Basic NMR Physics and MRIL Tool Physics.ppt

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1、10/9/2018,MRIL - Principles and Applications,1,Basic NMR Physics and MRIL Tool Physics,Outline Nuclear Magnetism Origin of the NMR signal Spin echoes and the CPMG pulse technique Relaxation times: T1 and T2 Commercial probe designs and investigation characteristics Experiment timing, nomenclature, a

2、nd basic data flow,10/9/2018,MRIL - Principles and Applications,2,Nuclear Magnetism,+,+,+,+,+,+,+,spin moment + charge magnetic moment m,Gyromagnetic Ratio ():,Spin quantum no. I = 0, 1/2 , 1, 3/2 , .,Nucleus has 2I + 1 spin “states”,Protons have I = 1/2 2 spin states,Quantum Mech. View (Energy),Cla

3、ssical View (Orientation),high E,low E,DE,Applied Magnetic Field, Bo,determines measurement frequencyh = Planks constant I = spin quantum number,10/9/2018,MRIL - Principles and Applications,3,Nuclear Magnetism, cont.,Many Spins,Spins precess about Bo at frequency .,Single Spin,At “high” temperatures

4、, net magnetization (M) = the parallel - the anti parallel protons to the Bo field. NMR is insensitive: 1016 to 1018 protons required for measurable M,z,M,y,x,Bo,z,y,x,m,Bo,q,10/9/2018,MRIL - Principles and Applications,4,Pulsed NMR Log Measurement Principle,maximum signal amplitude fluid-filled por

5、ositysignal decay time pore size, fluid props, flow props,1. Permanent magnet in tool polarizes hydrogen nuclei,2. Transmit train of RF pulses and records returning spin echo signals,3. Wait for recovery hydrogen magnetization,. the basic NMR “experiment” .,N,S,time,signal,RF pulses,10/9/2018,MRIL -

6、 Principles and Applications,5,The Resonance Effect,precession in xy plane induces FID signal in coil,at equilibrium,M,x,y,z,Bo,rf pulse generates magnetic field B1 B1 oriented normal to Bo B1 oscillates at Larmor frequency,B1,y,z,x,90x pulse,z,x,Excite transitions between spin states by irradiating

7、 at Larmor frequency:,y,10/9/2018,MRIL - Principles and Applications,6,T1S = f (S/V ratio (pore size) , relaxivity ),T1B = f (temperature, little pressure effect (liquids),T1 Polarization,Bulk Relaxation - intrensic property of fluid,Relaxation Mechanisms for T1,Surface Relaxation - Fluid-Rock inter

8、face,10/9/2018,MRIL - Principles and Applications,7,0.95,1.00,Polarized 95% often estimated as a multiple of 3 X T1,0 1 2 3 4 5 sec.,0,M(t)/Mo,Polarization Time / T1,T1 Polarization,10/9/2018,MRIL - Principles and Applications,8,T1 characterizes the rate at which longitudinal magnetization builds up

9、,T2 characterizes the rate at which transverse magnetization decays,ML,MT,T1 and T2,RF,10/9/2018,MRIL - Principles and Applications,9,A Single Spin Echo,time,RF field,time,0,t,2t,free-induction decay (FID) signal,spin-echo signal,signal amplitude,adapted, with permission, from Akkurt, 1990.,90,180,1

10、0/9/2018,MRIL - Principles and Applications,10,Carr-Purcell Gradient Field Relaxation Rate,10/9/2018,MRIL - Principles and Applications,11,Idealized CPMG Spin-Echo Train,Shortening inter-echo spacing (TE) . reduces diffusion-induced shortening of T2 improves resolution of short T2 components,Increas

11、ing number of echoes . increases signal-to-noise (SNR) improves resolution of long T2 components,time, t,2t,4t,6t,8t,envelope of spin-echo amplitudes ,TE,90 pulse,180 pulse,180 pulse,180 pulse,180 pulse,10/9/2018,MRIL - Principles and Applications,12,Data Acquisition .,+,-,1. Record CPMG trains in p

12、hase-alternate pairs (PAPs),2. Stack adjacent echo trains to improve signal-to-noise (SNR),alternate phase of first (p/2) pulsecorrects for baseline offset, driftreduces interference from ringingone tool also alternates frequency,running average,.,10/9/2018,MRIL - Principles and Applications,13,Rela

