Full-diameter core oil saturation testing measures the percentage of pore volume occupied by hydrocarbons in an intact, whole-diameter reservoir core sample, preserving natural fractures, vugs, and rock fabric that small plug samples cannot represent. This laboratory experiment is essential for original oil in place (OOIP) estimation, petrophysical log calibration, and reservoir characterization of heterogeneous and fractured formations. Accredited third-party laboratories and specialized core analysis facilities perform these tests following established practices such as API RP 40 and GB/T 29172.
Technical Overview
Full-diameter core oil saturation testing is a specialized laboratory experiment in which the hydrocarbon content of an intact cylindrical section of reservoir rock—typically recovered at a nominal diameter of 100 mm (4 inches)—is quantified and expressed as a percentage of the sample's total pore volume. The core parameter, oil saturation (So), is defined as the ratio of oil volume to pore volume, So = Vo / Vp × 100%, and is normally reported together with water saturation (Sw) and gas saturation (Sg), where gas saturation is obtained by difference (Sg = 1 − So − Sw).
Unlike conventional plug analysis, which uses small cylindrical samples (typically 25–38 mm in diameter) drilled from a larger core, full-diameter analysis preserves the complete rock fabric, including open and mineralized fractures, vuggy porosity, cross-bedding, laminations, and other macro-scale heterogeneities. As a result, whole-core saturation data are significantly more representative for vuggy carbonates, naturally fractured reservoirs, conglomerates, and strongly laminated sandstones, where plug-scale measurements can severely overestimate or underestimate hydrocarbon volume. The experiment is generally classified within routine core analysis (RCAL), although it frequently supports special core analysis (SCAL) programs and log calibration studies.
Testing Samples
Conventional Full-Diameter Sandstone Cores
Standard whole-diameter sandstone core sections, typically 80–130 mm in diameter and 80–150 mm in length, are submitted for analysis when the formation exhibits visible heterogeneity, coarse grain size, or dispersed shale laminae that make plug sampling unrepresentative. Samples are selected at regular stratigraphic intervals to build a continuous saturation profile across the reservoir interval.
Vuggy and Fractured Carbonate Cores
Carbonate reservoirs often contain moldic and vuggy porosity, natural fractures, and brecciated zones that dominate fluid storage and flow behavior. Full-diameter testing is the preferred—and frequently the only reliable—approach for these rocks, because the large sample volume integrates the contribution of vugs and fracture networks into the measured oil and water volumes.
Heterogeneous, Laminated, and Conglomeratic Cores
Interbedded sand-shale sequences, conglomerates, and gravel-rich intervals exhibit strong permeability and porosity anisotropy. Whole-core saturation analysis captures the bulk fluid content of these mixed lithologies, providing saturation values that can be directly compared with borehole log responses over the corresponding depth interval.
Preserved and Unconsolidated Cores
For cores containing volatile light hydrocarbons or reactive fluids, preserved samples sealed at the wellsite (e.g., in foil and wax, heat-sealed tubes, or frozen with liquid nitrogen) are analyzed to minimize evaporative and oxidation losses. Unconsolidated or friable whole cores are commonly stabilized by freezing and machined under cryogenic conditions so that the native fluid distribution remains intact through the extraction stage.
Testing Items
The full-diameter core oil saturation experiment typically delivers the following measurement parameters:
- Oil saturation (So), expressed as percent of pore volume
- Water saturation (Sw), expressed as percent of pore volume
- Gas saturation (Sg), calculated by difference
- Oil volume and water volume per unit mass or bulk volume of rock
- Whole-core pore volume and porosity (derived gravimetrically or by complementary measurement)
- Bulk density and grain density of the whole-core sample
- Residual oil saturation, where the test follows displacement or flooding sequences
- Vertical saturation profiles along the core interval for reservoir zonation
Testing Methods
Sample Receipt, Preservation Check, and Preparation
On receipt, whole-core samples are inspected, photographed, and logged under controlled conditions. Outer mud-cake and contaminated annular material are trimmed away to eliminate drilling-fluid invasion effects. Each sample is measured for length and diameter, weighed, and, where required, CT-scanned to document internal fractures and vugs before fluid extraction begins. Chain-of-custody and depth control records are maintained throughout.
Solvent Extraction and Azeotropic Distillation Method
The most widely used approach for whole core is a large-scale adaptation of the Dean–Stark azeotropic distillation–extraction technique. The full-diameter sample is placed in a dedicated extraction vessel and refluxed with toluene (or an equivalent hydrocarbon solvent), which vaporizes, condenses, and cycles through the pore system, dissolving and stripping the hydrocarbons. A polar co-solvent such as methanol or ethanol is subsequently used to remove formation water and salts. The distilled water volume is captured in a calibrated trap and read directly, while the extracted oil mass is determined gravimetrically by mass balance of the sample before and after cleaning, corrected for water removal. This method provides direct volumetric water measurement and high-accuracy oil determination.
