Hi Weal,
NMR (Nuclear magnetic resonance) is a very useful tool when applied in the correct zones and with the right purpose. It allows for detailed analysis of the material present within the porous; basically the ‘moving parts’ of a rock. Let me explain this a little bit more…
With moving parts I refer to the protons of fluids that can align with a potential magnetic field. Surrounding mineralogy (the rock) does generally not affect the NMR measurements, which is in contrast with conventional logging tools. Basically, the hydrogen protosn align with a magnetic field induced by the magnet in the tool. Then a radio frequency (RF) is emitted as a pulse that rotates the protons at a 90-degree angle. Often another pulse is applied and tips the protons in 180 degree angles. The magnitude of the NMR signal (which is caused by the RF pulses) is then measured and is proportional to the quantity of hydrogen in the volume that is probed. The tool, thus, provides a measure for Liquid filled porosity; effective porosity.
The hydrogen bound to clay or other minerals relax, and realign quickly, making them detectable. So basically irreducible water can also be measured. Pore size and Texture affect the decay rate of the NMR signal and the tool can thus estimate the rocks internal surface to volume ratio. This relates to the volume of irreducible water and provides a method for estimating grain-size distribution.
NMR tools (like MRIL) can thus be used for:-Identifying bound (irreducible) fluid
-Identifying moveable fluid (effective porosity).
-Determine permeability (through grain size distribution).
-Quantifying reservoir fluids like oil, gas, and water (via DHT).
We often use this checklist for identifying the need to use NMR technology in our logging suite.-Do we need to know how Porous the rock is?
-Should we identify the moveable and producible fluids?
-Is it necessary to know the producing fractions of gas, oil and water?
-Is it important to know what the height of each column and the transition zone is?
-Do we need to identify specific baffles for production, like pore-clogging clays and tar layers?
-Is it paramount to identify the properties of each producible fluid: How viscous is the oil? What is the gas/oil ratio?
I hope this answered your question and I welcome other approaches and usage of NMR technology.
