1 Scope
This document specifies the principles, test procedures, calculation formulas and data processing for the determination of the pore size distribution of coal and rocks by NMR. This document is applicable to the determination of pore size distribution of medium and high rank coals and sandstones, and the reference use of low rank coals and shales.
2 Normative references
The contents of the following documents constitute essential provisions of this document through the normative references in the text. Among them, note the date of the cited documents, only the date of the corresponding version applicable to this document; do not note the date of the cited documents, the latest version (including all the revision of the list) applicable to this document.
GB/T 474 Methods of preparing coal samples
GB/T 21650.1 Determination of pore size distribution and porosity of solid materials by mercury pressure and gas adsorption ﹑ Part 1; Mercury pressure method
GB/T 23561.3-2009 Methods for the determination of physical and mechanical properties of coal and rocks: Part 3: Determination of bulk density of coal and rocks GB/T 29171-2012 Determination of capillary pressure curves of rocks
GB/T 29172-2012 Methods for analysis of rock cores
SY/T 6490-2014 Specification for laboratory measurement of NMR parameters of rock samples
3 Terminology and definitions
The following terms and definitions are applicable to this document.
3.1
Free decay signal free induction decay
The NMR signal that oscillates according to the sinusoidal law and decays according to the exponential law.
3.2
T relaxation time T:time
The time constant describing the recovery of the transverse component of the nuclear magnetisation intensity.
Note 1: Also known as the transverse relaxation time or spin relaxation time.
Note 2: Reflects the decay of the horizontal component of the non-equilibrium magnetisation vector to zero, caused by the inhomogeneity of the local and static magnetic fields of the neighbouring nuclei.
The decay of the horizontal component of the non-equilibrium magnetization vector to zero.
3.3
T:cut off value
The limit on the relaxation time of a movable fluid and a bound fluid in a nuclear magnetic T.
Note: above this relaxation time, the pore fluid is called a movable fluid; below this relaxation time, the pore fluid is called a bound fluid. 3.4
Inverse inversion
The process of obtaining the relaxation signal for each pore space from the overall relaxation signal.
3.5
Nuclear magnetic resonance (NMR)
The physical process whereby an atomic nucleus with a non-zero magnetic moment undergoes a Seeman splitting of the spin energy levels in the presence of an external magnetic field and resonates to absorb a certain frequency of radio frequency radiation.
4 Principle of NMR measurements
When a core sample is subjected to a uniform static magnetic field, the interaction between the fluid-rich hydrogen nuclei in the core and the magnetic field generates a magnetisation vector. The NMR signal contains information on frequency, phase and amplitude, and the collected NMR signal can be inverted to obtain the T-harmonic diagram of the sample, which corresponds to the pore structure. saturation, pore size distribution, etc.
5 Reagents and materials
5.1 Standard oil or vegetable oil.
5.2 Anhydrous paraffin.
5.3 Deionised or distilled water.
5.4 Standard samples (0.05% CuSO,solution or ceramic powder) of different pore oscillations.
6 Equipment
6.1 Standard round hole sieve; sieve size 2 mm (10 mesh), 4 mm (5 mesh), 6.2 Blast drying oven; to meet the (room temperature - 25o ℃) ± 1 ℃ use conditions. 6.3 Vernier calipers; minimum index value o.02 mm.
6.4 Protractor; index value 2'.
6.5 Centrifuge; speed 8 ooo r/min or more.
6.6 Electronic balance: 0.001 g sensitivity.
6.7 Nuclear magnetic resonance analyser (see Figure 1), with:
7 Sample requirements
7.1 Sample water status 7.1.1 Natural water status
After preparation of the specimen, at room temperature, placed in a desiccator with water at the bottom for l d~2 d to maintain a certain temperature, but the specimen should not touch the water surface.
7.1.2 Drying condition
Rock samples should be dried at 105 ℃~110 ℃ for 24 h. Coal samples should be dried according to GB/T474.
