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Drilling problems · lost circulation · field units
Lost circulation: loss-rate classification, LCM and the calculations
Lost circulation is the loss of whole drilling fluid into the formation, so less mud returns up the annulus than is pumped down the drill string, or none at all. Losses are classified by rate in barrels per hour, commonly as seepage, partial, severe or total, and the treatment, from adding lost circulation material (LCM) to the mud to setting a cement plug, follows from that rate and from what kind of rock is taking the fluid.
Reference material only. Loss-rate thresholds, LCM limits and treatments are set by the operator's drilling programme, the mud engineer and the tool vendors on your rig. Total losses are a well-control concern: follow your approved well-control procedures.
1. The lost circulation formulas
Loss rate
Loss (bbl/hr) = (Pit drop − New hole volume) ÷ Time (hr)
New hole volume
Vhole (bbl) = Hole size² ÷ 1029.4 × Footage
Pressure lost when the level drops
ΔP (psi) = 0.052 × MW (ppg) × Level drop (ft)
Effective mud weight at TVD
EMW = MW × (TVD − Level drop) ÷ TVD
Volume to fill the annulus
V (bbl) = (Dh² − OD²) ÷ 1029.4 × Level drop
LCM for a pill
LCM (lb) = Concentration (ppb) × Pill volume (bbl)
Hole size, Dh and OD are in inches, 1029.4 converts in² × ft to barrels, MW is mud weight in pounds per gallon, TVD is true vertical depth in feet and ppb is pounds of LCM per barrel of mud. The level-drop formulas are the hydrostatic pressure formula applied to the missing column of mud; the fill-up volume uses the same annular capacity as annular velocity.
2. Loss-rate classification
| Category | Rabia (Well Engineering & Construction) | SLB Energy Glossary | What it usually means at the rig |
|---|---|---|---|
| Seepage | Up to 10 bbl/hr | Under about 20 bbl/hr | Slow loss, easily mistaken for filtrate loss or solids-control discard; often treated by adding fine LCM while drilling ahead |
| Partial | 10 to 50 bbl/hr | Over about 20 bbl/hr with some returns | Clearly reduced returns; usually LCM pills or background LCM, often with reduced pump rate or ECD |
| Severe | More than 50 bbl/hr | (included in partial) | Large volumes lost quickly; drilling normally stops to cure the losses |
| Total | No returns | No returns | The fluid level falls out of sight when pumps stop; hydrostatic pressure on open formations drops and the well must be kept full |
The two references disagree on the numbers, and many operators use their own. What matters for the morning report and for choosing a treatment is the measured rate in bbl/hr, whether it is measured static (pumps off) or dynamic (pumps on, with the flow rate stated) and whether the losses started gradually or suddenly. A sudden total loss usually points to a fracture or cavern; a gradual rise in losses as a sand is drilled points to permeable matrix.
3. Where the mud goes
Highly permeable rock
Unconsolidated sands, gravels and shell beds with pore throats large enough for whole mud to enter. Losses build gradually as more of the zone is exposed.
Natural fractures
Pre-existing fractures can take mud at low overbalance; losses are often sudden and may reverse as ballooning when pumps stop.
Vugs and caverns
Typical of karstic carbonates. Losses are sudden and often total, sometimes with the bit dropping; conventional LCM may not seal them.
Induced fractures
Created when wellbore pressure, including ECD and surge pressure, exceeds the fracture pressure, for example at the weakest point just below the last casing shoe.
Induced losses are the ones the drilling crew controls most directly. Running in too fast, breaking circulation hard after a trip, a packed-off annulus from poor hole cleaning and raising mud weight without checking the shoe strength all raise wellbore pressure toward the fracture limit established by the leak-off or formation integrity test.
4. Worked example: loss rate and the hydrostatic you lose
While drilling 8½-in hole at 9,000 ft TVD with 10.5 ppg mud, the active pit volume falls 18 bbl in 30 minutes. In that time 45 ft of new hole was drilled. The drill pipe is 5-in OD.
