NMR Logging in Formation Evaluation Notes

NMR Logging in Formation Evaluation

  • Presenter: Vikas Jain

    • Title: Global Domain Head - Petrophysics, Well Construction Measurements, SLB.

Introduction to NMR Logging

  • NMR (Nuclear Magnetic Resonance) logging measures the properties of formations to evaluate subsurface conditions.

  • Objective: Identify characteristics related to the presence of hydrocarbons and water in formations.

Key Measurements in NMR Logging

What to Measure?

  • Volume of Hydrogen Nuclei (v/v).

  • Pore Size Distribution:

    • Small pores to large pores analyzed through T2 relaxation times.

  • Frequency measurements (T2 in ms) across varying pore sizes.

Properties Influencing NMR Measurements

  • Environmental Effects:

    • Temperature: Affects the volume of interest (VOI).

    • Fluid type and properties (viscosity, etc.).

  • Magnetic field interactions influencing proton behavior.

Principles of NMR

  • Hydrogen Properties:

    • Protons act as positive charged spheres always spinning, giving rise to a net magnetic moment.

  • NMR Process:

    • In the absence of a magnetic field, nuclear magnetic moments are randomly oriented.

    • Under magnetic field, they align, creating longitudinal magnetization.

    • RF (Radio Frequency) pulses are used to manipulate the phase alignment of protons.

    • Relaxation processes (longitudinal and transverse) occur post RF pulse, providing measurement signals.

    • Exponential growth and decay described by time constants T1 and T2, respectively.

T2 Relaxation Mechanisms

  • Fundamental Decay Rate Influencers:

    • Bulk Relaxation: Interaction of protons with each other (fluid type and viscosity).

    • Surface Relaxation: Interaction of protons with pore walls, related to surface-to-volume ratios.

    • Diffusion Relaxation: Protons moving out of the tool's measurement volume.

NMR Tool Components

  • Logging Tools: Include wireline and LWD (Logging While Drilling) antennas with defined sensitive volumes.

  • Measurements are often taken using CPMG (Carr-Purcell-Meiboom-Gill) pulse sequences to enhance signal detection and accuracy.

Measurement and Inversion Principles

  • Data Acquisition Steps:

    1. Data cleanup and calibration.

    2. Signal processing involving separation of signal and noise.

    3. Inversion to obtain volumes at corresponding T2 from echo signal.

  • Stages of inversion: fitting known T2 decay curves to acquired signals.

Applications of NMR Logging

  • Evaluation of Reservoir Properties:

    • Determining lithology, fluid types, reservoir geometry, and hydrocarbon saturations.

    • Assessment of hydrocarbon flow efficacy through permeability measures.

    • Identification of pore sizes and distribution for suitable production methods.

Advanced NMR Techniques

  • Simultaneous T1-T2 Measurements: Provides insight into multiple fluid types present in the formations.

  • Factor Analysis in NMR: Helps analyze the complexities of fluid constituents and T2 distributions effectively.

Final Insights

  • NMR logging provides a comprehensive picture of subsurface conditions and aids in accurate reservoir modeling and production planning.

  • Essential for determining fluid behavior and decision-making in drilling and production areas.

Conclusion

  • NMR methodologies offer an indispensable tool for formation evaluation, delivering critical insights into the characteristics and behaviors of subsurface fluids.