Contents
Partially compliant

API RP 40: rock wettability by oil and brine contact angle in core analysis

API RP 40 sets recommended practices for core analysis, and asks that wettability be measured as soon as possible after the core leaves the formation. With the optional cuvette, the Dropometer reads wettability as an oil and brine contact angle on a flat rock or mineral surface, with ±0.35° accuracy. Contact angle on a flat surface is an alternative method, not the core plug Amott or USBM test many core labs use; if a contract or customer requires an API RP 40 wettability result, run a correlation study on your own samples first.

What brings you here?

Who this is for
Core labs, reservoir and petroleum engineers, and EOR chemical teams who need a wettability reading on reservoir rock or model mineral surfaces.
Compliance tier
Partially compliant: with the optional cuvette, the Dropometer reads wettability as an oil and brine contact angle on a flat surface. It does not measure saturation, porosity or permeability, or run Amott or USBM tests.
Measurement
Oil and brine contact angle, captive drop under a submerged flat sample with the optional cuvette, 10° to 175° range, ±0.35° accuracy (Dropometer datasheet).
Setup
About 2 minutes for the base unit. Battery powered, 1 kg, runs on the lab bench.
Last updated
October 18, 2026
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Free one page checklist for an API RP 40 wettability reference by contact angle: what to prepare, the settings to lock, the drop geometry and what to record. Made for core and reservoir labs.

Talk to an expert

Book a 15 minute demo on your rock and brine

Tell us your rock, crude and brine. We will show the captive drop measurement live and tell you plainly which parts of your core analysis the Dropometer covers and which it does not.

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View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against API RP 40
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box

Standard

API Recommended Practice 40, Recommended Practices for Core Analysis, second edition, February 1998. It replaced RP 40 (1960) and RP 27. API's store, run by Accuris, lists this edition as withdrawn, with no newer edition shown. It is still cited in core analysis work.

Compliance tier

Partially compliant. RP 40 section 2.6 aims to measure wettability as soon as possible after core recovery, as a reference. With the optional cuvette, the Dropometer gives that reading as an oil and brine contact angle on a flat rock or mineral surface. It does not run the saturation, porosity, permeability or supplementary tests, or core plug Amott and USBM tests.

Instrument performance

Contact angle range 10° to 175°, accuracy ±0.35°, display resolution 0.01°, from the Dropometer product specification. With the optional cuvette, the Dropometer also runs captive bubbles, captive drops under a submerged sample, and interfacial tension by pendant drop of one liquid in another.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

We have no published client paper or case study on reservoir rock wettability yet. For a related workflow, see wettability alteration screening.

What it does not tell you

A contact angle on a flat surface does not capture pore geometry, mixed wetting inside the pores or a whole plug's response. Amott and USBM indices do. Pair the angle with your core lab tests.

Executive summary

What this practice covers, who uses it and how the Dropometer fits

API RP 40 is the American Petroleum Institute's recommended practice for core analysis: how to plan coring, handle and preserve core, and measure saturation, porosity and permeability. Core labs, reservoir engineers and operators use it to keep core data comparable. Its section on preserving wettability asks for a wettability reading as soon as possible after the core is recovered. The Dropometer is partially compliant: with the optional cuvette it reads wettability as an oil and brine contact angle on a flat rock or mineral surface, with ±0.35° accuracy. It does not measure saturation, porosity or permeability, and it does not run Amott or USBM tests on core plugs.

Contact angle on a flat surface is an alternative method, not the core plug imbibition or centrifuge test many labs pair with RP 40; if a contract or customer requires a wettability index, run a correlation study on your own samples first. RP 40 warns that drilling fluid, coring fluid, clays and storage can all change wettability, so a quick early reading gives you a reference before those effects build up. For bench practice, see reproducible contact angle measurement.

The Dropometer contact angle and surface energy kit runs on battery power in the core lab. With the same optional cuvette it also measures oil and brine interfacial tension by pendant drop. To screen chemicals that shift rock wetting, see wettability alteration screening for surfactants and chemicals. For background, read the oil and gas guide and the contact angle measurement guide.

Further reading: water contact angle reference values for common surfaces, choosing a contact angle goniometer for industrial QA and the Leidenfrost effect, a wetting effect on very hot surfaces.

