Let’s start off with some basics about NMR or MRI as it is the technology underlying our products and services. NMR is an acronym for nuclear magnetic resonance and is generally the term that is used in the oil and gas sector. MRI stands for magnetic resonance imaging and is the acronym used in the medical field.
To understand how NMR works, we need to understand that each hydrogen in a rock or the fluid in the rock can be thought of like a tiny bar magnet. These hydrogen atoms are spinning like tiny tops and are commonly referred to as “spins”. Outside of an external magnetic field, these spins are randomly oriented in all directions, and so the whole rock has a net magnetization of zero. However, if the rock is placed in a strong magnetic field (B0), the spins will align either parallel or antiparallel to the main field. A slightly higher proportion will align with the field, giving a net magnetization vector. The problem is that this small magnetization cannot be measured. The spins need to be rotated away from the static field and then they can be observed returning to the desired state. This rotation can be done by applying a short burst of magnetization orthogonal to the static field and oscillating at the same frequency as the spinning spins. This short burst is known as a radio frequency (RF) pulse. As soon as the RF pulse is stopped, under the influence of the main field, the protons want to return to their original alignment with B0. When they “relax” back to this original orientation they give off a small RF signal, which is detected by the NMR system as the MR signal.
One of the most commonly measured substances with MRI/NMR is water because it is a rich source of hydrogen. Hydrocarbons can also be measured as they also have a large number of hydrogens. Most reservoir rock has more than one fluid in it. Generally a mixture of brine (water) and different hydrocarbons (liquid and gas).
The return to equilibrium where the spins are aligned with the main field is governed by two independent processes – T1 regrowth, and T2 decay (in NMR terms). These two exponential processes are different for each type of fluid, which allows us to distinguish between them when scanning. T1 regrowth is a measure of the time it takes for the spins to re-align with the main magnetic field. T2 is a measure of the time it takes for the spins to lose phase coherence.
It is not spatially resolved. It is bulk measurements of proton density, T2 , T1, etc. The imaging part of MRI allows that measurement to be spatially resolved.
As stated earlier, NMR/MRI gets it’s signal from hydrogen. If there is no hydrogen there is no signal. NMR is an excellent tool that can be coupled with CT and X-ray to give a complete picture of what is going on within a body (or rock) as they will have signal even when there is no oil, gas or water present.
MRI spatial resolves the hydrogen signals to form images. As an example, a 2D image of a human head is shown. We can also collapse one dimension and get a simple profile. This is what is done for rocks.
The NMR signal intensity depends directly on the amount of hydrogen present in the object. There are two main relaxation times – T1, and T2. There is also T2*, which is related to the static field homogeneity in the rock and is less important in petrophysical applications. These times are characteristic for different substances. For example, in humans, fat has a short T1 and a short T2. Water has a long T1 and a long T2. Also the lifetime of the signals depend on the environment of the hydrogens. So for example, in rocks fluid in small pores can be differentiated from the same fluid in larger pores.
NMR is a very valuable, multipurpose tool when analysing reservoir rock and fluids. It can measure the total oil and water volume, the oil and water content, the porosity, permeability, capillary pressure, wettability, and relative permeability. You can quantify the amount of bound water, clay bound water and free fluid index. You can understand about the pore connectivity. NMR is a powerful tool when trying to understand unconventional, tight reservoirs as it isn’t constrained by the small pore network in these rocks.
GITs in-house research team is continually adding new and innovative NMR-based measurements to our software packages. To learn more about what NMR can do for you, please contact our sales team.
In 2014, Dr. Derrick Green, CTO of Green Imaging Technologies visited ENI in Italy and presented the Basics of NMR Core Analysis to a large group. This presentation was recorded to be shared with others looking to learn the basic principles and capabilities of NMR/MRI technology in the Oil and Gas sector. Please find a link to this recording below.