Drilling Fluid Testing: API RP 13B-1 (2026) and 13B-2 (2023) Field and Laboratory Tests

Drilling fluid testing measures the density, flow, filtration, solids and chemistry of mud at the rig and in the laboratory, under API RP 13B-1 (6th edition, January 2026) for water-based and 13B-2 (6th edition, July 2023) for nonaqueous fluids. Sixteen tests in eight groups, from the mud balance to the static sheen test, cover the work, and the daily mud report collects their results.

Each result decides an additive treatment and shows what the formation is doing to the drilling fluid in the hole. A rising filtrate, a falling mud weight and a sulfide reading each call for a different additive class, so the useful question is which test answers which question, and under which practice.

The sections below give the test table, the four API Recommended Practices, the five standard conditions behind the common numbers, the result-to-treatment map, the test interval (every 30 to 45 minutes on water wells) and the three EPA discharge methods.

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 Drilling Fluid Testing?#

Drilling fluid testing is the routine measurement of a mud's properties against the fluid program, so that the mud engineer can see a change and treat it before it becomes a hole problem. The scope of API RP 13B-1 shows its breadth: density, viscosity and gel strength, static filtration, water, oil and solids, sand, methylene blue capacity, pH, alkalinity and lime, chloride, total hardness as calcium, low-gravity solids and weighting material, and HTHP static filtration through permeability plugging testing.

Tests measure properties, and properties are the targets the fluid program sets: the mud balance measures density, the viscometer rheology and the filter press filtration. The ranges and units of the drilling fluid properties are what each test reports against.

Field tests and laboratory tests#

Field tests run at the rig with a portable kit; laboratory tests age, heat and pressurize samples to answer questions the kit cannot. A rig kit holds the mud balance, the Marsh funnel, a rotational viscometer, a low-pressure filter press, a retort, a sand content kit and a titration set.

Laboratory testing of drilling fluid materials and of a fluid's physical, chemical and performance properties falls under API RP 13I (9th edition, 2020, with Addendum 1 of 2023). HTHP and permeability plugging filtration, shale inhibition and lubricity tests belong to that laboratory work.

Drilling Fluid Tests Compared: Instrument, Result and Practice#

Sixteen tests cover drilling fluid testing, grouped as density, flow, filtration, solids, chemistry, inhibition, friction and discharge checks. The table gives the instrument, the reported result and the governing practice of each, with the daily mud report as the 17th row.

Practice: the API Recommended Practice that holds the procedure, API RP 13B-1 for water-based and API RP 13B-2 for nonaqueous fluids.

The 16 tests are summarized below in table order.

• Mud balance: density in lb/gal (sg). The pressurized mud balance reads mud that carries entrained gas or air.
• Sag tests: settling of weighting material, read as a top-to-bottom density difference or a static sag factor.
• Marsh funnel: flow time in seconds per quart, the quickest viscosity check at the pits.
• Rotational viscometer: dial readings at 600, 300, 6 and 3 r/min give plastic viscosity, yield point and gel strengths.
• API and HTHP filter press: filtrate volume and filter cake.
• Permeability plugging test: HTHP static filtration through a ceramic or slotted disc instead of filter paper.
• Retort: water, oil and solids by volume, and from them the oil-water ratio of a nonaqueous mud.
• Sand content: the coarse, abrasive fraction of the solids, in volume % of the mud.
• Methylene blue test: reactive clay as lb/bbl bentonite equivalent (kg/m3). Methylene blue is a test reagent, not a mud additive.
• pH and alkalinity: Pf, Mf and Pm by titration with standard sulfuric acid.
• Chloride and total hardness: silver nitrate and EDTA titrations of the filtrate.
• Electrical stability: emulsion strength of a nonaqueous mud.
• Garrett gas train: sulfide and carbonate in the filtrate.
• Shale inhibition tests: hot-roll dispersion, linear swell and capillary suction time. Hot-roll recovery tops out near 85 to 86 % of the cuttings, because mechanical erosion counts as dispersion (AADE paper).
• Lubricity test: a coefficient of friction from an EP/lubricity tester, with no API or ISO lubricity test standard behind it.
• Static sheen test: free oil on mud or cuttings, run as EPA Method 1617.

The daily mud report collects these results in one record that the mud company hands to the operator each day.

Which API Practices Govern Each Test?#

Four API Recommended Practices govern drilling fluid testing: RP 13B-1 for water-based field tests, RP 13B-2 for nonaqueous field tests, RP 13I for laboratory tests and RP 13D for rheology and hydraulics. Their current editions on the API Standards Plan (checked 2026-10-27) are RP 13B-1, 6th edition (January 2026); RP 13B-2, 6th edition (July 2023) with Addendum 1 (April 2025); RP 13I, 9th edition (2020) with Addendum 1 (2023); and RP 13D, 8th edition (April 2026).

ISO 10414-1 (water-based) and ISO 10414-2 (oil-based) are the ISO counterparts of the two field practices. ISO 10416 is the counterpart of RP 13I, and ISO/DIS 10416 is at DIS stage, so ISO 10416:2008 stays current (ISO Open Data, 2026-10-21); a 10th edition of RP 13I is in development.

Recommended Practices describe how to run a test and are not specifications. No API or ISO lubricity test standard exists, and static sheen, SPP toxicity and retort on cuttings are EPA methods. The register of drilling fluid standards lists every edition beside the discharge rules.