How the Dropometer fits the workflow

A contact angle wettability reference for API RP 40 work, step by step, in our own words

From rock surface to a recorded oil and brine angle

These steps outline one common way to take a contact angle wettability reading with the Dropometer and the optional cuvette. They do not replace API RP 40 or your core lab methods. Set the surfaces, liquids, ageing and drop volume in your own written method.

±0.35°

Contact angle accuracy

From the Dropometer product specification. Display resolution is 0.01°.

10° to 175°

Contact angle range

Covers most water wet to strongly oil wet readings. A drop that spreads flat on clean quartz reads below range.

Cuvette

Optional accessory

Holds the brine and the submerged rock or mineral surface for the captive oil drop. The same optional cuvette runs oil and brine interfacial tension.

1

Prepare flat rock or mineral surfaces

Cut and polish flat slices from the rock, or use quartz or calcite plates as model surfaces. Clean and dry them the same way every time and handle them by the edges. RP 40 warns that cleaning and storage can change wettability, so record each step.

2

Age the surfaces in brine and crude

Soak the surfaces in formation brine or a matched synthetic brine, then in crude oil if your method restores reservoir wetting. Record ageing time and conditions. RP 40 notes that water saturation, temperature and time all affect restored wettability.

3

Set up the Dropometer and cuvette

Level the base plate, mount the phone and LED, and fill the optional cuvette with brine. Hold the surface face down in the brine. Custom holders are available on request.

4

Place the oil drop under the surface

Release the crude oil drop below the submerged surface in the optional cuvette, so it rises and settles. A captive drop under a submerged sample is a supported Dropometer setup. Keep the drop volume the same for every drop.

5

Capture and read the angle

Capture the drop at the reading time your method sets. Automatic edge and baseline detection reads the left and right angles, so two operators get the same reading from the same image. Report the angle through the water phase.

6

Repeat, record and correlate

Repeat on fresh areas. Record the mean, spread, rock, crude, brine, ageing and room temperature. Then compare the angles with Amott or USBM results on sister plugs. To screen chemicals that shift wetting, see wettability alteration screening.

7

Watch the full measurement

This video shows a complete Dropometer contact angle measurement, from setup to result. It covers the steps above in one run.

Dropometer contact angle measurement, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A. Published contact angle accuracy is ±0.35°.

Read the validation papers

Customer evidence

We have no published client paper or case study on reservoir rock wettability yet. The wettability alteration use case shows how oil and water contact angles screen chemicals that shift wetting.

Read the use case

Set your wettability bands from your own rock

API RP 40 · rock wettability · oil and brine contact angle

API RP 40 asks for an early wettability reference but, in the text we could review, gives no contact angle limits. A common reference table gives approximate bands, read through the water: water wet from 0° to about 60 to 75°, neutral from there to about 105 to 120°, and oil wet above that (source: AAPG Wiki, Wettability, Table 1).

Contact angle tests on model surfaces use cleaned quartz or calcite, aged in brine and then with a crude oil drop, as described in a Schlumberger review (source: Abdallah et al., Oilfield Review, 2007). The same review notes that Amott and USBM indices run from about +1 for strongly water wet to about minus 1 for strongly oil wet.

So set your own bands from a correlation study. Measure angles on flat surfaces cut from the same rock as sister plugs, then compare them with the Amott or USBM results from your core lab. Repeat the study when you change rock type, crude, brine or ageing method.

Example output

Example data for illustration only. These are not measured results. Your own rock, crude, brine, ageing and bands decide the numbers.

Example: oil drop under flat rock and mineral surfaces in brine, angle through the water

Surface Treatment Drops (n) Mean angle through water (°) Std dev (°) Example band
Quartz plateBrine only528.61.9Water wet
Quartz plateAged in crude A584.13.2Neutral
Calcite plateAged in crude A5131.74.0Oil wet
Calcite plateAged, then surfactant B soak592.53.6Neutral
Carbonate rock sliceAged in crude A5118.35.8Oil wet

How to read this table

Each row is the mean of five drops on fresh areas. The band column uses example cut points of 75° and 105°, inside the approximate ranges in the AAPG Wiki table. Your own bands come from a correlation study with core plug tests.

Example output chart

The same example data as the table, plotted against example wettability bands at 75° and 105°.

Bar chart of example oil and brine contact angles on quartz, calcite and a carbonate rock slice, before and after crude ageing and a surfactant soak, with example wettability bands at 75 and 105 degrees

Example data, not measured results.