7.1.3 Saturated state
Water saturation according to GB/T 23561.3-2009, 3.3.2~3.3.6. 7.2: Preparation of dehydrated samples
The rock samples were dewatered by centrifugation or repellent devices at temperatures of 15 °C to 28 °C to produce dewatered samples of different saturation levels.
7.3 Core preparation
7.4 Preparation of coal cores
Coal cores were drilled in the direction of the laminae with a diameter of not less than 26.4 mm and a length of 1.5 to 2 times the diameter. The core shall be drilled to a length of 1.5 times to 2 times the diameter.
7.5: Chip sample preparation
7.5.1 Crushing
Samples of naturally dried coal or rock shall be crushed to less than 10 mm by mechanical or manual crushing.
7.5.2 Sieving
The samples were sieved using a combination of 2 mm and 4 mm sieves and those between 2 mm and -4 mm were bagged and stored in a desiccator for use.
Note: Core or coal core samples were preferred for testing. When core samples could not be prepared, chip samples were substituted and shaped for testing.
8 Nuclear magnetic resonance T:spectroscopy
8.1 Instrument commissioning
8.1.1 Magnet thermostat
The magnet temperature should be set according to the requirements of the instrument and should remain stable for at least 8 hours after the magnet temperature has reached the set value to ensure that subsequent operations such as finding the centre frequency can be carried out properly.
8.2 Acquisition of T. 8.2.1 Pre-treatment of the sample
The dried core or chip samples from Chapter 7 should be vacuumed for 12 h or more and then saturated with deionised water for 18 h or more, at the same pressure as the formation pressure of the core. If the core contains highly water-sensitive minerals, anhydrous paraffin may be used for saturation.
8.2.2 Sample information entry
After the sample has been saturated with fluid, the sample is drained using filter paper until there is no visible water on the surface, the diameter and length of the core sample is measured using vernier calipers and the total volume of the chip sample can be measured by draining. Basic sample information, such as size, volume and well depth, is entered into the software.
8.2.3 Saturated sample determination
The saturated sample is placed in the sample chamber and the NMR signal is measured by selecting a line of similar porosity to that of the sample to be measured.
8.2.4 Determination of centrifugation force
The optimum centrifugal force should be determined before centrifugation of the sample. The centrifuge speed can be set at 3 000 r/min to 10 000 r/min. The conversion between centrifugal force and speed was performed according to 4.4 of GB/T 29171-2012.
9 Data processing
9.1 Pore radius (r) - relaxation time (Tz) conversion (indirect method)
9.1.1 Pore radius to T. Conversion equation for spectrograms
10 Quality requirements
10.1 Relative uncertainty of the standard sample
The relative uncertainty of the NMR longitudinal relaxation time T, and the transverse relaxation time T;,of the characteristic parameters of the standard sample measurements and the characteristic parameters of the standard spectrum should be less than 3%.
10.2 Relative uncertainty of samples
The relative uncertainty in the repeatability and reproducibility of the measured coal and rock samples shall be less than 8% for the longitudinal relaxation time T, and the transverse relaxation time T.
Appendix A (informative) T: Characteristic parameters and NMR porosity calculations
Appendix B (informative) NMR signal data log sheets
Appendix C (Informative) NMR Data Fitting Plot
Appendix D (informative) Mercury piezometric data log sheet-
Appendix E (informative) T:-r data log sheets
Appendix F (informative) Coal and rock pore size distribution junction diagrams
1 Scope
2 Normative references
3 Terminology and definitions
4 Principle of NMR measurements
5 Reagents and materials
6 Equipment
7 Sample requirements
8 Nuclear magnetic resonance T:spectroscopy
9 Data processing
10 Quality requirements
Appendix A (informative) T: Characteristic parameters and NMR porosity calculations
Appendix B (informative) NMR signal data log sheets
Appendix C (Informative) NMR Data Fitting Plot
Appendix D (informative) Mercury piezometric data log sheet-
Appendix E (informative) T:-r data log sheets
Appendix F (informative) Coal and rock pore size distribution junction diagrams
