NMR - What you need to know for operations and quality control

What is Nuclear Magnetic Resonance (NMR) and what is it used for?

Nuclear Magnetic Resonance (NMR) is a physical phenomenon that occurs when certain atomic nuclei, most notably hydrogen nuclei (protons), are placed in a strong magnetic field and exposed to specific radiofrequency (RF) pulses. This interaction results in the alignment of these nuclei with the magnetic field, followed by their disturbance through the RF pulses. The subsequent relaxation of these nuclei back to their original state generates measurable signals. These signals provide critical information about the molecular environment of the nuclei, enabling a calculation to determine formation ?such as porosity, permeability, pore size distribution, free and bound fluid volumes, and fluid type within the pore space, providing key insights into the structure and producibility of reservoirs

What are the key measurements from Magnetic Resonance Tools?

Magnetic resonance tools primarily measure the behaviour of hydrogen nuclei in pore fluids. By analysing the precession ("wobble") of these nuclei following an RF pulse, the tools provide valuable information about the reservoir's characteristics and potential productivity:

• Porosity: The amplitude of the initial signal produced by the tool is proportional to the number of hydrogen nuclei in the pore fluids. This directly correlates with the total porosity of the rock, making the measurement independent of lithology, which is particularly beneficial in complex formations.
• T2 Distribution: The T2 distribution indicates the range of pore sizes within the rock. Short T2 times are associated with smaller pores, while longer T2 times correspond to larger pores. This distribution is essential for distinguishing between different fluid types and provides insights into permeability.
• Free-Fluid and Bound-Fluid Volumes: These tools differentiate between free-fluid porosity (the portion of the pore space that can produce fluids) and bound-fluid porosity (where fluids are trapped in small pores and cannot flow freely) - see image below from Pore size distribution from the CMR, (after Coates, et al., 2000). This distinction is crucial for understanding the reservoir's productivity.
• Permeability Estimation: Although the tools do not directly measure permeability, they can estimate it using empirical models that relate porosity and T2 distribution to the reservoir's ability to transmit fluids. These models provide an indication of flow potential. (see also Timur-Coates, Schlumberger-Doll Research (SDR) models and Free Fluid Models).
• Fluid Type Identification: By analysing relaxation times, specifically T1 and T2, magnetic resonance tools can distinguish between different fluids such as water, oil, and gas. This capability is especially valuable for identifying gas zones, where other conventional logging methods might struggle due to the unique properties of gas

The Physics Behind NMR

The key principle of NMR lies in the interaction between atomic nuclei and an externally applied magnetic field. When certain nuclei, such as the hydrogen atoms in water and hydrocarbons, are placed in a magnetic field, they align along the field’s direction. These aligned nuclei can be excited by a pulse of RF energy, causing them to “wobble" around the direction of the magnetic field. When the RF pulse is turned off, the nuclei return to their original alignment (or equilibrium) at a rate known as relaxation.

NMR measures two key types of relaxation:

1. T1 (longitudinal relaxation time): The time it takes for the nuclei to realign with the magnetic field after being disturbed by the RF pulse.
2. T2 (transverse relaxation time): The time it takes for the nuclei’s precession signal to decay after the RF pulse is applied.

These relaxation times depend on the properties of the fluids and the pore sizes in the surrounding medium. In NMR logging, this information is used to deduce the nature of the subsurface rock formations and their fluid content.

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What are the main operating parameters to be aware of when planning to run an NMR tool?

Wireline programme's will often include a set of detailed operating paramaters, like those shown in the image below. Many people are not aware of what much of this means, we will try and address some of those parameters here.

The performance of magnetic resonance tools is highly dependent on the configuration of several operational parameters. The following are some of the key settings that need to be optimised based on the characteristics of the reservoir being logged:

