Nuclear magnetic resonance logging

NMR logging, theory, NMR log application, geophysics, well logging

1. NUCLEAR MAGNETIC RESONANCE LOGGING 23-Mar-17 BARCTS-AMD ‹#› AVADHESH KUMAR SHUKLA TSO-121/2016 BARCTS-AMD, HYDERABAD BY

2. What is NMR?.. Response of nuclei in the presence of Magnetic field Nuclei having odd number of proton/neutron: in our case it is Hydrogen. Each proton is like spinning bar magnet These tiny bar magnet are aligned along in external magnet field applied When external field is switch off precession starts like gyroscope in gravity field. 23-Mar-17 2 BARCTS-AMD

3. This precession has certain frequency known as Larmor frequency and is given by f = yH where y is the gyromagnetic ratio of the proton ( y = 4.2576 x 10^3 Hz/G ). The precession of the spins results in a varying magnetic field which induces an AC voltage (at the precession frequency ). 23-Mar-17 3 BARCTS-AMD

4. Principle of NML The alignment of these protons is called polarization but this does not happen immediately it grows with a time constant called longitudinal relaxation time T 1 After T 1 an oscillating magnetic field is applied, sending pulses of radio-frequency energy into the formation The initial pulse is perpendicular to B o and aligns the spins in the transverse direction in phase with one another 23-Mar-17 4 BARCTS-AMD

5. As the pulse dies, protons losses their energy/precession by coming to the initial state. This exponential decay is sinusoidal , this is called relaxation(T 2 ) T his signal is seen in the receiver coil This very rapid decay is referred to as free induction decay (FID) Both during build up magnetisation and relaxation there is transfer of energy. How this energy is transferred into the surrounding material form the basis for NML 23-Mar-17 5 BARCTS-AMD

6. The quantities measured are signal amplitude and decay Signal amplitude is proportional to hydrogen nuclei present and calibrated to give porosity However decay of NMR signal(Relaxation time) during each cycle gives information about pore size Small pores short relaxation time Clay bound Large pores large relaxation time readily producible fluid Relaxation times and their distribution may be interpreted to know: Permeability Producible porosity Irreducible water saturation Capillary pressure curve Hydrocarbon identification Distinction b/w free fluid and capillary bound porosity 23-Mar-17 6 BARCTS-AMD

7. NMR Logging Tool The modern Schlumberger N MR apparatus consists of two samarium cobalt magnets, each about one foot (30 centimeters) long, that project a magnetic field into the formation. An antenna located in a cylindrical well on the face of the apparatus generates the oscillating magnetic field, which is also projected into the rock The two magnetic fields are substantially perpendicular within the volume of rock being examined, which extends about an inch (2.5 cm) into the formation The antenna is used as both transmitter and receiver The transmitter operates at the Larmor frequency, which depends on the strength of the permanent magnet on the tool. On a CMR, for hydrogen, this is 3.8 MHz. 23-Mar-17 7 BARCTS-AMD

8. Sandy Interval The NMR Log 23-Mar-17 8 BARCTS-AMD

9. 23-Mar-17 BARCTS-AMD 9 Porosity logs in mixed-lithology carbonates show the advantage of NMR lithology-independent total porosity. NMR total porosity is not sensitive to lithology and reads correctly over the entire interval. The density tool reads correctly only in intervals in which the assumed matrix density (lithology) used to compute the density-log porosity matches the actual matrix density of the formation. Dolomite Limestone

10. Advantages of NMR Only fluids are visible to NMR technology so porosity measurement is independent of the lithology Producible zones with high percentage of clay-bound water can be identified A better measurement of permeability is possible than traditional plots In-situ measurement of oil viscosity Differentiation of oil/gas zones 23-Mar-17 10 BARCTS-AMD

11. Disadvantages of NMR Tools Any diamagnetic or paramagnetic ions present in the formation can affect the tool response Expensive Slower logging speeds Slimhole tools are not available Shallow depth of penetration Permeability measurement is actually an empirical measurement and should only be used to compare to permeabilities 23-Mar-17 11 BARCTS-AMD

12. Summary NMR Logging uses the energy given off from hydrogen protons as they precess in a magnetic field to infer measurements of a formation’s porosity, permeability, pore space distribution, etc. Logs can be used to interpret zones of high porosity and producibility Main tools used are the Halliburton’s MRIL and Schlumberger’s CMR tool Gives lithology independent porosities but is more expensive than conventional tools 23-Mar-17 12 BARCTS-AMD

13. Thank You! 23-Mar-17 13 BARCTS-AMD

14. Questions Please… 23-Mar-17 14 BARCTS-AMD