Nuclear magnetic resonance (NMR) logging

Nuclear magnetic resonance (NMR) has been, and continues to be, widely used in chemistry, physics, and biomedicine and, more recently, in clinical diagnosis for imaging the internal structure of the human body. The same physical principles involved in clinical imaging also apply to imaging any fluid-saturated porous media, including reservoir rocks. The petroleum industry quickly adapted this technology to petrophysical laboratory research and subsequently developed downhole logging tools for in-situ reservoir evaluation.

Purpose of NMR logging

NMR logging, a subcategory of electromagnetic logging, measures the induced magnet moment of hydrogen nuclei (protons) contained within the fluid-filled pore space of porous media (reservoir rocks). Unlike conventional logging measurements (e.g., acoustic, density, neutron, and resistivity), which respond to both the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR-logging measurements respond to the presence of hydrogen protons. Because these protons primarily occur in pore fluids, NMR effectively responds to the volume, composition, viscosity, and distribution of these fluids, for example:

• Oil
• Gas
• Water

NMR logs provide information about the quantities of fluids present, the properties of these fluids, and the sizes of the pores containing these fluids. From this information, it is possible to infer or estimate (Fig.1):

• The volume (porosity) and distribution (permeability) of the rock pore space
• Rock composition
• Type and quantity of fluid hydrocarbons
• Hydrocarbon producibility

• 

  Fig.1 – NMR logging-tool response compared to conventional logging tools. NMR porosity is independent of matrix minerals, and the total response is very sensitive to fluid properties. Differences in relaxation times and/or fluid diffusivity allow NMR data to be used to differentiate clay-bound water, capillary-bound water, movable water, gas, light oil, and viscous oils. NMR-log data also provide information concerning pore size, permeability, hydrocarbon properties, vugs, fractures, and grain size.

NMR logging provides measurements of a variety of critical rock and fluid properties in varying reservoir conditions (e.g., salinity, lithology, and texture), some of which are unavailable using conventional logging methods (Fig.1) and without requiring radioactive sources (Table 1). Whether run independently as a standalone service or integrated with conventional log and core data for advanced formation and fluid analyses, NMR logging has significantly contributed to the accuracy of hydrocarbon-reservoir evaluation. During the past decade, a new generation of wireline-logging devices has been introduced into commercial service. In the past few years, logging-while-drilling (LWD) devices and downhole NMR spectrometers have also been introduced.

• 

  Table 1

Whether used as a standalone service or in combination with other logs and core data, NMR logs can provide an improved understanding of reservoir petrophysics and producibility. However, NMR logs are the most complex logging service introduced to date and require extensive prejob planning to ensure optimal acquisition of the appropriate data needed to achieve the desired objectives.

Historical development

Within a few years after the first successful observations of NMR in 1946, and the demonstration of free-precession NMR in the earth’s magnetic field in 1948, the petroleum industry recognized the potential of NMR measurements for evaluating:

• Reservoir rocks
• Pore fluids
• Fluid displacement (flow)

In the early 1950s, several companies—particularly California Research (Chevron), Magnolia (Mobil), Texaco, Schlumberger, and Shell—began extensive investigations to understand the NMR properties of fluids in porous media for the purpose of characterizing reservoir rocks (porosity, permeability, and fluid content). In addition to laboratory research, these investigations included proposals for logging devices and the development of well-logging methods to permit formation evaluation in situ. Although a number of patents for logging tools were issued in the 1950s, it was not until Chevron completed an experimental Earth’s field nuclear-magnetic-log (NML) logging device in 1958 that a functioning device was actually developed. Limited commercial service of these devices was introduced in 1962 by Atlas, using the Chevron centralized design, and followed in 1965 by Schlumberger, using a pad-type tool of its own design. An improved version of the Schlumberger tool was introduced in 1978. Although the potential applications for this measurement were significant, particularly in the shallow, heavy-oil fields of the San Joaquin Valley, in general, they did not live up to expectations and were not commercially successful. Tool reliability and operational limitations proved to be major obstacles:

• The tool was not combinable, it required high (surface) power
• The signal level varied geographically and was generally very low as a result of the low-operational frequency (2 kHz)
• The borehole had to be doped with powdered magnetite to suppress the proton signal from the mud.

The final version of the Schlumberger NML tool—a centralized tool introduced in 1984—proved reliable and commercially successful and was in service until the advent of modern pulse-echo tools in 1994.