Business impact

What changes when the wettability reference is a measured angle

Metric Before Dropometer With Dropometer Results
Early wettability reference None, or judged from field behaviour Oil and brine contact angle on a flat surface from the same rock API RP 40 section 2.6 aim
Result format Water wet or oil wet, by judgement An angle per drop, 0.01° display, ±0.35° accuracy Dropometer product specification
Record Lab notebook entry Drop image and angle saved per measurement No published benchmark
Wettability index on core plugs Amott or USBM in a core lab Still need the core lab Abdallah et al., Oilfield Review 2007

See the measurement on your own rock

Bring a flat rock slice or mineral plate, your crude and your brine. We will measure the angle live under brine.

Book a 15 minute demo Request a quote
ROI estimate

Estimate the cost of the core data problems an early wettability reading can help catch

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

Estimate what wetting related failures cost you.

Example only. Use what you see in a trial on your own samples.
Your own figure.
Your own estimate.

Result

~0
Estimated annual cost today
~0
Estimated annual cost avoided
~0
Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Method settings

Lock these before you compare rocks, wells or treatments

Parameter Recommended Setting Technical Rationale
Surface Flat, polished slice of the rock, or a quartz or calcite plate A drop needs a flat surface. Pores and roughness change the apparent angle.
Bulk phase Brine in the optional cuvette Most crudes are opaque. A clear bulk phase lets the camera see the drop outline.
Drop phase and geometry Crude oil drop under the submerged surface, captive drop in the optional cuvette The oil rises onto the surface. A water drop under clear model oil is the other common layout.
Angle convention Through the water phase, stated in the report Most wettability studies report it this way. Higher angles then mean more oil wet.
Ageing As your method sets, recorded for every surface Crude components adsorb over time, and RP 40 notes time and temperature affect restored wettability.
Drop volume As your method sets, fixed for every drop The automatic dropper add on doses down to 0.05 µL.
Static or moving contact line Static angle at a fixed reading time; advancing and receding if your method needs them The Dropometer reads advancing and receding angles by adding or withdrawing liquid through the needle.
Environment Room temperature, recorded The Dropometer has no temperature control, does not test under pressure and runs at 10 to 45 °C.

Which wettability or core test do you need?

Pick the method by the question you need answered

Start: What do you need to know about your rock and fluids?

Wettability as an angle on a flat surface

Signals:

You want an early wettability reference, or a quick comparison of rocks, crudes, brines or chemicals.

Rule-out:

Use an oil and brine contact angle, this page. The Dropometer is partially compliant and runs it as a captive oil drop under the submerged surface, with the optional cuvette.

Wettability index on a core plug

Signals:

You need an Amott or USBM index for reservoir models or a contract.

Rule-out:

Use a core lab with imbibition cells, a flood cell or a centrifuge. The Dropometer does not run these tests. The angle can help you choose which plugs to send.

Saturation, porosity or permeability

Signals:

You need routine core analysis data on plugs or whole core.

Rule-out:

Use the API RP 40 core lab methods. The Dropometer does not measure these properties.

Oil and brine interfacial tension

Signals:

You study how a surfactant or brine changes oil and water behaviour.

Rule-out:

The Dropometer measures it by pendant drop with the optional cuvette. See ASTM G205 for oil and water wetting on steel.

Decision tree at a glance

Start from the question you need answered. Blue boxes are measurements the Dropometer runs, with the optional cuvette.

Decision tree for choosing between an oil and brine contact angle on the Dropometer, Amott or USBM core plug tests, API RP 40 routine core analysis and pendant drop interfacial tension with the optional cuvette

Which wettability or core test do you need?

