Plug core saturation testing is a fundamental laboratory procedure in petroleum reservoir evaluation that quantifies the distribution of oil, gas, and water within the pore system of cylindrical rock samples recovered from wells. Accurate saturation data underpins reserves estimation, reservoir characterization, and petrophysical log calibration, making it indispensable throughout the entire field life cycle. This article explains the testing principles, sample requirements, analytical methods, applicable standards, and practical applications of core plug saturation analysis.
Technical Overview
Plug core saturation testing refers to the laboratory determination of fluid saturation—the volumetric fraction of pore space occupied by oil, gas, and water—in right cylindrical rock samples (core plugs) drilled from full-diameter cores recovered during well drilling. Core plugs are typically 25.4 mm (1 inch) or 38.1 mm (1.5 inches) in diameter, with lengths of approximately 1.5 to 2 times the diameter, and are selected at regular intervals along the recovered core to represent the reservoir interval.
Fluid saturation is defined as the ratio of the volume of a given fluid phase within the plug to the total pore volume. Because the three principal phases jointly occupy the pore system, the saturations must satisfy the relationship where water saturation, oil saturation, and gas saturation sum to unity. Direct laboratory measurement on core plugs provides the only physically recovered evidence of in-situ fluid distribution, and the resulting data serve as a benchmark against which indirect measurements such as well log interpretation and formation testing are calibrated.
A key consideration in saturation work is the preservation of native fluids. During coring and core recovery, mud filtrate invasion, pressure depletion, and fluid expansion can alter the original saturation state. Accordingly, industry practice, as codified in API RP 40, GB/T 29172, and SY/T 5336, emphasizes rapid preservation through sealing, freezing, or wrapping of cores at the wellsite, followed by plug drilling under preserved conditions in accredited third-party laboratories.
Testing Samples
Conventional Core Plugs
Standard cylindrical plugs are drilled from whole core, usually with water- or nitrogen-cooled diamond bits to minimize fluid contamination. Horizontal plugs, drilled parallel to bedding, are preferred for saturation and routine petrophysical analysis in conventional reservoirs, while vertical plugs may be acquired to evaluate saturation anisotropy and vertical fluid distribution.
Preserved and Frozen Core Plugs
Where the objective is to quantify native or remaining oil saturation, plugs are cut from cores that were sealed at the wellsite using foil, wax, heat-shrink tubing, or immersion in sealed containers, or that were frozen to immobilize fluids. Preserved plugs are analyzed without prior cleaning so that the original fluid distribution remains intact until extraction.
Restored-State Plugs
For special core analysis programs, cleaned plugs are re-saturated under controlled conditions to reproduce wettability and saturation states representative of the reservoir. These samples support aging, displacement, and relative permeability studies in which saturation endpoints must be reproduced reliably.
Sidewall and Unconventional Samples
Rotary-side wall plugs and plugs from tight sandstone, carbonate, and shale formations can also be analyzed, though smaller sample volumes and low permeability require adapted extraction protocols and longer distillation or soaking cycles.
Testing Items
Typical saturation-related deliverables from plug analysis include:
- Water saturation, expressed as a percentage or fraction of pore volume
- Oil saturation, determined directly or by difference after extraction
- Gas saturation, calculated by difference once liquid saturations are established
- Fluid volumes per unit mass of rock, reported in milliliters per gram or per 100 grams
- Pore volume, porosity, and grain density measured on the same plug to convert fluid volumes to saturations
- Bulk and grain density profiles supporting lithology and log interpretation
- Remaining oil saturation and residual oil saturation for waterflood and enhanced oil recovery evaluation
- Saturation profiles along the core depth interval to identify transition zones, fluid contacts, and bypassed pay
Testing Methods
Dean-Stark Azeotropic Distillation
The Dean-Stark method is the reference technique for water saturation on preserved plugs. The plug is placed in a distillation apparatus where refluxing toluene (or an equivalent solvent) vaporizes and co-distills the pore water, which is condensed and collected in a calibrated sidearm. Water volume is read directly, while the oil content is obtained gravimetrically from the mass difference of the plug before and after extraction, combined with measured oil density. Gas saturation is then determined by difference. The method provides high accuracy for water content and allows subsequent porosity and permeability measurement on the same cleaned plug.