In 1978, the Los Alamos Natl. Laboratory developed a logging tool that employed permanent magnets and used a pulsed radio frequency (RF) (pulse-echo) NMR method. Although this particular design had serious limitations—such as a low signal-to-noise ratio (S/N) that prohibited continuous, nonstationary logging—the concept set the stage for the development of modern commercial NMR tools. This advance was soon followed by improvements in magnet and coil design that enabled continuous logging. During the 1980s, while developing a commercial logging tool, industry also carried out laboratory experiments to further understand NMR behavior in fluid-filled porous media and to develop petrophysical interpretations from these data. Ultimately, two wireline tools using different magnet and coil configurations emerged from these efforts:

• Numar’s mandrel device (MRIL)
• Schlumberger’s skid (sometimes called "pad") design [combinable magnetic resonance (CMR) tool]

Commercial logging began with these tools in 1991 and 1995, respectively. These wireline-tool designs continue to evolve (see section on Tool Design in this chapter). Recent improvements allow simultaneous acquisition of more measurements, operation in a wider range of borehole conditions, and faster logging speeds. Detailed accounts of the historical development of NMR and NMR logging are available in several published references.

In addition to improvements in wireline tools, new acquisition schemes and processing methods have improved the resolution, quality, and utility of the acquired data and enabled enhanced interpretation methods and data analysis. Concurrent with wireline improvements, LWD NMR logging services were being developed and have been introduced in the past few years. In a related development, a downhole NMR spectrometer is now available for use with a formation-testing tool for in-situ fluid analysis.

NMR physics

Atomic nuclei spin, and this angular moment produces a magnetic moment (i.e., a weak magnetic field). The NMR technique measures the magnetic signal emitted by spinning protons (hydrogen nuclei are the protons of interest in NMR logging) as they return to their original state following stimulation by an applied magnetic field and pulsed radio frequency (RF) energy. These signals, which are observed (measured) as parallel or perpendicular to the direction of the applied magnetic field, are expressed as time constants that are related to the decay of magnetization of the total system.

NMR devices—both laboratory spectrometers and logging tools—use strong magnets to create a static magnetic field, B₀, that aligns (polarizes) the protons in the pore fluid from their resting (random) state to the direction of the imposed magnetic field (Fig.2).

• 

  Fig.2 – Because of their inherent nuclear magnetism, hydrogen nuclei (left) behave as though they are tiny bar magnets aligned with the spin axes of the nuclei. In the absence of an external magnetic field, the nuclear-magnetic axes (right) are randomly aligned.

Polarization is not instantaneous—it grows with a time constant, which is called the longitudinal relaxation time, denoted as T₁. Once full polarization (magnetic equilibrium) has been achieved, the applied static magnetic field, B₀, is turned off.

The protons begin to lose energy as the imposed magnetization, M₀, decays and the protons fall out of alignment, back to their original orientation and low-energy state. The protons’ angular momentum causes them to behave like tiny gyroscopes, and the loss of energy occurs during a wobbling or axial rotation (called precession) in the direction of the applied magnetic field. M₀, also known as the bulk magnetization, provides the signals measured by NMR devices. The frequency at which the energy is emitted or is initially absorbed, f, called the Larmor or resonance frequency, is proportional to the strength of the external magnetic field, B₀, (Fig.3). The Larmor frequency is used to tune a NMR probe, permitting it to image very thin slices of a sample at different distances from the tool.

• 

  Fig.3 – In an external magnetic field, the precessional frequency (f) of a nucleus depends on the gyromagnetic (γ) of the nucleus and the strength of the external field (B₀).

An antenna detects and records the decaying magnetic field generated by the precessing nuclei. At any given time, t , the strength of this magnetic field, M_z, is proportional to the number of protons, the magnitude of B₀, and the inverse of the absolute temperature (Eq.1):

....................(1)

where M_z(t) = the magnitude of magnetization at t, M₀ = the final and maximum magnetization at a given magnetic field, and t = the time that the protons are exposed to the B₀ field.

The signal recorded parallel to the direction of the applied magnetic field (z plane) is called T₁, or longitudinal (spin-lattice) relaxation. T₁ describes how quickly the protons align within the static magnetic field. The T₁ curve is an exponential curve that characterizes the rate of change of the proton magnetization (Fig.4).

• 

  Fig.4 - T₁ relaxation (polarization) curves indicate the degree of proton alignment, or magnetization, as a function of the time that a proton population is exposed to an external magnetic field.

T₁ is the time at which the magnetization reaches 63% of its final value, and three times T₁ is the time at which 95% polarization is achieved. Full polarization of typical reservoir-pore fluids may take several seconds. Large values of T₁ (measured in milliseconds) correspond to weak coupling between the fluid and its surrounding environment and a slow approach to magnetic equilibrium, whereas, small T₁ values represent strong coupling and a rapid approach to equilibrium. Different fluids, such as water, oil, and gas, have very different T₁ values. T₁ is directly related to pore size and viscosity.

