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How advanced NMR techniques unveil capillary pressure and fluid type

Nuclear Magnetic Resonance (NMR) not only enables simple core analysis, it can also unlock crucial data relating to capillary pressure and fluid type more quickly and cost-effectively than traditional methods.

NMR is an incredibly valuable tool, which can determine porosity and pore size distributions easily and quickly. What many core analysts might not realise is that it can also be used to accurately determine capillary pressure and fluid types – when used in conjunction with the right software.

Under pressure

High capillary pressure makes liquid extraction difficult and therefore expensive. In order to assess pressure levels, laboratories typically look to two methods, of which porous plate is the gold standard for accuracy. However, although it undisputedly provides the most accurate outcome, it is a process that can take months to produce results.

Using a centrifuge in tandem with a stroboscope is much quicker but still takes a few days to generate only eight to ten data points. The additional equipment required makes this method comparable in cost to an Oxford Instruments NMR solution, which also provides further useful information.

Our patented GIT-CAP technique uses a combination of conventional centrifuge and a technique called 1-D Profiling, which records NMR signals in one dimension only – along the axis of the core. The result is a detailed measurement of the distribution of fluid along the length of the core. This process is repeated at one more centrifuge spin speed, then the data is analysed. Capillary pressure is calculated from knowledge of the centrifuge spin speeds and the changes in the saturation profile.

The advantage of this approach is that, with only two centrifuge spins required, it is much quicker, typically taking hours rather than days. It also yields up to 40 data points, making it more detailed than traditional centrifuge plus stroboscope. And, because it doesn’t require a stroboscope or fluid collection vessel, it is less expensive. If you already have a centrifuge in your lab, simply adding our GeoSpec instrument with integrated GIT software will speed up and improve your capillary pressure analysis without any specialist knowledge required.

Let it flow

When it comes to ascertaining the types of liquid contained within a core sample, it is no exact science. In fact, many oil industry veterans rely on gut instinct rather than scientific method, simply because few are aware of the reliable scientific techniques that are already available.

For example, by adding an accessory called Pulsed Field Gradients (PFG) to our GeoSpec2+ NMR instrument, you can carry out measurements based on the diffusion, flow, or distribution of the fluids within the core sample, making the identification of fluid type faster and simpler.

Essentially, NMR signals are generated from liquids (oil or brine) when the sample is placed in a magnetic field and then excited with a brief pulse of radio frequency (RF) energy. Immediately after the pulse, an NMR signal appears, which then dies away with a characteristic relaxation time or decay rate known as T2. The amplitude of the signal immediately after the pulse is an indication of the total amount of fluid present, while the T2 of the signal gives valuable information about the physical environment of the liquids.

However, different fluids, such as heavy oils and partially-bound water, may exhibit similar T2 values, which makes it difficult to identify which fluid is which. Nevertheless, even if they have similar T2 values, they are unlikely to also have similar diffusion characteristics, so a measurement that collects both T2 and diffusion data can identify the fluid type.

GIT Systems software, together with the PFG accessory on our GeoSpec2+ NMR instrument, enables such measurements to be done. The result is a two-dimensional data map illustrating the relative proportions of the different fluid types in the sample, giving you previously inaccessible visibility of the oil-water ratio.

Putting NMR at the core of your analysis

We’ve seen that NMR complements basic sample analysis in our previous post and now, it is clear that more advanced NMR techniques can unlock deeper mysteries within the rock. If you would like to find out more information or arrange a demonstration to find out how NMR can improve your productivity, speed analysis and reduce lab costs, please contact us.

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