Petroleum Reservoir Rock Properties & Core Analysis

Learning Outcomes

• By the end of the lecture students should be able to:

  • Describe the nature of petroleum, define hydrocarbon families, and recognise physical states (gas, liquid, solid).

  • Explain what a petroleum reservoir / trap is and why rock properties control hydrocarbon accumulation & production.

  • List, differentiate and justify the use of whole cores, core plugs, side-wall cores.

  • Outline core‐analysis workflows (Routine vs. Special), including sample preparation and preservation requirements.

  • Recognise laboratory equipment and field tools used for coring, scanning, cleaning and measurement.

Nature of Petroleum

• Chemical classification

  • All natural compounds ⇒ organic or inorganic.

  • Organic = contain carbon covalently bonded to other atoms.

• Hydrocarbons

  • Composed solely of C + H.

  • Molecular size range: from methane $CH_4$ to heavy asphaltenes (hundreds of C atoms).

• Petroleum mixture

  • "Hundreds of thousands" of distinct compounds.

  • Exists as:

  • Natural gas (mainly light HC, eg. $CH_4$).

  • Crude oil (liquid phase).

  • Tar / Bitumen (solid, very high molecular weight & viscosity).

• Occurrence

  • Generated over millions of years within source rocks.

  • Migrates and is ultimately trapped in sedimentary formations (sandstone, limestone, etc.).

Petroleum Reservoirs & Traps

• Petroleum trap = Geological structure + petrophysical conditions preventing further migration.

• Typical geometry: anticline fold; other possibilities include fault traps, stratigraphic pinch-outs, salt domes.

• Migration & accumulation sequence

  1. HC generated at depth migrates upward with buoyancy.

  2. Displaces formation water (HC density \rho{hc} < \rho{water}).

  3. Accumulates beneath an impermeable seal (e.g.
     anhydrite, shale).

• Economic threshold

  • Two key metrics required before labelling a trap a reservoir:

  1. Original Hydrocarbon in Place (OHIP) – "quantity".

  2. Deliverability / Production rate – "rate".

  • Both heavily controlled by rock & fluid properties.

Significance of Rock & Fluid Properties

• Determine OHIP (volumetrics)

  • Porosity $\phi$ : fraction of void space.

  • Saturation $S<em>{w}, S</em>{o}, S_{g}$ : fluid distribution.

  • Net-to-gross (NTG) & reservoir thickness.

• Control recovery factor

  • Permeability $k$ (absolute & effective).

  • Wettability, capillary pressure $P_c$.

  • Compressibility, rock strength (compaction drive).

• Control flow rate (Darcy’s law) $Q = -\dfrac{kA}{\mu}\dfrac{\mathrm{d}P}{L}$.

• Lecture introduces basic rock & fluid properties used to rank reservoir quality.

Source of Rock Samples

• Downhole coring remains the primary way to obtain direct, physically representative samples.

• Picture examples: sandstone, carbonate cores.

Core Types & Dimensions

1. Whole / Full-Diameter Core

• Continuous section removed during conventional rotary coring.

• Diameter usually 3–4 in (≈7.5–10 cm) & length up to tens of metres.

• Advantages

  • Captures macro-scale heterogeneity (bedding, fractures).

  • Ideal for facies description, SCAL on heterogeneous rocks.

• Disadvantages

  • Requires large test apparatus; longer stabilisation times.

  • Cleaning, drying and pressure control more complex.

  • Expensive – only justified when plugs deemed non-representative.

2. Core Plugs

• Cylindrical subsamples drilled from whole core with plug mill.

• Diameters: 1.0 in or 1.5 in (≈2.54 cm / 3.81 cm); length ≈ 3 in.

• Orientations

  • Vertical/HV plug: axis parallel to bedding.

  • Horizontal plug: axis perpendicular.

• Pros: shorter tests, easier pressure/temperature control, cheaper.

3. Side-Wall Cores (Percussion or Rotary)

• Retrieved after main drilling by guns or mini-rotary tools.

• Good for zones where full coring not feasible.

Core Condition Terminology

• Fresh core

  • Immediately preserved to minimise evaporative loss.

  • Coring fluid (OBM or WBM) recorded.

• Preserved core

  • Stored with protective methods: shrink-wrap, freezing, wax coating, etc.

• Cleaned core

  • Fluids removed via solvent sequences (details must be reported: solvent type, temperature, order).

  • Special care for friable samples (e.g. critical-point drying for clays).

• Restored-state core

  • After cleaning, re-saturated with reservoir fluids to restore native wettability & $S_{wi}$.