Pulse NMR devices use precisely timed bursts (pulse sequences) of RF energy that generate an oscillating magnetic field (B₁) that tilts or "tips" the aligned protons perpendicular (x-y plane) to the direction of the applied magnetic field. The application of B₁ results in a change in energy state that causes the protons to precess in phase to one another. These changes are known as NMR.

When the B₁ field is turned off, the precessions of the protons are no longer in phase with one another, and the net magnetization decreases. In this situation, a receiver coil (antenna) that measures magnetization in the transverse direction will detect an exponential decaying signal called free-induction decay (FID); see Fig.5. NMR-logging tools use the same antenna to transmit the RF pulse (kilowatt scale) and receive the decay signal (nanovolt scale).

• 

  Fig.5 – After application of a 90° pulse, the proton population dephases and an FID signal can be detected.

The FID signal measured in the x-y plane is called T₂ —the transverse or spin-spin relaxation. In contrast to T₁, T₂ of hydrocarbons is much shorter (see Table 2) in an inhomogeneous magnetic field. The process of spins lossing their coherence due to magnetic field inhomogeneity is not a true "relaxation" process and is dependent on the location of the molecule in the magnet field distirbution. Therefore, the FID decay constant is often referred as T₂^* rather than T₂.

• 

  Table 2

The primary objectives in NMR logging are measuring T₁ signal amplitude (as a function of polarization), T₂ signal amplitude and decay, and their distributions. The total signal amplitude is proportional to the total hydrogen content and is calibrated to give formation porosity independent of lithology effects. Both relaxation times can be interpreted for pore-size information and pore-fluid properties, especially viscosity.

In the laboratory, T₁ is generally measured by either of two pulse sequences: inversion recovery or saturation recovery. Inversion recovery consists of a 180° spin inversion followed by a variable recovery time and then a 90° read pulse. The magnetization vector is entirely in the longitudinal range and, thus, has a higher dynamic range than the other method. Saturation recovery uses a 90° pulse, followed by a 90° read pulse. Saturation recovery is generally considered the more robust and efficient method. Although the actual T₁ sampling sequence is very short—involving several short echoes trains, each of which requires only a few milliseconds—the total amount of time required to obtain the number of samples sufficient to define the T₁ spectrum is significantly greater.

Depending on the activation used, the computation of a T₁ spectrum requires at least 25% more, and sometimes double, the time needed for the computation of a T₂ spectrum. In NMR logging, T₁ measurement initially required either a stationary mode or very slow logging speeds. With the latest multifrequency tools, a technique used for speeding up T₁ measurements is to make simultaneous measurements of the individual steps observed during a T₁ recovery experiment in adjacent volumes; at least two such volumes are required. This technique enables T₁ acquisition in less time, thereby permitting faster logging speeds.

T₂ measurement uses the spin-echo technique, in which the protons are first tipped into the transverse (x-y) plane by a 90° RF pulse and then inverted (flipped) by a subsequent 180° RF pulse at a fixed-time interval to rephase the dephasing protons. Rephasing the protons creates a detectable signal called a spin echo (Fig.6).

• 

  Fig. 6 – NMR spin echo: (1) to generate a spin echo, a 90° B₁ pulse is first applied; (2) after cessation of the 90° pulse, dephasing starts; (3) at τ, a 180° B₁ pulse is applied to reverse the phase angles and, thus, initiate rephrasing; (4) rephrasing proceeds; and (5) rephrasing is complete, and a measureable signal (a spin echo) is generated at 2τ.

In practice, a sequence of pulses is used to generate a series of spin echoes (echo train) in which echo amplitude decreases exponentially with the time constant, T₂. A variety of multiple-echo pulse sequences have been developed for different purposes. In well logging and petrophysical studies, the most widely used is the Carr-Meiboom-Purcell-Gill (CMPG) sequence. A polarization period is followed by a 90° tip pulse, which in turn is followed by a series of alternating RF pulses and measurements of echo amplitudes detected by the logging-tool antenna. Successive 180° pulses are applied at a fixed-time interval (echo spacing, T_E), and the echoes are recorded between the pulses (Fig. 7). By recording an echo train, T₂ can be calculated from the decay in the height (amplitude) of successive echoes using Eq.2:

....................(2)

where M_x(t) = the amplitude of the transverse magnetization (i.e., the amplitude of the spin-echo train) at time t, and M_0x = the magnitude of the transverse magnetization when t = 0 (i.e., the time at which the 90° pulse stops).