13、xation Times: T1 , T2 , and T2*,90 pulse,at equilibrium,After pulse is switched off . rapid loss of phase coher- ence, time constant = T2*,recovery of longitudinal magnetization, time constant = T1,decay of transverse magnetization, time constant = T2,Pulse has two effects: 1. Increases thermal ener

14、gy(spin temperature) 2. Introduces phase coherence,T2 can be measured faster and thus is more practical for logging applications than T1,10/9/2018,MRIL - Principles and Applications,14,NMR Experiment Timing,Mo,0,M | to Bo (longitudinal component),M to Bo (transverse component),Mo,0,RF field,0,B1,0,0

15、.5,1.0,1.5,2.0,2.5,3.0,3.5,4.0,4.5,5.0,time, seconds,TW,TE,TX,adapted from Murphy, D.P., World Oil, April 1995,T1 = 400 msec,T2 = 250 msec,10/9/2018,MRIL - Principles and Applications,15,Tw,Te,Tw = wait time Te = interecho time Ne = Number of echoes RA = running average,time,time,Data Acquisition,10

16、/9/2018,MRIL - Principles and Applications,16,Solids,Fluids,Invisible to NMR,10,3,1,0.3,Polarization (ideal),Time in sec.,Clay Bound Water,rock matrix,clay matrix,clay bound water,capillary bound water BVI,movable water,hydrocarbons,Effects of Pore Fluids on T1,Capillary Bound,Movable Water,Hydrocar

17、bons,Variability in T1 due to Fluid Types,10/9/2018,MRIL - Principles and Applications,17,T1 Build-up,T2 decay,Low Porosity Clean Cgr Sandstone,Low Porosity Shaly Fgr Sandstone,Higher Porosity Shaly Cgr Sandstone,Time, sec.,Effects of Chemistry and Texture on T1 and T2 (water filled),10/9/2018,MRIL

18、- Principles and Applications,18,3 T1,95 % Polarization,Rule of Thumb for T1 build-up,Must use the correct Tw (wait time) to see full porosity,10/9/2018,MRIL - Principles and Applications,19,Type of Measurements by MRIL,T1,f: MRIL PorosityEffective porosityT2: Transverse Relaxation Time Differentiat

19、es capillary bound water from free fluids; important for permeability estimate.T1: Longitudinal Relaxation TimeIdentifies hydrocarbon fluids in the non-wetting phase.D: Fluid DiffusivityDifferentiates between gas phase and liquid phase.,10/9/2018,MRIL - Principles and Applications,20,Resonant Freque

20、ncy,N,S,Field Strength diminishes with increased distance from magnet.Proton “spin” rates are proportionalto Magnetic Field Strength.F = g B0 For Hydrogen: g = 4258 Hz / Gauss F = Frequency (Hz) B0 = Field Strength (Gauss)Applying a radio frequency signal equal in frequency to the proton “spin” rate

21、 causes the protons to resonate.,distance,field strength,10/9/2018,MRIL - Principles and Applications,21,MRIL Gradient Magnetic Field,6”,16”,Formation,Borehole Wall,mud,Magnet,Antenna, NUMAR Corp., 1996,B0(r),B0(r),Sensitive Volume,Frequency Bandwidth,10/9/2018,MRIL - Principles and Applications,22,

22、Measurement Principle,Sensitive Volume,Mud Cake,Damaged Zone,typ. 12 cm,10/9/2018,MRIL - Principles and Applications,23,MRIL in Wellbore,MRIL Probe,Borehole,Sensitive Volume Cylinders (each 1 mm thick at 1 mm spacing),24 “, NUMAR Corp., 1995,16”,10/9/2018,MRIL - Principles and Applications,24,f1,f2,

23、Using Multiple Frequencies,f3,Improves Logging SpeedIncreases Signal to Noise,10/9/2018,MRIL - Principles and Applications,25,A New Multiband Generation of NMR Logging Tools,MRIL-Prime,Features9 discrete measurement volumesAccelerated polarization - 12 sec in 6 secmore robust electronics and new son

24、de,BenefitsSingle-pass, fast, total porosity & T2 distribution24 fpm logging speed orData quality of C tool x 3Multi-parameter acquisition (T1 curve, etc.)Familiar outputs and interpretation,10/9/2018,MRIL - Principles and Applications,26,MRIL-Prime Shells,16” 250F,MRIL Probe,Borehole,9 Sensitive Vo