• Echo Spacing (TE): This is the time between consecutive radiofrequency (RF) pulses. Shorter echo spacings (as low as 0.2 milliseconds) are used to capture faster-decaying signals from small pores or more viscous fluids. Longer echo spacings (up to 1 millisecond) are employed to focus on slower-decaying signals from larger pores or lighter fluids.
• Number of Echoes: The number of echoes recorded affects the resolution and depth of investigation. For continuous logging, 600 to 1800 echoes are generally sufficient, while more detailed measurements at specific depths (such as in station logs) may require between 3000 and 5000 echoes.
• Wait Time (TW): This parameter determines how long the tool waits between measurements, allowing the hydrogen protons to realign with the magnetic field. Short wait times (5–10 seconds) are necessary for full polarisation in more viscous fluids like heavy oil whereas gas would have longer wait times.
• Sampling Interval: The vertical resolution is directly related to the distance between successive measurements, typically defined by the sampling interval. For example, if the sampling interval is set to 6 inches, the tool measures and records data every 6 inches as it moves along the borehole. A smaller sampling interval results in finer vertical resolution, allowing for better identification of thin beds or smaller features within the formation.
• Logging Speed: The speed at which the tool moves through the borehole directly impacts data quality. Slower speeds (typically 600–750 feet per hour) are recommended to ensure complete polarisation of the formation and that measurements are taken when the tool is in full contact with the rock (see note below for more detail)
• Tool Positioning and Centralisation: Proper positioning of the tool is crucial to minimise signal interference and ensure accurate data collection. The type of tool will determine tool positioning (CMR, MR Scanner, MagniPh and MREX are all eccenered tools). This is typically achieved using bowstring or calliper devices to centre the tool within the borehole.

What paramters should be monitored for Quality Control when running an NMR tool?

• Signal-to-Noise Ratio (SNR): Achieving a high SNR is vital for clear and usable data. SNR can be improved by stacking multiple measurements—repeating the process several times and averaging the results.
• Logging Speed: ?A consistent and appropriate speed (typically above 600ft/hr based on experience) ensures that the tool remains in proper contact with the borehole walls and that measurements are taken at regular intervals without missing critical data. Logging too fast may result in incomplete polarization of the hydrogen nuclei, while logging too slow can introduce noise into the data or cause the tool to move in a “stop-start” manner. If the tool moves erratically or inconsistently through the borehole, variations in the tool’s alignment with the borehole or formation can cause fluctuations in the magnetic flux.
• Magnetic Flux - Magnetic flux changes can have a significant impact on the quality of Nuclear Magnetic Resonance (NMR) logging data. Magnetic flux refers to the amount of magnetic field passing through a given area, and in the context of NMR logging tools, it is vital for ensuring consistent and reliable measurements of the hydrogen nuclei in pore fluid. Magnetic flux changes can also arise from the surrounding borehole environment, especially in wells drilled with steel casing or in areas with magnetic minerals. It is essential to use a ditch magnet and circulation to ensure the hole is as clean as it can be.
• Depth Control (@haraldbolt) - Accurate depth control is vital for correlating NMR data with other logs and geological markers. Depth errors can occur due to cable stretch, tool movement, or changes in borehole conditions. Proper calibration of the depth system and regular checks ensure that the data corresponds accurately to the formation being logged.
• Calibration and Tuning - Tool calibration must be performed regularly to ensure that the NMR tool operates within its specified accuracy limits. Calibration / tuning checks should occur before and after each logging pass to accommodate any changes in magnetic flux due to magnetic degradation at temperature or environmental changes in flux.

In addition to the above, the pre-job parameters setting should be confirmed and monitored throughout the log to esnure data is acquired effectively.

A note on the Importance of Logging Speed and Smooth Movement for NMR logging

It is common for parameter settings to be setup in the planning phases to "cover all bases" and capture the maximum amount of data possible. For instance, the sampling rate may be set to its highest level. However, unless the measurement is aimed at identifying thin layers, detecting small changes in porosity, or distinguishing between fluid types in thin beds, a higher sampling rate may not be necessary and could even negatively impact the measurement. The following are critical considerations in obtaining a "quality" log:

Logging Speed

Logging speed controls how long the tool stays at each point in the borehole, influencing how well the hydrogen nuclei polarize. If the tool moves too quickly, there isn’t enough time for full polarization, which can lead to underestimating porosity or distorting T2 distributions, especially in viscous fluids or larger pores. On the other hand, if the tool moves too slowly, data quality may not improve, but efficiency is lost.

Smooth Tool Movement

In addition to speed, smooth, continuous tool movement is crucial. Erratic motion, such as stopping and starting, can degrade data quality by causing incomplete polarization, leading to inaccurate readings. Poor tool movement can also create artifacts in the log data, making it difficult to trust the measurements.

Data Quality

Both speed and movement directly impact the signal-to-noise ratio (SNR) and vertical resolution. Inconsistent speed or movement can lead to increased noise, while smooth, steady operation allows for better resolution and clearer distinctions between different pore sizes and fluid types.

