- [Ravinath] Why is a T1-T2 measurement interesting, important and what can it give us? Over here I plot the relaxation time versus viscosity. Now the relaxation time that I plot could be T1, or it could be T2, both of which are used commonly in petrophysics. Now the x-axis over here is nothing but a correlation time. What does this mean? If you have a very small correlation time, it means you're dealing with objects which move very fast, which means the viscosity is very, very low, which means you're talking about gas, bulk water, bulk fluids, bulk oil, light oil. And as you go towards the right, you're moving to smaller and smaller correlation times, which means you're going into increased viscosity. So as you go down you will encounter more bound species, species in wetting environments, species which cannot move much. And then you would go into very, very viscous environments, such as heavy oil, bitumen, et cetera. So in the T1-T2 plot, you have this area corresponding to low viscosity, bulk water, bulk fluids. You have an area corresponding to bound fluids, and more viscosity corresponding to bitumen as you move from the left to the right. And if you look at how T1 and T2 behave, you see that for these bulk species or low viscous species, the T1 and the T2 are close to each other, they can be comparable. This means that the T1 over T2 ratio is close to one because the two species, T1 and T2, are comparable, so T1 divided by T2 are two identical numbers, the ratio's equal to one. As you go to more species which are bound, the T1 slowly diverges from the T2. So the T1 becomes bigger than the T2, so the T1 over T2 will start to have bigger and bigger values compared to one. So it could become 1.25, two. And as you go towards the right, there is an even bigger diversion between the T1 and T2, leading to much bigger T1-T2 ratios, five, 10, 50, hundred, et cetera. So based on the T1-T2 ratio, we can try to use that as a mobility or a sensitivity parameter, and extract out information about our system. So this is what we're gonna do. And I'm gonna show you how to do that. Now basically the first thing that I want to set, and I don't want to get too deep into the math at the moment because that's not important, the main thing that I want to set the stage for is that the T1 and T2 have two different spectral density functions. In other words, what it means is that the sensitivity of T1 and T2, the frequencies are different. Frequencies are indirectly timescales, they are indirectly lin scales. So what it means is, if you make a measurement with a tool, and the tool has a frequency of two megahertzs, then the T1 is sensitive to omega, which is the tool frequency of two megahertz. So the Schlumberger CMR, for example, is two megahertz, so the tool would be sensitive to T1, so T1 would be sensitive to two. T2, on the other hand, irrespective of whatever tool you use, is mainly sensitive to slow motions, or in other words zero frequency, which means very, very slow motions, or very long timescales. When you make a T1-T2 2D plot, it means that you're sensitive from the tool frequency to zero motion. So you have a window of frequencies, or a range of lin scales or a range of motions you're sensitive to. And that is why this maps are much more useful in comparison to only T1 or only T2. So that is the main message, firstly, to see why these maps are useful.