Water, Oil and Solids Test

The water, oil and solids test (also called the retort test) is a quantitative distillation analysis performed on a drilling fluid sample in which a measured volume of mud is heated in a sealed retort chamber to vaporize and collect its water and oil components, leaving the dry solids behind, and the recovered volumes of water and oil are read directly from graduated collection tubes to determine the volume fractions of water, oil, and total solids in the mud, enabling calculation of individual solids components and monitoring of mud composition and solids control efficiency.

Key Takeaways

The retort test measures three primary outputs: percent water by volume, percent oil by volume, and percent total solids by volume (calculated as 100 minus percent water minus percent oil), providing the fundamental composition data for all subsequent solids analysis calculations.
From the retort water and oil volumes and independently measured mud density, the low-gravity solids content (bentonite equivalents plus drill solids), high-gravity solids content (barite), and the low-gravity solids-to-bentonite ratio can all be calculated using standard material balance equations, guiding solids control and dilution decisions.
For oil-based muds, the retort test determines the oil-water ratio (OWR), which governs emulsion stability, filtration control, and shale inhibition performance — a target OWR of 70:30 to 90:10 (oil to water by volume) is typical for HTHP oil-based mud systems.
The retort temperature must be high enough to vaporize all water and oil in the sample (typically 500 to 600 degrees C for the solids heating cycle) but low enough to avoid thermal decomposition of the solid components, which would produce erroneous gas volumes counted as liquid.
The retort test is a standardized procedure under API RP 13B-1 (Section 3, Retort Analysis) and is one of the most frequently performed mud tests on the rig floor, typically run at least once per 24-hour period and whenever significant formation fluid influx, dilution, or mud weight changes are made.

Fast Facts

Standard retort cell volumes are 10 mL (small, faster) and 20 mL (larger, more precise); API RP 13B-1 specifies both the 10 mL and 20 mL procedures. The typical precision of the retort test is approximately ± 0.5 to 1 volume percent for each component. For an oil-based mud with a target oil-water ratio of 80:20, a retort result showing 75 percent oil and 25 percent water versus total fluids indicates the ratio has drifted to 75:25, requiring an addition of base oil to restore the design OWR. The accuracy of all solids calculations depends directly on the retort test accuracy, so retort calibration (using water and oil standards) and technique standardization are critical to reliable mud engineering decisions based on retort data.

What Is the Water, Oil and Solids Test?

The composition of a drilling fluid — how much water, how much oil, and how much solid material it contains — is one of the most fundamental pieces of information needed for mud engineering decisions. Dilution rates, solids control equipment settings, weighting material additions, and emulsifier treatments all depend on knowing the current mud composition accurately. The water, oil and solids test provides this composition data through a direct physical measurement: distillation of a known volume of mud to separate and measure the liquid components.

The instrument used is a retort (also called a mud still), a small laboratory distillation apparatus consisting of a heated retort cup that holds the mud sample, a condenser tube that cools the vapors, and a graduated glass collection tube that measures the recovered liquid volumes. The entire apparatus is compact enough to operate on the rig floor during active drilling operations, making it possible to track mud composition in real time without sending samples to an offsite laboratory.

How the Retort Test Is Performed

The retort cup is filled to the exact calibrated volume mark with well-mixed mud sample and sealed. The cup is heated to approximately 400 to 500 degrees C (for the oil and water vaporization cycle) and the vapors travel through the condenser, where they are cooled and condensed, and drip into the graduated collection tube. Water and oil separate in the collection tube due to density difference — water sinks to the bottom, oil floats on top — and their individual volumes are read after the distillation is complete.

The collected water volume is read from the bottom of the tube, the total liquid volume is read from the top of the tube, and the oil volume is calculated as the difference. Total solids volume percent is calculated as 100 minus water percent minus oil percent. For example, if a 10 mL retort sample yields 3.5 mL water and 2.5 mL oil, the total liquids are 6.0 mL and total solids are 4.0 mL, giving 35 percent water, 25 percent oil, and 40 percent solids by volume.

These raw retort volumes, combined with the measured mud density and the known densities of the base oil (or diesel), water (or brine), and high-gravity solids (barite density approximately 4.25 g/cm3), are used in material balance equations to calculate low-gravity solids content, barite content, and the bentonite-to-drill-solids ratio. These derived values guide solids control centrifuge settings, dilution water additions, and barite top-up quantities.

