the circulating mud, because the opening is wider than a blend can bridge. The 2014 review names three settable pills: cement, gunk and deformable viscous cohesive plugs (Alsaba et al. 2014).
A gunk plug is a slurry of bentonite, cement or polymers in oil, commonly bentonite in diesel, that turns into a sticky gunk when it meets water downhole. US offshore discharge rules allow no diesel oil in discharged drilling fluid and cuttings (40 CFR 435.13, Subpart A).
High-fluid-loss, high-solids squeeze pills seal depleted and highly permeable zones by losing their liquid and leaving a plug of solids in the opening. Diatomaceous earth forms the base of high-fluid-loss squeeze pills, and these pills work poorly in low-permeability shale and may underperform in non-aqueous fluids (Alsaba et al. 2014). Cement slurry design belongs to well cementing, a separate field.
Temperature limits of loss treatments#
Cellulosic LCM has a general stability limit of 300 degF (149 degC) (Tuttle, Stanford Geothermal Workshop 2025), and conventional LCM starts to degrade from 200 degF (93 degC) upward in roller-oven tests (Saleh et al., Stanford Geothermal Workshop, 2020).
Softening temperatures from a 1990 laboratory data set separate the organic solids. Thermoset rubber softens at 110 to 192 degF (43 to 89 degC), gilsonite at 345 to 375 degF (174 to 191 degC) and black walnut shells at 360 to 500 degF (182 to 260 degC) (Loeppke 1990, cited by Lee, Stanford Geothermal Workshop 2021). Each limit is a test result for a group of materials, not a rating of one product.
What Lost Circulation Costs and How Often Treatments Work#
Lost circulation added more than 100 h of nonproductive time and about USD 185,000 or more in rig cost per well across 38 US geothermal wells drilled since 2009 (Cole et al., Stanford Geothermal Workshop, 2017). Unprogrammed nonproductive time in general, with lost circulation its largest cause, adds 20 % or more to geothermal drilling cost. Lost circulation accounts for about 10 % of total well cost in mature geothermal fields on average and can exceed 20 % of well cost in exploration and developing fields (Saleh et al., Stanford Geothermal Workshop, 2020).
Failed treatments take the larger share of that time. In 15 wells with remediation records, 1,734.5 of 2,434 lost-circulation hours (71.3 %) over 211 events went to methods that failed (Cole et al. 2017). LCM mixed into the mud succeeded in 24.7 % of its applications, LCM pills in 27.8 % and cement plugs in 27.6 % (32 of 116 plugs).
Failure rates by class were 71.25 % for seepage and partial losses, 68.5 % for severe and 83.6 % for total losses, with success defined as a loss rate below 25 bbl/hr (3.97 m3/h) for severe and total losses (Cole et al. 2017, cited by Saleh et al. 2020). An earlier study of about 4,500 loss zones found about 65 % were total losses, cured in about 15 % of cases with cement and about 10 % with LCM (Hyodo et al. 2000, cited by Saleh et al. 2020).
Which Properties and Additive Classes Tie into Losses?#
Lost circulation ties into two additive classes and three mud properties: lost circulation materials and viscosifiers carry the cure, while density, ECD and rheology set the pressure that opens the loss path.
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The table below lists the classes, materials, properties, test and tools behind a lost circulation treatment, with the role of each.
| Item | Role in lost circulation |
|---|---|
| Class: lost circulation materials | Blends, sweeps and pills that bridge and seal the loss zone |
| Class: drilling fluid viscosifiers | Bentonite pills and gel that carry LCM and form gunk |
| Sized calcium carbonate | Acid-soluble bridging solid in background blends and pills |
| Resilient graphitic carbon (RGC) | Resilient bridging solid in blends and pills |
| Ground nut shells (nut plug) | Granular LCM in blends and pills |
| Bentonite | Pill and gel base, and the solid in diesel gunk |
| LCM fibers | Fibrous LCM in blends |
| Mica flakes | Flaky LCM that mats over openings in blends |
| Diatomaceous earth | Base of high-fluid-loss squeeze pills |
| Property: mud weight | Sets the static pressure at the zone |
| Property: ECD | Adds annular pressure loss while circulating; ECD calculator |
| Property: drilling fluid rheology | Raises ECD and surge pressure |
| Calculation: hydrostatic pressure | Pressure of the mud column on true vertical depth |
| Calculation: equivalent mud weight | Leak-off result written as a density |
| Test: permeability plugging test | Slotted or ceramic discs for corrective LCM evaluation |
| Tool: bridging agent calculator | D50 and D90 sizing of bridging solids |
Additive makers and service companies sell these materials by grade; compare additive suppliers by the materials they carry.
In the North-East Atlantic, calcium carbonate, including marble or limestone, is on the PLONOR list (OSPAR Agreement 2013-06, Update 2026).
How Is Lost Circulation Support Scoped?#
Lost circulation support is scoped by the loss rate, the zone type and the hole section: a mud engineer records the class, tests the planned blend or pill and sizes it to the zone before it is pumped. Operators scope this work as mud engineering services for the section at risk.
A request names the application, location, hole size and depth or bore length, formation notes (loss rate in bbl/hr, zone type, mud type and weight), timeline and a company email.
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How Does Lost Circulation Relate to Other Drilling Fluid Problems?#
Lost circulation relates to the other drilling fluid problems as a cause or as a neighbor in the same pressure window, as the table below compares.
| Problem | How it ties to lost circulation |
|---|---|
| Stuck pipe | A consequence of losses, once the mud column drops |
| Wellbore stability | The collapse side of the same operating window |
| Hole cleaning | A consequence of losses, with less mud to lift cuttings |
| Drilling-induced formation damage | Losses of mud and solids into the reservoir |
Lost circulation materials are the solids and pills added to a mud to stop the loss. Their types, particle sizing and pill design are grouped under lost circulation materials as an additive class.
Completion and production chemistry, including completion fluid-loss pills, is a separate field.
Is fluid loss the same as lost circulation?#
No: fluid loss is filtrate passing into a permeable wall while the solids build a filter cake, and lost circulation is whole mud leaving through openings too large for a cake to form.
Fluid loss is measured on the filter press in mL per 30 min as part of filtration and fluid loss, while lost circulation is a rate in bbl/hr. A mud with a low fluid loss can still lose whole mud to a fracture.
Lost circulation in HDD, water wells and geothermal wells#
Lost circulation occurs outside oil and gas wells too, with different fluids, doses and limits in each of the three applications below.
- In HDD drilling fluids, bentonite at 35 to 45 lb/100 gal (14.7 to 18.9 lb/bbl; 42 to 54 kg/m3) serves for lost-circulation control (typical value of MAX GEL (Wyo-Ben) data sheet).
- In water well drilling mud, LCM pills are mixed to about 50 s/qt Marsh funnel viscosity (Water Well Journal), and in foam drilling fluids the foamer may reach 5 % of the injection fluid in fractured loss zones (The Driller, 2018).
- In geothermal drilling fluids, hot fractured rock combines severe losses with the temperature limits and low cure rates set out above.
Air drilling removes the liquid mud column and may eliminate lost circulation, but it gives no control of formation-fluid inflow.