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API fluid loss and filter cake: how drilling fluid filtration tests work
The API fluid loss is the volume of filtrate, in millilitres, that a mud loses through filter paper with a 7.1 in² filtering area in 30 minutes at a 100 psi differential pressure and room temperature. The high-pressure, high-temperature (HPHT) test uses a 500 psi differential at elevated temperature in a half-area cell, so its 30-minute volume is doubled before it is reported. Under static conditions filtrate volume grows with the square root of time, which is why V30 ≈ 2 × V7.5 and why the spurt loss can be estimated as Vsp ≈ 2 × V7.5 − V30. This page explains both tests, the filter cake, and how to read the numbers.
Reference material only. Fluid-loss limits and treatments are set by the drilling fluids programme and the mud engineer. Test procedures are defined in API RP 13B-1 (water-based) and API RP 13B-2 (oil-based); follow the procedure and equipment in use on your rig.
1. The filtration formulas
Square-root-of-time law
V2 = V1 × √(t2 ÷ t1)
30-minute volume from 7.5 minutes
V30 ≈ 2 × V7.5
With spurt loss
V = Vsp + m × √t
Spurt loss from two readings
Vsp ≈ 2 × V7.5 − V30
HPHT reported filtrate
HPHT (mL/30 min) = 2 × V30 in half-area cell
Filtrate viscosity effect
V2 ÷ V1 ≈ √(μ1 ÷ μ2)
V is cumulative filtrate volume in mL, t is time in minutes, Vsp is the spurt loss (filtrate that passes before a cake has formed), m is the slope of filtrate against √t, and μ is filtrate viscosity. The square-root law holds for static filtration through a cake that keeps growing; it ignores spurt loss, which is why the spurt term is added. The 2 × V7.5 shortcut follows from √(30 ÷ 7.5) = 2.
2. The two standard tests
| Test | Differential pressure | Temperature | Filter area | Reported |
|---|---|---|---|---|
| API (low-pressure, low-temperature) | 100 psi | Ambient (room) | 7.1 in² (about 45.8 cm²) | Filtrate in mL after 30 min; cake thickness |
| HPHT | 500 psi | Elevated, up to about 380 °F with a standard cell | Half of the API area | 30-min filtrate doubled; cake thickness |
The API test is the routine check for water-based mud. The HPHT test is used for oil-based and synthetic muds and for water-based muds in hot hole sections, because it exposes the mud to temperature. The SLB Energy Glossary notes that HPHT cells are half the size of the ambient filtration area, so 30-minute volumes are doubled; that total cell pressure should not exceed 700 psi; and that the test simulates downhole temperature but not downhole pressure. When a half-size cell is used in the ambient test, its filtrate volume is doubled in the same way.
3. Worked example
An API filter press test on a water-based mud gives 4.2 mL of filtrate at 7.5 minutes and 7.4 mL at 30 minutes.
- Shortcut estimate: 2 × 4.2 = 8.4 mL/30 min, about 14% above the measured 7.4 mL, because the shortcut assumes there is no spurt loss.
- Spurt loss: Vsp ≈ 2 × 4.2 − 7.4 = 1.0 mL
- Slope: m = (7.4 − 4.2) ÷ (√30 − √7.5) = 3.2 ÷ (5.477 − 2.739) = 1.17 mL/√min
- Check at 15 minutes: V15 ≈ 1.0 + 1.17 × √15 = 1.0 + 1.17 × 3.873 = 5.5 mL. A reading well away from this suggests a leaking cell, torn paper or a cake that is not behaving as a static cake should.
- HPHT: the same mud collects 9.0 mL in 30 minutes in a half-area HPHT cell. The reported HPHT fluid loss is 2 × 9.0 = 18.0 mL/30 min.
Plotting cumulative filtrate against √t gives a straight line whose intercept is the spurt loss and whose slope reflects how quickly the cake controls further loss. A curved plot is a sign that the cake is being disturbed or compressed, or that the test is not static.
4. Why HPHT filtrate is higher
Two effects push HPHT results above API results for the same mud. First, temperature thins the filtrate. Water viscosity falls from about 0.91 cP at 75 °F to about 0.18 cP at 300 °F, so on viscosity alone the filtrate volume could rise by about √(0.91 ÷ 0.18) ≈ 2.2 times. Second, temperature can degrade polymers and change clay flocculation, which makes the cake itself more permeable. Pressure matters less than many expect: for an incompressible cake, filtrate would grow with √ΔP, about 2.2 times from 100 to 500 psi, but most mud cakes compress under higher pressure and lose permeability, so the real increase is usually much smaller. That is why the HPHT test is run at temperature: it reveals fluid-loss control that will break down in a hot hole.
5. The filter cake
What it is
The layer of mud solids left on the filter paper, or on the borehole wall, as filtrate passes into a permeable formation. A good cake is thin, tough and of low permeability.
How it is reported
Cake thickness is measured after the test and usually reported in 32nds of an inch or in millimetres (1/32 in ≈ 0.8 mm), with a note on texture: firm, soft, rubbery or brittle.
Static vs dynamic
In a static test the cake keeps growing because nothing erodes it. While circulating, the mud erodes the cake surface, so downhole cakes tend toward a thinner equilibrium thickness.