Interpreting results

What the angle tells you, and what it does not

Higher angle through water, more oil wet: When oil holds the mineral surface, water cannot spread on it, and the angle through the water rises.
Neutral angles need a second test: Angles between about 75° and 105° sit between the common bands. Confirm with an Amott or USBM test on a sister plug.
Wetting changes with ageing: Crude components adsorb on minerals over time. Compare surfaces only at the same ageing and reading time.
A flat surface is not a pore: The angle shows how the mineral surface wets. It does not show mixed wetting inside the pores of a whole plug.
Expert perspective

Why the oil and brine angle on rock tracks wettability

Wettability is set by the top five to ten ångström of a surface: what the liquid actually sees. It doesn't matter what lies beneath; that outermost layer is what you are really measuring.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls and limits

Honest limits of the method and of the Dropometer

Opaque crude in the bulk phase hides the drop. Put the crude in the drop, or use a clear model oil on crude aged surfaces.
The Dropometer has no temperature control and does not test under pressure. Results are at room temperature, not reservoir conditions.
Clean quartz under brine can read below 10°, the bottom of the range, and strongly oil wet surfaces can pass 175°. Report those drops as out of range.
Porous rock slices can soak up the drop or pin it on rough grains. Polish the surface and read at a fixed time.
Oil and brine angles, captive drops and interfacial tension all need the optional cuvette, so order it with the base unit.
The camera runs at 10 frames per second. That suits a reading at a fixed time, not millisecond spreading studies.
FAQ

Frequently asked questions

What does API RP 40 cover?

API RP 40 sets recommended practices for core analysis in eight sections: planning a coring program, wellsite handling and preservation, core screening and preparation, fluid saturation, porosity, permeability, supplementary tests and reporting. It excludes two and three phase relative permeability. The second edition dates from February 1998.

Does API RP 40 include a wettability test?

RP 40 treats wettability mainly as something to protect. It warns that drilling fluid, clays and storage can change it, and section 2.6 aims to measure wettability as soon as possible after recovery as a reference. In the text we could review it names no Amott, USBM or contact angle procedure. Check your copy before you write a method.

Can the Dropometer run API RP 40?

In part. With the optional cuvette, the Dropometer gives a wettability reading as an oil and brine contact angle on a flat rock or mineral surface, with ±0.35° accuracy. It does not measure saturation, porosity or permeability, so you need core lab equipment for the rest of RP 40.

Can the Dropometer give an Amott or USBM wettability index?

No. Amott uses spontaneous and forced imbibition on a core plug, and USBM uses a centrifuge, so both need a core lab. Contact angle on a flat surface is an alternative method, not the core plug Amott or USBM test; if a contract or customer requires an API RP 40 wettability result or a wettability index, run a correlation study on your own samples first.

What contact angle means oil wet rock?

No single limit is published in API RP 40. A common reference table, read through the water, gives water wet up to about 60 to 75°, neutral up to about 105 to 120°, and oil wet above that. See the AAPG Wiki wettability page. Set your own bands from sister plug tests.

Do I need the optional cuvette?

Yes. A wettability angle looks at oil and brine on the rock at the same time, so the surface must sit in brine. The optional cuvette holds the brine and the submerged surface, and the oil drop is released under it as a captive drop. The same optional cuvette runs captive bubbles and oil and brine interfacial tension by pendant drop.

Is API RP 40 still current?

API's store, run by Accuris, lists the 1998 second edition as withdrawn, and no newer edition is listed. Many labs and contracts still cite it. Check with API, or with your client, which edition or replacement method your work must follow.

Legal note

This page summarises API RP 40 in our own words and does not reproduce it. Buy the edition your contract names before you write a procedure. The Dropometer is partially compliant: it gives a contact angle wettability reading with the optional cuvette, not saturation, porosity, permeability or Amott and USBM indices. Droplet Lab does not certify compliance. Your own method validation does.

View the official standard Book a 15 minute demo

How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

API RP 40 listing, Accuris (API store): 2nd edition, 1998, withdrawn; API RP 40 listing, GlobalSpec: February 1998, inactive; API RP 40, Recommended Practices for Core Analysis, 2nd edition (Energistics copy; sections 1 to 2.6.1 reviewed); Abdallah et al., Fundamentals of Wettability, Oilfield Review, Summer 2007, the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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Correction Request

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References

1. API RP 40, 2nd edition (1998). store.accuristech.com/standards/api-rp-40?product_id=3553
2. API RP 40 listing, GlobalSpec: February 1998, inactive. standards.globalspec.com/std/630065/api-rp-40
3. API RP 40, Recommended Practices for Core Analysis, 2nd edition (Energistics copy; sections 1 to 2.6.1 reviewed). energistics.org/sites/default/files/2022-10/rp40.pdf
4. Abdallah et al., Fundamentals of Wettability, Oilfield Review, Summer 2007. slb.com/-/media/files/oilfield-review/p44-61-english
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/