• 

  Fig. 7 – The CPMG pulse sequence is used in NMR logging to generate a spin-echo train; a 90° B₁ pulse is followed by a sequence of 180° B₁ pulses. Spin echoes of decreasing amplitude follow the 180° B₁ pulses.

A single T₂-pulse sequence may involve several hundred or thousand echoes. Only the amplitude (peak) of each spin echo is measured and stored. A series of echo trains is recorded and the signals stacked to improve S/N, especially at shorter relaxation times.

When recording multiple CMPG sequences, the time period between spin-echo recovery and the next 90° CMPG excitation—during which the protons are repolarized by the static magnetic field—is called the wait time, T_W (Fig. 8). Each CMPG sequence may use a different wait time, echo spacing, and number of echoes. An additional advantage of the CMPG sequence is that a small echo spacing, T_E, in the CMPG sequence can minimize the diffusion effect on T₂. CMPG measurement sets are always collected in phase-alternate pairs (PAP) to preserve the signal and to eliminate low-frequency electronic offsets. In general, pulse NMR offers better methods to measure relaxation times and quantify liquid displacement in rock.

• 

  Fig. 8 – CPMG pulse sequences (top) and alternating polarization (T₁-relaxation) curves and spin-echo train acquisition (bottom).

In-gradient diffusion

FID is caused by inhomogeneities in the magnetic field that are primarily caused by the existence of magnetic-field gradients. Gradients in the magnetic field occur, in part, because of the distance from the magnet to the sensitive (measurement) volume. For a given geometry, the gradient is inversely related to magnetic-field strength. Compared to laboratory and clinical NMR devices, NMR-logging tools produce a relatively weak and inhomogeneous static magnetic field. In the case of reservoir rocks, differences between the magnetic properties of the rock matrix and pore fluids may also contribute to a magnetic-field gradient. T₂, but not T₁, is affected by this phenomenon, which is called diffusion. In the presence of high magnetic-field gradients, diffusion effects make T₂ interpretation difficult. However, because the gradients produced by NMR-logging tools are relatively constant, they can be accounted for in T₂ interpretation. In fact, the existence of these field gradients has actually proved beneficial in NMR logging. Magnetic resonance imaging (MRI) is the process by which NMR measurements are obtained in a gradient magnetic field.

Nomenclature

B0	=	static magnetic field, gauss
B1	=	amplitude of the oscillating magnetic field perpendicular to B0, gauss
M0	=	macroscopic magnetization, gauss/cm3
M0x	=	magnitude of the transverse magnetization at t = 0, gauss/cm3
Mx(t)	=	transverse magnetization at time t, gauss/cm3
Mz(t)	=	longitudinal magnetization at time t, gauss/cm3
t	=	time, seconds
x, y, z	=	cartesian space coordinates
T1	=	longitudinal relaxation time, seconds
T2	=	transverse relaxation time, seconds
TE	=	CMPG interecho spacing, seconds
TW	=	polarization (wait) time, seconds

Noteworthy papers in OnePetro

NMR logging: Georgi, D. T., & Chen, S. (2007, January 1). What Does NMR Really Contribute To Formation Evaluation? Offshore Mediterranean Conference.

Freedman, R. (2006, January 1). Advances in NMR Logging. Society of Petroleum Engineers. doi:10.2118/89177-JPT

Coates, G. R., Gardner, J. S., & Miller, D. L. (1994, January 1). Applying Pulse-Echo NMR To Shaly Sand Formation Evaluation. Society of Petrophysicists and Well-Log Analysts.

Matthias Appel (Shell E&P Technology Applications Research) | J. Justin Freeman (Shell E&P Technology Applications Research) | Rod B. Perkins (Shell E&P Technology Applications Research) | Niels P. van Dijk (Shell E&P Technology Applications Research) - Reservoir Fluid Study By Nuclear Magnetic Resonance SPWLA 41st Annual Logging Symposium, 4-7 June, Dallas, Texas

Jerosch-Herold, Michael, .Thomann, Hans, Thompson, A.H.- Nuclear Magnetic Resonance Relaxation in Porous Media 22861-MS SPE Conference Paper - 1991

Howard, James J., Kenyon, William E., Straley, Chris - Proton Magnetic Resonance and Pore Size Variations in Reservoir Sandstones 20600-PA SPE Journal Paper - 1993

Knight, Rosemary, Stanford University - Nuclear Magnetic Resonance: From Pore-Scale Physics to Field-Scale Hydrogeophysics 2011-3750 SEG Conference Paper - 2011

External links

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See also

PEH:Nuclear_Magnetic_Resonance_Applications_in_Petrophysics_and_Formation_Evaluation