Tests for Water-Based and Nonaqueous Fluids#

Water-based muds are tested under API RP 13B-1 and nonaqueous oil- and synthetic-based muds under API RP 13B-2, and the two lists differ where the continuous phase changes what can be measured. The 13B-2 list covers density, rheology, HTHP filtration, the retort, alkalinity, chloride, calcium, electrical stability, lime and salinity, and low-gravity solids.

Electrical stability and the oil-water ratio from the retort belong to nonaqueous fluids alone and are never cited to 13B-1. The methylene blue test, filtrate alkalinity and total hardness are the characteristic water-based checks.

A water-based system keeps water as its continuous phase, so its chemistry is read in the filtrate. The system types and additives of water-based mud follow from that water phase.

Standard Conditions Behind Common Test Results#

A drilling fluid test result means something only under its standard conditions, and five conditions sit behind the numbers most quoted on a mud report. The five are listed below.

• API filtrate: 100 psi (690 kPa) for 30 min on 7.1 in2 (45.8 cm2) of filter area.
• HTHP filtrate: 500 psi (3,447 kPa) differential in a cell of 3.5 in2 (22.6 cm2), with the filtrate doubled to the API area.
• Marsh funnel: 1 US quart (946 mL) of fresh water drains in 26 +/- 0.5 s, so the funnel viscosity of water is 26 s/qt.
• Viscometer: 600 r/min shears the sample at 1,022 s-1, and gel strength is the maximum 3 r/min reading after 10 s and 10 min.
• Sand: particles above 75 µm, the opening of a No. 200 sieve, in volume % of the mud.

Raw HTHP filtrate is therefore never reported as API HTHP fluid loss. Yield point and gels carry lbf/100 ft2 (Pa) and are never written in cP.

Rheology models and hydraulics calculations under API RP 13D build on the same viscometer readings. The models and flow regimes of drilling fluid rheology start from these dial readings.

How Test Results Drive Additive Treatments#

A test result outside the program points to one additive class: high filtrate to fluid-loss additives, low mud weight to weighting agents, rising hardness to calcium treatment and a sulfide reading to H2S scavengers. The six result-to-treatment pairs are listed below.

• High API or HTHP filtrate: fluid-loss additives such as polyanionic cellulose, CMC and starch.
• Low mud weight: weighting agents such as barite.
• Rising methylene blue capacity: reactive clay from drilled shale or excess bentonite, answered by dilution and solids removal.
• Rising calcium or total hardness: calcium treatment with soda ash or sodium bicarbonate.
• Sulfide in the gas train: H2S scavengers such as zinc oxide or zinc carbonate.
• Falling electrical stability of a nonaqueous mud: emulsifiers and wetting agents.

Results are read as trends rather than single points, and the daily mud report keeps that trend visible to the mud company and the operator.

Thinners, alkalinity agents, lubricants and biocides are dosed from the same field tests. The functions of every class of drilling fluid additives start from the result that called for them.

How Often the Mud Is Tested and Who Tests It#

The mud is checked several times a shift while drilling: water-well drillers test every 30 to 45 minutes (The Driller), and a detailed mud test on an HDD project takes about 1 hour (Trenchless Technology, 2020).

The daily mud report carries the mud properties and specifications, the solids analysis, the rheology and hydraulics and the products and volumes used, according to an AADE presentation on reading the report. The same presentation calls the report the link between the mud company and the operator.

The mud engineer is the person responsible for testing the mud at the rig and prescribing the treatments. Duties and service scope of a mud engineer follow from that testing role.

Tests That Decide Offshore Discharge#

Three EPA test methods in 40 CFR 435 decide whether US offshore mud and cuttings may be discharged: the static sheen test (Method 1617), the SPP toxicity test (Method 1619) and the retort for base fluid on cuttings (Method 1674). Sheen and toxicity decide whether a water-based mud leaves the platform, and the retort decides how much synthetic base fluid goes overboard on cuttings. The four discharge tests compare as follows.

EPA Method 1674 is Appendix 7 to 40 CFR 435 Subpart A and is derived from API RP 13B-2, but it is an EPA method, never an API one. OSPAR Decision 2000/3 also bars diesel-based fluids in the North-East Atlantic.

Toxicity, cuttings and waste rules apply by jurisdiction and by base fluid. The rules on drilling fluids and the environment start from these test results.

Where Drilling Fluid Testing Stops#

Drilling fluid testing ends at the mud: the cement slurry and the completion brine are not drilling fluid tests and follow other practices.

Cement slurry testing is not mud testing and belongs to well cementing, a separate field. Completion brine testing is not mud testing and belongs to completion chemistry, a separate field.

The fluid system decides which test list applies, 13B-1 or 13B-2. The types of drilling fluid systems set that choice.

Bored piles, water wells and other non-oilfield bores#

Support fluids for bored piles and diaphragm walls are tested to the EFFC/DFI Guide to Support Fluids for Deep Foundations, 1st edition (March 2019).

Water-well and HDD crews run a subset of the rig kit: mud balance, Marsh funnel, sand content and pH. A crew that needs a specialist can find a drilling fluid service provider for testing by describing the bore.

Testing the mud versus testing the materials#

API Spec 13A tests materials such as barite, bentonite and polyanionic cellulose before they enter the mud. The text in force is API Spec 13A, 19th edition (2019) with Addendum 1 (2020), Addendum 2 (2022) and Errata 1 (2024), the current edition (API Standards Plan, checked 2026-10-27); a 20th edition is in development. As a specification for 13 materials, API Spec 13A checks the dry product, while the RP 13B tests check the mixed fluid.