Why it matters
Thick cakes narrow the hole, raise swab and surge pressures and increase the contact area for differential sticking; high filtrate can damage productive formations.
Thick, soft cake opposite a permeable sand is one of the conditions behind differential sticking: the pipe embeds in the cake and the hydrostatic overbalance holds it against the wall. See stuck pipe and free point for how differential and mechanical sticking differ, and swab and surge pressure for why tight hole raises tripping pressures.
6. Reading the results
| What you see | Likely meaning | Usual direction of treatment |
|---|---|---|
| Filtrate rising, cake thicker | Fluid-loss control weakening: drilled solids, contamination or degraded additives | Check solids control; review fluid-loss additive and bentonite or polymer content with the mud engineer |
| Low API but high HPHT | Fluid-loss control that is not stable at temperature | Temperature-stable fluid-loss additives for the hot section |
| High spurt loss | Too few bridging particles sized to the formation pores | Review bridging material and its particle size distribution |
| Water in an oil-based mud HPHT filtrate | Emulsion instability | Check emulsifier and electrical stability with the mud engineer |
These are illustrative patterns, not limits. The acceptable API and HPHT values are set per well and hole section in the fluids programme, and they are read together with rheology and solids. For the rheology side, see PV, YP and gel strength from Fann readings; contamination that flocculates a water-based mud often raises yield point, gels and fluid loss together.
7. Running the API test well
- Use a clean, dry cell and fresh filter paper of the specified grade; a torn or poorly seated paper gives a large, false spurt.
- Apply the 100 psi differential quickly and start timing when pressure is applied.
- Read the filtrate at 7.5 and 30 minutes. Recording both gives the spurt loss and a check on the result for no extra time.
- After the test, release pressure, remove the paper, rinse loose mud gently and measure the cake thickness at several points.
- Report the temperature of the sample, because filtrate viscosity changes with it.
Permeability plugging tests, which use ceramic discs sized to the formation instead of filter paper, are used where the concern is sealing a specific formation rather than the routine paper test.
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 drilling fluid filtration results are measured and read; it does not describe a DrillX filtration calculator. For the mud weight itself see mud weight up and dilution. DrillX is in final testing; email info@drillx.app for a demo or early access.
Questions
What is API fluid loss in drilling mud?
API fluid loss is the volume of filtrate, in millilitres, that a drilling fluid loses in 30 minutes through filter paper with a 7.1 in² filtering area at a 100 psi differential pressure and room temperature, using a static filter press. It indicates how well the mud builds a low-permeability filter cake.
How is the HPHT fluid loss test different from the API test?
The HPHT test runs at a 500 psi differential pressure and an elevated temperature, up to about 380 °F with a standard cell, and is used for oil-based muds and hot hole sections. Its cell has half the API filter area, so the 30-minute filtrate volume is doubled before it is reported.
Why is the 7.5-minute filtrate doubled?
Under static filtration the filtrate volume grows with the square root of time, and √(30 ÷ 7.5) = 2, so twice the 7.5-minute volume estimates the 30-minute volume. The shortcut ignores spurt loss, so it reads high when spurt is significant; measuring at both 7.5 and 30 minutes avoids that.
What is spurt loss?
Spurt loss is the filtrate that passes through the filter medium before a filter cake has formed. It can be estimated from two readings as twice the 7.5-minute volume minus the 30-minute volume; with 4.2 mL at 7.5 minutes and 7.4 mL at 30 minutes the spurt loss is about 1.0 mL.
How is filter cake thickness reported?
Filter cake thickness is measured on the filter paper after the test and usually reported in 32nds of an inch or in millimetres, where 1/32 in is about 0.8 mm. A good cake is thin, tough and of low permeability; a thick, soft cake raises the risk of tight hole and differential sticking.
Why does fluid loss increase with temperature?
Higher temperature lowers filtrate viscosity, and filtrate volume rises roughly with the square root of the viscosity ratio; water at 300 °F is about five times thinner than at 75 °F. High temperature can also degrade polymer fluid-loss additives and change clay behaviour, making the cake more permeable.
Sources and further reading
- SLB Energy Glossary, API fluid-loss test: static filter press at ambient temperature, 100 psi differential, filter paper with 7.1 in² filtering area; half-size cell filtrate doubled.
- SLB Energy Glossary, HPHT filtration test: 500 psi differential, up to about 380 °F (450 °F with a special cell), total cell pressure not above 700 psi, half-area cell so 30-minute volumes are doubled; simulates downhole temperature but not pressure.
- SLB Energy Glossary, static filtration: cake keeps growing with no erosion; filtrate volume theoretically increases with the square root of time, ignoring spurt loss.
- API RP 13B-1 and RP 13B-2: field testing of water-based and oil-based drilling fluids, including low-temperature and HPHT filtration procedures and cake measurement.
- A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986), chapter 2: static filtration theory, spurt loss and the effect of time, pressure and filtrate viscosity.
Reference summary only. Worked examples are illustrative arithmetic, not property limits. Water viscosity values are standard physical data for pure water and are used only to show the size of the temperature effect.