In practice, adjustments to logging speed and tool setup (such as centralizers) are often necessary to ensure smooth, consistent movement, especially in challenging well conditions. By carefully managing these factors, high-quality, reliable data can be achieved, leading to better reservoir understanding and decision-making.

A note on station logs and fluid typing

Magnetic resonance stations are specialised measurement setups used in nuclear magnetic resonance (NMR) logging to gather detailed information about the fluid properties within a reservoir. These stations involve acquiring precise NMR signals at specific locations in the borehole, where the tool pauses or slows down to ensure high-quality data collection. The length of time each stations takes varies depending on formation properties, however a typical station will take 20-30 minutes.

What Are These Stations?

These stations are points during the NMR logging process where the tool performs detailed measurements. Instead of continuously logging while moving through the borehole, the tool stays in one location to capture more accurate data about the formation and fluids. This method provides higher-resolution insights compared to standard continuous logging.

At these stations, several key properties are measured:

• Fluid Properties: The relaxation times (T1 and T2) and molecular diffusion of fluids (water, oil, gas) are measured, enabling the tool to differentiate between various fluid types in the formation.
• Viscosity: By analysing relaxation times and molecular diffusion, the tool can estimate the viscosity of fluids, which is crucial for determining the flow behaviour of oil and other fluids.
• Fluid Saturation: These measurements help determine the volumes and saturations of different fluids such as brine, hydrocarbons, and gas in the formation.
• Porosity: The tool provides an accurate assessment of porosity, including distinguishing between fluids that are mobile (free fluid) and those that are bound within the rock's pore space.
• Permeability: By using specific models, the data gathered can be used to estimate the formation’s permeability, which is essential for understanding how fluids will flow through the reservoir.

How Do These Stations Work?

• These stations employ specialised techniques to capture detailed data. The tool sends out spin-echo pulses with varying echo spacings and wait times to collect comprehensive data about the fluids in the formation. Key components of how these stations operate include:
• Pulse Sequences: The tool uses multiple pulse sequences, each with different parameters, to capture a range of relaxation and diffusion data, providing detailed insights into the fluids.
• Inversion Modelling: Once the data is collected, it is processed using inversion algorithms to model the behaviour of different fluids, helping to separate and identify them accurately.
• Extended Measurement Time: Since these stations take longer measurements than continuous logging, they can capture more accurate data for slow-relaxing fluids, such as viscous oils, by allowing enough time for full polarisation.

Purpose of These Stations

The primary purpose of these specialised stations is to deliver a more detailed and accurate evaluation of the reservoir. The key objectives include:

• Fluid Differentiation: These stations help distinguish between water, oil, and gas in the formation, especially in complex zones where conventional logs may struggle.
• Enhanced Porosity and Permeability Estimates: The detailed fluid measurements improve the accuracy of porosity and permeability estimates, particularly in hydrocarbon-bearing formations.
• Oil Viscosity Measurement: These stations provide essential data on oil viscosity, aiding in the evaluation of production potential and planning for recovery operations.
• Flushed Zone Analysis: By providing detailed data on fluid saturations and volumes near the borehole, these stations assist in the assessment of the flushed zone, which is critical for accurate formation evaluation.

It should be noted that some tools can perform fluid typing measurements whilst moving. Stations measurements with increased data stacking can provide useful reference points across the well section to validate the depth log data.

What tools are available for NMR measurements on wireline

The table below provides an overview of the main tools from the 3 largest vendors.

one&zero Technical Advisory

Incorporating the expertise of a third-party technical advisory company, such as one&zero, can be crucial when deciding why and how to run complex services like NMR logging. With the multitude of available settings and options, having an independent advisor ensures that the service is tailored precisely to your specific reservoir needs.

one&zero adds value by helping in the early planning stages, ensuring that the right data acquisition parameters are selected to optimize results while avoiding unnecessary complications.

At the wellsite our experience in overseeing operations (wireline witness service) means that you can avoid the pitfalls of incorrect configurations, incorrect parameter settings and failure to observe red flag quality control indicators. By leveraging the expertise of one&zero, operators can make informed decisions and have confidence that their data is being acquired safely and effectively. one&zero ensure data accuracy whlst reducing operations risk throughout the entire process.

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