25、lume Cylinders (each 1 mm thick at 1 mm spacing),24”,1”,760kHz,580kHz,10/9/2018,MRIL - Principles and Applications,27,12,13,14,15,16,17,18,19,20,500,550,600,650,700,750,800,850,900,950,1000,Frequency (kHz),Diameter (Inches),50 F.,100 F.,150 F.,200 F.,250 F.,300 F.,MRIL Diameter of Investigation 6” P

26、robe,300 F.,250 F.,200 F.,150 F.,100 F.,50 F.,10/9/2018,MRIL - Principles and Applications,28,MRIL Calibration Tank, NUMAR Corp., 1996,Side View,End View,Faraday Cage,Borehole Chamber,Formation Chamber,MRIL Tool,Sensitive Volumes,10/9/2018,MRIL - Principles and Applications,29,T2 Relaxation,10/9/201

27、8,MRIL - Principles and Applications,30,Idealized Echo Train, NUMAR Corp., 1995,T2R,Time,Amplitude,10/9/2018,MRIL - Principles and Applications,31,Hydrogen in Matrix & Clay Bound Water, NUMAR Corp., 1995,Hydrogen in ClayBound Water,Hydrogen in Lithology,10/9/2018,MRIL - Principles and Applications,3

28、2,NMR Relaxation in a Water-Filled Pore,fast-relaxing spins in thin film on pore surface S, . . film thickness l 10 ,1H,slow-relaxing spins in pore volume, V,Diffusion and surface relaxation in an isolated, water-filled pore,Water in rocks relaxes 101 to 104 faster than bulk fluid value (T1 3.5 sec

29、at 25C) Enhanced relaxation on pore surfaces due to additional relaxation mechanisms . - paramagnetic centers (T1 and T2 ) - hindered molecular motion (T1 and T2 ) - fluid/solid bulk magnetic susceptibility contrast (T2 only)Overall relaxation rate on the surface characterized by “relaxivity” r Diff

30、usion mixes slow-relaxing H spins in V with fast relaxers on S,10/9/2018,MRIL - Principles and Applications,33,T2,T2,T2,T2,T2,time,time,time,time,time,Pore Size and T2 (Water),10/9/2018,MRIL - Principles and Applications,34,Coarse Grain Response, NUMAR Corp., 1995,MPHI = 36, MFFI = 30, MBVI = 6, MPE

31、RM = 4200 md,0,5,10,15,20,25,30,35,40,0,50,100,150,200,250,Time (ms),Porosity,10/9/2018,MRIL - Principles and Applications,35,Fine Grain Response, NUMAR Corp., 1995,MPHI = 36, MFFI = 6, MBVI = 30, MPERM = 6.7 md,-5,Time (ms),0,5,10,15,20,25,30,35,40,0,50,100,150,200,250,Porosity,10/9/2018,MRIL - Pri

32、nciples and Applications,36,MAP Processing, NUMAR Corp., 1995,10/9/2018,MRIL - Principles and Applications,37,0.00,0.50,1.00,1.50,2.00,0.1,1,10,100,1000,10000,T2 msec,Incremental Porosity pu,0,2,4,6,8,10,12,14,16,Cumulative Porosity pu,Data Processing,Spin-echo data,T2 Spectrum,time,multiexponential

33、 fit to spin-echo amplitudes,large-pore (mobile fluid) signal,small-pore (irreducible fluid) signal,clay-bound water,NMR porosity,MAP “Inversion” Processing,Water-saturated rock: rT2 = V/S“Invisible Region” is a function of signal to noise and echo spacing,Capillary Bound Fluid - BVI,Free Fluid - FF

34、I,Invisible Region,Clay Bound Water - CBW,10/9/2018,MRIL - Principles and Applications,38,NMR - Porosity Model,Neutron ,Density ,rock matrix,clay matrix,clay bound water,capillary bound water BVI,movable water,hydrocarbons,MR porosity (effective),MR porosity (total short TE),Resistivity Sw,NMR BVI,N

35、MR FFI,Integration of MR Log and Resistivity Log Interpretation,nonmovable water,Producible hydrocarbon,will produce some water,10/9/2018,MRIL - Principles and Applications,39,Spin Echo Attenuation by Diffusion in a Gradient Magnetic Field,only stationary spins are completely rephased by p pulses in

36、 a CPMG expt spins diffusing in a gradient magnetic field undergo unrecoverable dephasing. echo attenuation transverse relaxation mechanism,. two sources of magnetic field gradients .,Applied . MRIL uses strong gradient magnetic field to perform “slice selection” . known, well-defined gradient gives predictable T2 shifts that depend only on diffusion,Rock Grain,Rock Grain,Rock Grain,