- New hole volume: 8.5² ÷ 1029.4 = 0.0702 bbl/ft × 45 ft = 3.2 bbl, which filled new hole and was not lost.
- Loss rate: (18 − 3.2) ÷ 0.5 hr = about 30 bbl/hr. That is partial loss under the Rabia scheme and also partial (over about 20 bbl/hr) under the SLB definition.
- Losses turn total: the pumps are stopped and the annulus level falls 300 ft below the flowline.
- Pressure lost: 0.052 × 10.5 × 300 = 164 psi less at every depth below the fluid level.
- Effective mud weight at TVD: 10.5 × (9,000 − 300) ÷ 9,000 = 10.15 ppg. If any open zone needs more than about 10.15 ppg to stay static, it can now flow.
- Filling the annulus: open-hole annular capacity (8.5² − 5²) ÷ 1029.4 = 0.0459 bbl/ft, so 300 ft takes 13.8 bbl. Filling with 8.33 ppg water instead of mud restores 0.052 × 8.33 × 300 = 130 psi of the 164 psi, which is why the annulus is often topped up with water or base oil and the volumes pumped are recorded.
This is the link between lost circulation and well control: once losses are total, the bottomhole pressure falls by the missing column, and a zone that was safely overbalanced can kick. The kick tolerance and kill sheet pages cover what happens next if it does.
5. Lost circulation materials
| Type | Examples | Typical use |
|---|---|---|
| Granular | Sized calcium carbonate, graphite, ground nut shells, gilsonite | Bridging pore throats and fracture mouths; sized carbonate is acid-soluble and often preferred across reservoirs |
| Flaky | Mica, cellophane flakes | Lying across openings to build a mat on the wall |
| Fibrous | Cellulose and wood fibre, cotton seed hulls, shredded paper | Forming a mesh that traps other particles; often in pills |
| Blends | Mixed granular, flaky and fibrous products | General-purpose pills when the loss mechanism is not yet known |
| Reinforcing plugs | Cross-linked polymer pills, high-fluid-loss squeezes, gunk (bentonite in oil) squeezes, cement plugs | Severe and total losses into fractures and caverns that conventional LCM does not seal |
Particle size matters more than product name. For permeable rock, a common guide from Abrams (1977) is that the median bridging particle should be at least about one third of the median pore size and present at a few percent of the solids volume; for fractures, the particle distribution is matched to the estimated fracture width. Coarse LCM must also pass the bit nozzles, mud motor, MWD and LWD tools, so tool vendors publish maximum LCM size and concentration, and coarse pills are often pumped through open-ended pipe or a circulating sub.
6. Worked example: LCM pill
A 50 bbl pill at 40 ppb of a fibrous and granular blend is mixed for partial losses.
- LCM required: 40 ppb × 50 bbl = 2,000 lb. Divide by the sack weight printed on the product to get the number of sacks.
- Spotting: the pill is pumped to cover the loss zone with some excess. With 5-in drill pipe of 0.01776 bbl/ft capacity, displacing the pill to the bit at 9,000 ft takes about 0.01776 × 9,000 ≈ 160 bbl, from the drill pipe capacity. The pump strokes follow from the pump output.
- Pill height: in the 0.0459 bbl/ft annulus from the example above, 50 bbl covers about 50 ÷ 0.0459 ≈ 1,090 ft of open hole, less once the pill starts entering the zone.
Check the actual capacity of your drill string and BHA from the tally; the numbers above only show the method. The 0.01776 bbl/ft figure is for 5-in, 19.5 lb/ft pipe with a 4.276-in ID.
7. Prevention and first response
- Know the margin: mud weight plus ECD and surge against the formation integrity at the shoe and the expected fracture gradient of the open hole.
- Control trip speed and break circulation gently after connections and trips, rotating first and bringing pumps up in stages.
- Keep the hole clean so that cuttings beds and pack-offs do not spike annular pressure.
