Detailed Study Notes on Fundamental Petrophysical Properties of Reservoir Rocks

CHAPTER 2 – FUNDAMENTAL PETROPHYSICAL PROPERTIES OF RESERVOIR ROCKS

  • Presenter: Engr. Manilyn Vicencio-Calapatia, MEng. ME, Petroleum Engineer

  • Institution: Batangas State University - Alangilan, The National Engineering University

Overview of Petrophysical Concepts

  • Examination of how reservoir saturation behaves after hydrocarbon migration, with an emphasis on the geological context.

    • Components of interest in the figure illustrating saturation distribution include:

    • Shale (Seal)

    • Oil

    • Irreducible Zone

    • Transition Zone (Sw=100%)

    • Water (Aquifer)

    • Source Rock (Shale + Kerogen)

    • Essential questions include:

    • What volume fraction of the pore space is occupied by oil, gas, and water?

      • Concept of Porosity

    • Fluid types present in the rock: Is it oil, gas, or water?

    • Rate of fluid production from the rock: How quickly can fluids be expelled from the reservoir?

Porosity

  • Definition:

    • The ratio of the volume of pores (empty spaces) in a rock to the total volume of the rock sample.

    • Mathematical Expression: ext{Porosity} = \frac{V{pores}}{V{sample}}

  • Types of Pores:

    • Unconnected Pore Spaces: Not contributing to permeability.

    • Connected Pore Spaces: Contribute to flow.

    • Non-porous/Non-permeable: No effective pore spaces available.

    • Porous/Permeable: Effective transmission of fluids.

  • Distinct conditions affecting porosity include:

    • Degree of saturation (how much of the pore space is filled with fluid).

Key Formulas

  • Porosity calculation concerning total volume and specific mineral volumes.

    • Related Definitions:

    • Total Volume of Solid Minerals: V_{s}

    • Bulk Volume: V_{b}

    • Volume of Pores: V_{p}

  • Trends of Porosity:

    • Typically decreases from high porous materials (e.g., unconsolidated sediments) to denser materials (e.g., fractured igneous rocks).

    • As observed in different rock types, from sandstones to limestones, to dense rock types.

Factors Governing the Magnitude of Porosity

  1. Uniformity of Grain Size

    • Consistency in size allows for increased porosity due to proper packing.

  2. Grain Size Distribution

    • Example cases showcasing how varying grain sizes affect porosity levels.

    • Case 1: Higher porosity with optimal size sorting.

    • Case 2: Lower porosity due to mixed size distribution.

  3. Degree of Cementation or Consolidation

    • Process of how sediments are compacted and cemented into solid rock.

  4. Amount of Compaction during and after Deposition

    • Factors affecting the loss of pore volume as sediments accumulate and exert pressure on lower layers.

  5. Methods of Packing

    • Various packing methodologies (e.g., Cubic, Tetragonal) showing how arrangement impacts porosity.

Quantitative Use of Porosity

  • Key Equation for the use of porosity in assessments:

    • N = 7,758 \times A{s} \times h \times \phi \times (1-S{oi}) where:

    • A_{s} = surface area of the reservoir (acres)

    • h = thickness of the formation (feet)

    • \phi = porosity (fraction)

    • S_{oi} = initial oil saturation (fraction)

Types of Permeability

  • Primary Permeability: Inherent to the rock structure itself without post-depositional modifications.

  • Secondary Permeability: A result of deformation pathways, including fracturing or cementation changes.

Darcy's Law and the Darcy Equation for Permeability

  • Darcy Equation:

    • u = \frac{Q}{A} = k \frac{\Delta P}{L}

    • Where:

    • u = volumetric flow rate (m³/s)

    • k = permeability of the medium (m²)

    • A = cross-sectional area perpendicular to flow (m²)

    • \Delta P = pressure difference across the length (Pa)

    • L = length of the medium (m)

Factors Affecting the Magnitude of Permeability

  • Includes physical characteristics and treatments like:

    1. Shape and Size of Sand Grains

    2. Lamination

    3. Cementation

    4. Fracturing and Solution Processes

Reservoir Heterogeneity

  • Microscopic Heterogeneity: Variations at the grain scale.

  • Mesoscopic Heterogeneity: Variations at the small reservoir scale.

  • Macroscopic Heterogeneity: Large-scale variations, such as between different rock layers.

  • Megascopic Heterogeneity: Comprehensive regional variations across large areas.

  • Gigascopic Heterogeneity: Variations observable across massive geological scales.

Rock Compressibility

  • Mechanics of compaction encompass:

    • Rotation and Closer Packing: Importance of grain arrangement for density.

    • Ductile vs. Brittle Deformation: Different responses of rock types to compaction forces.

    • Pressure Solutions and Grain Contacts: Mechanisms by which individual grains compress and adhere in response to applied pressure.