Retort (High-Temperature Distillation) Method
In the retort method, the whole-core or representative splits are heated in stages—first to approximately 110–120 °C to recover water and light hydrocarbons, then to 500–650 °C to retort the remaining oil from the mineral matrix. Fluids are condensed, separated, and volumetrically measured. Retort analysis is fast and suited to large sample throughput, but results require correction for water of crystallization, oil cracking losses, and evaporative shrinkage, which are applied through established calibration factors.
Supplementary and Non-Destructive Techniques
Where the sample must be preserved for subsequent SCAL work, non-destructive methods such as low-field nuclear magnetic resonance (NMR) relaxometry and X-ray computed tomography can be used to estimate fluid saturations and their distribution before any extraction occurs. Gravimetric monitoring during controlled desaturation and centrifuge or porous-plate capillary pressure tests may also be combined with whole-core saturation measurements to construct capillary pressure–saturation relationships on representative material.
Data Reduction, Quality Control, and Standards Compliance
Measured oil and water volumes are normalized by pore volume obtained from complementary whole-core porosity determinations (summation of fluids, or helium porosimetry on the cleaned, dried sample). Quality control checks include saturation closure (So + Sw + Sg ≈ 100% ± acceptable tolerance), solvent blank controls, duplicate extractions, and calibration of all volumetric glassware and balances. Reputable accredited third-party laboratories conduct these operations in accordance with recognized industry practices, including API RP 40 (Recommended Practices for Core Analysis), the equivalent Chinese national standard GB/T 29172, and applicable SY/T oil-industry procedures for conventional core analysis, ensuring traceable and defensible results.
Applications
Full-diameter core oil saturation data are used across the entire reservoir development workflow:
- Estimation of original oil in place (OOIP) and volumetric reserves calculations, particularly in fractured and vuggy carbonate reservoirs
- Calibration and validation of petrophysical log interpretation models, including Archie saturation exponent verification and water-saturation log benchmarking
- Identification of productive pay intervals and transition zones in heterogeneous or low-contrast reservoirs
- Input to reservoir simulation, where whole-core saturations define initial condition distributions
- Screening and design of enhanced oil recovery (EOR) projects, including residual oil saturation baselines for waterflood, gas injection, and chemical flooding studies
- Evaluation of drilling-fluid invasion depth and coring-fluid compatibility through comparison of interior and exterior core saturations
- Support for asset transactions, reserves audits, and regulatory submissions requiring independent laboratory evidence
FAQ
What is the difference between full-diameter core analysis and plug analysis?
Full-diameter analysis tests the entire recovered core section, integrating fractures, vugs, and laminae into the measurement, whereas plug analysis uses small samples drilled from the core that may miss or misrepresent these features. Whole-core results are generally more reliable in heterogeneous, fractured, or vuggy formations; plug analysis remains appropriate and more economical for relatively homogeneous sandstones.
Is the test destructive?
The solvent extraction and retort methods are destructive in the sense that native fluids are removed from the sample; however, the rock matrix is preserved for subsequent porosity, permeability, and petrographic measurements. Non-destructive NMR-based saturation estimation is available when the sample must remain in its native fluid state for later SCAL experiments.
How should whole-core samples be preserved before testing?
Samples should be sealed at the wellsite immediately after recovery—using foil-and-wax or heat-sealed barrier wrap for conventional cores, or freezing with liquid nitrogen for unconsolidated and volatile-oil cores—and transported with a documented chain of custody. Proper preservation prevents evaporation of light ends and oxidation, which would otherwise bias saturation results.
What accuracy and turnaround can be expected?
With proper calibration and quality control, whole-core saturation measurements are typically repeatable within a few saturation percentage points, with water volumes read volumetrically to high precision. Turnaround depends on sample size, rock properties, and extraction difficulty—dense or low-permeability carbonates may require extended reflux cycles—so projects are commonly scheduled over one to several weeks per batch.
Which standards govern the experiment?
Procedures follow recognized core analysis practices such as API RP 40 and GB/T 29172, together with applicable SY/T industry standards for conventional core analysis. These documents specify requirements for sample handling, solvent purity, apparatus calibration, calculation methods, and reporting formats, ensuring that results from accredited third-party laboratories are comparable and technically defensible.