Retort Distillation (Fluid Summation)
In retort analysis, the plug is heated in stages to vaporize water and oil, which are condensed and measured volumetrically. Total fluid volumes, combined with pore volume from companion measurements, yield individual saturations. Corrections must be applied for clay-bound and structural water released at high temperature and for oil cracking effects. The method is fast and yields all three fluid volumes, making it suitable for high-throughput screening.
Solvent Extraction and Gravimetric Analysis
Plugs may be cleaned by Soxhlet extraction or miscible solvent flushing, and the saturation is inferred from the mass loss of the sample, corrected for water recovered separately. Gravimetric methods are often integrated with Dean-Stark results to close the material balance.
Nuclear Magnetic Resonance (NMR)
Low-field NMR relaxation measurements on as-received plugs detect hydrogen-bearing fluids and provide fluid typing and saturation distribution within the pore network. NMR is non-destructive for bound fluid assessment and is particularly valuable in unconventional and low-permeability reservoirs where distillation times are long.
Quality Control and Data Validation
Robust programs include duplicate plug analyses, material balance checks confirming that extracted fluid volumes are consistent with pore volume, cross-comparison with retort and Dean-Stark results, and reconciliation of laboratory saturations with log-derived values. All procedures should be executed in accredited third-party laboratories following API RP 40, GB/T 29172, SY/T 5336, and related national and international standards.
Applications
Plug core saturation data are used across the reservoir life cycle:
- Original oil and gas in place estimation, where saturation is a primary input to volumetric reserve calculations
- Petrophysical log calibration, including validation and tuning of Archie and other saturation models used to interpret openhole logs field-wide
- Identification of fluid contacts, transition zones, and movable hydrocarbon intervals
- Waterflood performance evaluation and remaining oil distribution studies in mature fields
- Enhanced oil recovery screening, wettability assessment, and endpoint saturation determination for special core analysis
- Unconventional reservoir characterization, where NMR-based saturation profiling supports assessment of movable versus bound fluids in tight rocks
- Geological modeling, in which plug saturation data condition three-dimensional property models and reservoir simulation
FAQ
What is the standard plug size for saturation testing?
The most common plug diameter is 25.4 mm (1 inch), followed by 38.1 mm (1.5 inches), with length-to-diameter ratios between 1.5 and 2.0. Larger diameter plugs are preferred for heterogeneous or vuggy carbonates to improve representativeness.
Which method gives the most reliable water saturation?
The Dean-Stark azeotropic distillation method is generally regarded as the most accurate and reproducible technique for water saturation, because water is collected and measured directly in a calibrated receiver. Retort and NMR methods serve as valuable complements depending on throughput and sample objectives.
How is gas saturation determined if gas escapes during core recovery?
Gas saturation is almost always calculated by difference once water and oil saturations have been measured and normalized to pore volume. Because gas expansion during core retrieval can expel liquids, saturation values should be interpreted together with coring, mud, and pressure data, and corrections may be applied where invasion analysis is available.
Which standards govern plug core saturation analysis?
Widely applied references include API RP 40 for recommended core analysis practices, GB/T 29172 and SY/T 5336 for core analysis methodology, together with related ISO and ASTM procedures covering fluid density, porosity, and petrophysical measurement. Testing should be performed by accredited third-party laboratories with documented quality management systems.
How is mud filtrate invasion accounted for?
Invasion effects are managed through the use of oil-based or specifically tracer-tagged drilling fluids, wellsite preservation, analysis of companion and offset plugs, and comparison of shallow versus deep plug saturations. Tracer analysis of the extracted water can distinguish native formation water from filtrate contamination.
What is the typical turnaround time for saturation testing?
Retort and NMR screening can be completed within days of sample receipt, while full Dean-Stark extraction, particularly on low-permeability or oil-wet samples, may require one to several weeks per batch, followed by companion porosity and permeability measurements on the cleaned plugs.