Chapter 10 NMR

NMR Logging Nuclear Magnetic Resonance

By

Jorge Salgado Gomes

Applications of NMR

NMR provides

• Mineralogy-independent porosity

• T2 distributions

Use of T2 distributions

• Pore-size distributions

• Irreducible water saturations

• Permeability

NMR Free Fluid & Porosity

NMR Principle

• Gives unprecedent information about both porosity and pore size distribution that is used to successfully derive continuous permeability logs, notably in siliciclastic formations.

• The physical principle called nuclear magnetic resonance refers to the response of atom nuclei to externally applied magnetic fields.

• Many atom nuclei have a magnetic moment, i.e. behave like tiny spinning magnets. These spinning nuclei can interact with a magnetic field, producing detectable signals. For most elements, nevertheless, the measured signals are weak, but hydrogen that is abundant in both water and hydrocarbons contained in the pore space of rocks, has a relatively large magnetic moment.

NMR Principle (cont.)

• Molecules in fluids are in constant Brownian motion. Besides the relaxation by molecular diffusion in magnetic field gradients that the CPMG pulse sequence is compensating for, there exist two main NMR relaxation mechanisms, i.e. bulk fluid relaxation and grain surface relaxation. Both mechanisms result from molecular interactions and create the irreversible dephasing that can be observed by means of the decaying amplitude of spin echoes.

• The initial NMR signal amplitude is thus proportional to porosity; its overall decay is the sum of the individual decays, which reflects pore size distribution. Separating out ranges of T2 values by a mathematical inversion process produces the T2 distribution curve.

NMR - Theory

• The area under the curve represents the porosity and the curve shape the distribution of pore sizes. This inversion process normally requires stacking, in order to improve the signal-to-noise ratio, which slightly degrades the vertical resolution

NMR Principles • NMR counts # hydrogen protons aligned with an external magnetic field (EMF) •Monitors the dephasing of protons’s spin with time • # protons aligned with EMF is proportional to porosity •Change of amplitude of subsequent echoes, known as the transverse relaxation, is characterized by a exponential decay with a characteristic time (T2) • the initial NMR signal amplitude depends only on the H proton density in the pore space. •To measure T2 decay, a series of RF pulses are used to rephase the transverse magnetizaton. •These RF pulses recall the NMR signal (echo)

Comparison between a spinning top precessing in a gravitational field, and a spinning nucleus

precessing in a magnetic field (Courtesy of SPE)

NMR Principles (cont.)

• The dominant factor on the T2 relaxation process is controlled by nuclear magnetic interactions occurring on the pore wall. Including diffusion effects, T2 relaxation is described by:

1/T2=(1/T2bulk)+(λ/T2surf).(S/V)+(1/T2D) Where: T2bulk = relaxation time of the bulk fluids in the pore space T2surf = relaxation time of the fluid in a few molecular layers next to the pore surface λ = thickness of the surface fluid layer S, V = pore surface and pore volume T2D = relaxation time due to diffusion

• If diffusion effects are negligible, and because T2bulk for water is on the order of seconds, while the observed T2 is only a few hundred milliseconds or less, the above equation can be approximate by:

1/T2= ρs (S/V) Where: ρs = surface relaxivity (0.003-0.03 cm/s) for clastics and (<0.003 cm/s) for carbonates.

• In water filled pore systems, T2 is a measure of the S/V ratio. • Large pores will have a small S/V ratio and will exhibit long T2s • Small pores will have a large S/V and short T2s

NMR Principles (Recap)

• Rocks with a spectrum of pore sizes give rise to a corresponding spectrum of T2 relaxation times.

• NMR echo data are converted to a T2 spectrum that is closely related to the pore-size distribution for a single-phase fluid.

• The T2 spectrum, derived from the echo data, measures the porosity fraction associated with each T2.

• The integral of the T2 spectrum is the NMR porosity. • The advantages of T2 spectrum is the partition into fast decaying fluids (bound

water; 4-32 ms for clastics) and free fluids (> 32 ms, for clastics). • T2 time separating moveable and irreducible water is referred as T2 cut-off time. kNMR= C. T2gm

2.φ4 ( Kenyon et al. 1988); overestimate k in HC zones

K=(100 φ/C)4 (Smov/Swir)2 (Coates et at 1991); unaffected by HC

Decay of Precessing Nuclei

De-phasing of nuclei

Pulse Echo

Spin Echo Train

Relaxation theory

Relaxation times

Decay distributions

Echo Profile for Sandstone

Halliburton MRIL tool

TOP VIEW OF MRIL

Schlumberger CMR

NMR & Permeability Estimations

Kozeny 1927 (modified by Carman in 1937)

• K=αφ3/S2

S= grain surface area per bulk volume

Φ= porosity

α = empirical constant

It describes permeability in packs of spheres of uniform size where grain surface area is known. In real life, grain surface area is difficult to obtain. It fails in heterogeneous formations.

Wyllie and Rose (1957)

• Grain surface area can be, in water-wet formations, approximately related to the irreducible water saturation, Swirr

Timur (1968)

• K 1/2 = φ2.25/Swirr

work OK in clean sandstones only

T2 distributions for two sandstones with same porosity but different permeabilities and pore sizes

Free fluid porosity

T2 cutoff of 33 msec for sandstones

Schlumberger-Doll Research Equation

• KNMR = C (φNMR)4 (T2,log)2

KNMR = Estimated permeability from NMR

φNMR = CMR total porosity

T2,log = The logaritmic mean of T2 distribution

C = is a constant (4 for sandstones and 0.1 for carbonates)

Comparison of CMR porosity and CMR permeability with core measurements

NMR in Carbonates

• The interpretation model assuming that, in water-saturated reservoir rocks, the T2 and pore-size distributions are directly related.

• It explains why NMR T2 curves are successfully used to characterize sandstones containing mixed pore-size distributions.

• However, there is some concern within the oil industry that NMR does not work as well in carbonate reservoirs. First, NMR responses in carbonates differ from those in sandstones.

NMR in Carbonates

• Pore surfaces in carbonates are not equally effective in relaxing hydrogen nuclei and carbonates are about three times less efficient in relaxing the nuclear magnetism than sandstones.

• For carbonates, relaxation times therefore tend to be three times longer and a 100 msec cutoff was proposed for free-fluid porosity.

• This cutoff value has often to be locally adapted. For instance, in the Thamama formations of Abu Dhabi, permeable grainstones could be distinguished from lower permeability packstones and mudstones with a 225 msec cutoff.

• But, while carbonate formations contain mixed pore-size distributions, e.g., intergranular porosity and vugs, NMR logging data in these formations nevertheless frequently yield unimodal T2 distributions, which often results in inconsistent T2 cutoff values to distinguish bound and free fluids, and leads to unreliable permeability predictions.

Comparison of CMRPlus high-resolution permeability with FMI borehole electrical images