- On losses: record time, depth, rate (static and dynamic) and pit volumes; reduce pump rate if the programme allows; keep the hole full and monitor the well on the trip tank.
- Report losses with an IADC time code and as non-productive time so their cost is visible; see IADC daily drilling report codes and NPT in drilling.
Lost circulation and stuck pipe are linked: losses can leave cuttings and LCM behind the BHA, and the fix for one can trigger the other. See stuck pipe and free point.
8. In DrillX
DrillX brings daily drilling reports with IADC-style time logs, a rig fleet view, NPT insights and engineering calculators into one tool. The calculators cover hydrostatic pressure, ECD, annular pressure loss, triplex pump output, annular capacity and velocity, buoyancy and the kill sheet. This page explains how lost circulation is measured, classified and treated; it does not describe a DrillX lost circulation calculator. DrillX is in final testing; email info@drillx.app for a demo or early access.
Questions
What is lost circulation in drilling?
Lost circulation is the loss of drilling fluid into the formation instead of returning up the annulus. It happens when the bit drills into natural fractures, vugs, caverns or highly permeable rock, or when the pressure in the wellbore exceeds the fracture pressure and opens new fractures. Returns at the flowline fall below the pumped rate or stop altogether.
How are lost circulation losses classified?
Losses are classified by rate. One widely used scheme (Rabia) calls up to 10 bbl/hr seepage, 10 to 50 bbl/hr partial and more than 50 bbl/hr severe, with total losses meaning no returns at all. The SLB Energy Glossary instead puts seepage at under about 20 bbl/hr. Operators set their own thresholds, so always quote the rate in bbl/hr, not just the category.
How do you calculate the mud loss rate?
Measure the drop in the active pit system over a known time, subtract the volume that went into new hole drilled in that time (hole capacity in bbl/ft times footage) and divide by the time in hours. For example, an 18 bbl drop in 30 minutes while drilling 45 ft of 8½-in hole is (18 − 3.2) ÷ 0.5 ≈ 30 bbl/hr.
How much bottomhole pressure is lost when the annulus level drops?
The pressure lost equals 0.052 × mud weight (ppg) × level drop (ft). A 300 ft drop in a 10.5 ppg mud removes 0.052 × 10.5 × 300 ≈ 164 psi from every point below the fluid level, which can let a higher-pressure zone flow. That is why the annulus is kept full during total losses.
What are the main types of lost circulation material?
Lost circulation materials are grouped as granular (for example sized calcium carbonate, graphite, nut shells), flaky (mica, cellophane flakes) and fibrous (cellulose, cotton seed hulls, wood fibre), and are usually blended. Severe and total losses can need reinforcing plugs such as cross-linked pills, gunk squeezes or cement instead of conventional LCM.
Why can't you pump any size of LCM through the bit?
Coarse LCM must pass the bit nozzles and downhole tools such as mud motors and MWD or LWD tools, or it can plug them. Pills with coarse material are often pumped through open-ended pipe or a circulating sub above the BHA, and tool vendors publish maximum LCM size and concentration for their tools.
Sources and further reading
- SLB Energy Glossary, lost returns: seepage under about 20 bbl/hr, partial over about 20 bbl/hr with some returns, total with no returns; the hole may not stay full and another zone may flow.
- H. Rabia, Well Engineering and Construction, Entrac Consulting (2002), pp. 505–508: seepage up to 10 bbl/hr, partial 10–50 bbl/hr, severe above 50 bbl/hr, total losses; LCM types and size limits from nozzles and downhole tools (as summarised in Wikipedia: Lost circulation).
- A. Abrams, "Mud design to minimize rock impairment due to particle invasion", Journal of Petroleum Technology (1977): bridging particle size of about one third of the pore size.
- A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986): hydrostatic pressure, annular capacity and lost circulation.
- IADC Drilling Manual: lost circulation prevention and treatment practice.
Reference summary only. Worked examples are illustrative arithmetic, not limits or recommended concentrations.