Water, Oil and Solids Tests Across International Jurisdictions

Canada (AER / WCSB): AER well operations require that mud logs including retort test results be available for the period of drilling each hole section, and completion reports may include mud composition data from retort tests as part of the drilling fluid program documentation. WCSB heavy oil and oil sands drilling programs, which use oil-based or synthetic muds extensively, rely on retort OWR data to manage emulsion stability in challenging conditions where formation water influx can dilute the water phase and shift the OWR from target values. Mud loggers on WCSB rigs routinely run retort tests every 6 hours as part of the standard mud monitoring program.

United States (API / BSEE): API RP 13B-1 (water-based mud) and API RP 13B-2 (oil-based mud) both include standardized retort test procedures that are the reference methods for all US oilfield retort analysis. BSEE offshore drilling regulations require that mud properties be monitored and logged throughout drilling operations; retort test results are part of the mud log data that supports drilling permit compliance documentation. The RRC in Texas and similar state agencies may require retort data as part of environmental compliance documentation for disposal of water-based mud and associated drill cuttings.

Norway (Sodir / NORSOK): NORSOK D-010 references API RP 13B test procedures including retort analysis as the standard for evaluating oil and water-based mud properties on NCS wells. Equinor's mud monitoring programs for NCS wells specify retort test frequency (minimum every shift during drilling) and acceptance limits for OWR in oil-based muds. The OWR result from retort tests is a key input to Equinor's emulsion stability monitoring protocol, which tracks the water-to-oil phase balance that governs mud behavior at HTHP conditions in deep North Sea wells.

Middle East (Saudi Aramco): Saudi Aramco's drilling operations standards reference API RP 13B-2 retort procedures for oil-based and synthetic-based mud programs used in deep Khuff and Jurassic completions. Retort test results are included in the daily mud report submitted by the mud engineer and reviewed by Aramco's drilling supervisor and mud technologist for compliance with the approved mud program OWR and solids loading specifications.

The water, oil and solids test is most commonly referred to as the retort test or retort analysis in day-to-day rig operations. The instrument is called a retort or mud still. Related terms include retort, oil-water ratio (OWR), low-gravity solids, solids control, mud density, and API RP 13B. The "solids" in the test name refers to all non-liquid components — clay, barite, formation cuttings, and any other solid additives.

Tip: When retort water volumes are unexpectedly high in an oil-based mud with no increase in bottoms-up cuttings or other indicators of water influx, check for condensed steam from an unsealed sample or incomplete emulsification of the mud before sampling. A poorly mixed mud sample that has separated into oil-rich and water-rich layers will give erroneous retort results depending on which layer was sampled. Always agitate the mud pit thoroughly before taking the retort sample, and sample from the suction side of the active pit where mixing energy is highest. Compare the retort result to the mud weight: if the density is consistent with the design formulation but retort water is unexpectedly high, the excess water may indicate phase separation in the mud rather than true water influx.

FAQ

Can retort results be used to calculate bentonite content directly?
Retort results give total solids volume percent, which includes all solids: bentonite, drill solids, and barite. To separate these three components, the retort total solids value is combined with the measured mud density in a three-component material balance that assumes two low-gravity solid densities (bentonite density approximately 2.6 g/cm3 and drill solids density approximately 2.6 g/cm3, often treated as equivalent) and barite density (approximately 4.25 g/cm3). The result is total high-gravity solids (barite) and total low-gravity solids, but the retort alone cannot distinguish bentonite from drill solids. The methylene blue test (MBT) for reactive clay content is used alongside the retort to estimate what fraction of the low-gravity solids is bentonite versus non-reactive drill solids.

Why does the retort test sometimes show more water volume than expected in an oil-based mud?
Excess retort water in an oil-based mud can result from: fresh water influx from formation aquifer contact (if confirmed by increased mud weight and pit gain); water phase expansion from temperature changes in the mud system (warm formation water may have entered); or poor mud sampling technique where the sample was taken from a stratified zone. Additionally, high-density brines used as the water phase (calcium chloride or formate brines) may produce slightly different apparent volumes than fresh water in the retort due to their dissolved salt content, which should be accounted for in retort calibration for brine-containing systems.

Why the Water, Oil and Solids Test Matters

The retort test is the compositional foundation of all quantitative mud engineering analysis. Without knowing the exact volumes of water, oil, and solids in the mud, dilution calculations, solids control adjustments, and chemical treatment programs are based on inference rather than measurement. In oil-based mud systems where emulsion stability, lubricating performance, and shale inhibition all depend critically on maintaining the target oil-water ratio within narrow limits, the retort test is run at every shift to confirm that the OWR is on specification and to trigger corrective action before the OWR drifts outside the design range that governs the mud's downhole performance and environmental compliance profile.