• Pressure-retained core

  • Maintained at reservoir pressure $P_{res}$ during retrieval ⇒ avoids saturation changes & gas breakout.

Coring Equipment & Retrieval Process

• Rotary core barrel

  • Components: drop-ball valve, inner/outer barrels, core catchers, core bit.

• Percussion Corgun®

  • Fires bullets into borehole wall to capture side-wall cores.

• Ocean drilling example (Chikyu)

  1. Run core bit to depth.

  2. Drop inner core barrel through drill string (wireline).

  3. Drill advances; 9.5 m core enters plastic liner.

  4. Barrel retrieved; core liner cut into 1.5 m sections on deck.

• Key design: prevents lost core at bit face; minimises disturbance.

Initial Core Examination & Description

• Slabbing / photographing for permanent record.

• Sedimentological logging: lithology, grain size, bedding, structures, colour.

• Thin-section petrography: pore types, diagenesis, micro-fabrics.

• Outputs feed facies models, identify reservoir/non-reservoir intervals.

• Provides calibration for wireline logs (e.g.
  gamma ray, density, sonic).

Whole-core scanning tools

• Natural Gamma Scanner: detects K, U, Th radiation for lithological changes.

• X-ray / CT (CAT) scanner: non-destructive imaging of density contrasts, fractures, vugs.

Core Cleaning & Drying (Sample Preparation)

• Goal: remove hydrocarbons + brine without altering pore structure.

• Solvent extraction methods

  • Dean-Stark reflux (toluene/methanol): simultaneous water & oil quantification.

  • Soxhlet extraction.

  • Centrifuge with solvents.

• Drying options: oven at $105\,^{\circ}\text{C}$, vacuum, humidity-controlled chambers.

Core Analysis Categories

1. Routine Core Analysis (RCAL)

• Basic petrophysical parameters at ambient conditions.

• Typical measurements & links:

  • Porosity $\phi$ – Helium Boyle’s law porosimeter.

  • Permeability $k$ – gas (Klinkenberg correction to obtain $k_{0}$).

  • Grain density $\rho_g$ – helium pycnometer.

  • Dean-Stark fluid saturations $S<em>w, S</em>o$.

  • Retort / distillation oil saturation.

2. Special Core Analysis (SCAL)

• Measurements addressing complex flow & electrical behaviour, often at overburden stress & reservoir T/P.

• Key suites:

  • Electrical properties: Formation factor $F = a \phi^{-m}$, cementation exponent $m$, saturation exponent $n$.

  • Capillary pressure $P_c(S)$: Mercury injection, porous-plate, centrifuge techniques.

  • Relative permeability $k_{r\alpha}(S)$: steady / unsteady state, drainage vs.
    imbibition.

  • Wettability indices: USBM, Amott-Harvey.

  • Rock compressibility $C_r$, acoustic velocity, mechanical strength, clay chemistry.

Typical Workflows

RCAL (unpreserved core)

1. Retrieve → 2. Cleaning & drying → 3. Measure $\phi, k, S<em>{wi}, \rho</em>g$.

SCAL (pressure-preserved core)

1. Retrieve under pressure → 2. Overburden loading → 3. Special cleaning / re-saturation → 4. Measure electrical, $P<em>c$, $k</em>r$, wettability, etc.

Practical / Ethical / Industrial Implications

• Economic decisions: Accurate core data influences $\text{NPV}$ and field development plans.

• Safety & environment: Core-based rock mechanics prevent wellbore collapse, uncontrolled HC release.

• Data integrity: Following API RP40 (1998) ensures standardised, reproducible measurements.

• Digital transformation: High-resolution CT & machine learning now augment traditional descriptions (Industry 4.0 alignment noted in lecture title).

Quick Reference – Equations & Symbols

• Darcy’s Law (linear) $Q = -\dfrac{k A}{\mu} \dfrac{\Delta P}{L}$.

• Porosity $\phi = \dfrac{V<em>p}{V</em>b}$.

• Formation Factor $F = a \phi^{-m}$.

• Capillary pressure $P<em>c = \dfrac{2 \gamma \cos\theta}{r</em>c}$.

Key Take-Away Messages

• Petroleum systems = source + migration + trap + seal; only a subset qualifies as producible reservoirs.

• Rock & fluid properties ultimately govern both volumes and flow rates of hydrocarbons.

• Core acquisition, preservation, description, cleaning and analysis form a chain – compromise in one link reduces data quality.

• Distinguish RCAL (basic, quick) from SCAL (advanced, condition-specific) and know when each is required.

• Adhering to industry standards (API RP40) and leveraging new scanning technologies supports reliable, transferable results.

End of Lecture 1 Notes – Ready for Exam Revision 🎓