Nuclear Magnetic Resonance (NMR) technique uses the response of atomic nuclei in a magnetic field. A spinning proton acts as a loop of current and therefore produces a magnetic field along the spin axis.
As hydrogen nuclei have only a proton, it produces relatively large magnetic moment when it spins. In a bulk of fluid that contains hydrogen, as water, these spinning protons are aligned randomly.
By applying a strong external magnetic field, the spinning protons can be aligned in the direction of the applied magnetic field. If the field is turned off, the aligned protons begin to spin in random direction again. Transverse relaxation time, T2 is the characteristic time that proton aligned by a magnetic field take to flip in the direction of another magnetic field which is perpendicular to the initial magnetic field.
Transverse relaxation rate, 1/T2 in a NMR experiment is proportional to the surface to volume ratio (S/V):
- where, r is relaxivity and Sf is the specific surface for pore space.
When fluid containing hydrogen wets grain surfaces, the fluid molecules continuously collide with the grain surface due to diffusion. As a result, relaxation of protons close to the solid surface is faster than of protons in the free flowing fluid. Thus, immobile water has faster relaxation rate than the free flowing water.
NMR setup at CERE
NMR measurement at CERE is performed by a GeoSpec 2/53 DRX-HF digital spectrometer comprising pulse programmer, acquisition with high speed sampling, pre-amplifier optimized for low frequency operation and up to 128000 echoes capability. It also includes probe tuning facility and electronics rack with magnet temperature controller external to magnet case. It includes LithoMetrix rock core applications software to allow routine operation of the instrument for determination of:
- T1 and T2 distributions
- Free Fluid Index (FFI)
- Bound Volume Index (BVI)
- Clay Bound Water (CBW)
- Effective Porosity
- T2 cutoff
Technical specification
- Nominal frequency: 2 MHz
- Magnet temperature: 35 °C
- Time -to –Echo (tau): minimum 50 microseconds
- Number of echoes (NECH): up to 128000
- Signal to noise ratio (SNR):
- Theoretical: 100
- Achieved: 200 (in brine saturated chalk)
- Sample size:
- Length: up to 75 mm
- Diameter: up to 37.5 mm
Publications
Alam, M.M, Hjuler, M.L., Christensen, H.F. and Fabricius, I.L. 2011: "Impact of Supercritical CO2 Injection on Petrophysical and Rock Mechanics Properties of Chalk: An Experimental Study on Chalk from South Arne Field, North Sea". Presented in the SPE Annual Technical Conference and Exhibition, 30 October-2 November 2011, Denver, Colorado, USA. SPE paper number 147056.
Hossain, Z., Grattoni, C.A., Solymar, M. and Fabricius, I.L. 2011 “ Petrophysical properties of greensand as predicted from NMR measurements” Petroleum Geoscience 17 111–125.
Alam, M.M. and Fabricius, I.L., 2010, "NMR as a Tool for Estimation of Excess Conductivity in Chalk". Poster presented in the 72nd EAGE Conference and Exhibition, Barcelona, Spain 14-17 June 2010.
For further details contact: Monzurul Alam