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  3. Thomas-Stieber Method

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- [Instuctor] So, looking at this information from the coal, I came to a conclusion that there is some pay that is missing in the even sand for which I need to do the thin bed analysis approach. One of the methods is Thomas-Stieber method, and you can refer to my paper where I have given the references to the method, and also to the Thomas-Stieber paper. So, on the left, what we see here is the gamma ray histogram and how it looks after normalization. I've given the end-point that I have used for normalization, so those are 34 and 115. It is very crucial that what happens is when we have a lot of wells with different wind ages, in this study, we had wells ranging from 1984 to 2007. We had a range of different tools used and also different mark tides. We have to be careful when we have wells ranging from, for over 20 years, begin. Also, because of the cause of the wind age, it is also possible that we sometimes do not get all the information associated with those logs. That's why it is very crucial that, before we start such a study, it is important that all the information, for example the gamma ray, they need to be normalized so that we look at them with a consistent approach. So, the best effort that I had tried to normalize the gamma ray gave me the result, which is shown here. On the right, I'm showing the exemptions that have gone into the Thomas-Stieber method. This has been taken from the paper. Only two rock types are considered. So, this is assumption, that I thought we had high porosity clean sand and low porosity pure shale. There is nothing in between. Which is, I know that is not true, but we have to go ahead with the assumptions when we run this model. The mineralogy in the investigated interval has to be same in the bounding shales, which is true for this bounds. The gamma ray is related to the number of radioactive events and we assume that the shale grain and the sand grain densities are comparable. Again, the background radiation is assumed to be constant, and the most important is, that the counting yield will not change as the rock types are intermixed. So, here I'm showing the end point selection for the Thomas-Stieber method. So, when we do the normalization, then, that is when I got the point, as I showed in the previous slide of gamma ray sand as 35 and gamma ray shale as 115. When I use those end points for this plot here, it is hydration method I started with gamma ray sand 35 and gamma ray shale 115. But, when I started using those end points, the porosities that were computed using these end points did not match the core porosity. That's why I had to go back and change my end points. So, these points, end point for the sand and shale, have been obtained after numerous hydrations. So, this process has gone through more than 50 hydrations to see which end point selection gives me the best match with the core porosity. So, I keep changing the end points, then rate the porosity of the sand fraction, and then all laid on the core porosity, and then see how the match is. If the match is not good, then I go and change this. It is also crucial that how the net-to-gross changes when you change the end points. So, in Thomas-Diever approach, I have seen that where whenever, which is very logical, that when I would change the porosity, the net-to-gross will change. If you try to decrease your porosity, you look at high net-to-gross because then, you are actually putting more dirty stuff in there. So, as you increase your porosity, your net-to-gross will go down. As you decrease your porosity, the net-to-gross will go up. So, they are both inversely proportionate to each other. But, this is the main driver of the results. The end point selection. It is crucial that at this stage, we spend as much time as possible, because I have seen, also works in other fields of using containment method and Thomas-Diever approach. Well, the calibration is very crucial because if this is highly interpretative. It depends on the interpreter to come up with the end points. The end points drive everything. It can change your porosity, your net-to-gross, and of course, then it changes the saturations. That's why people keep saying that this is highly interpreter-dependent, and we can get different results, which is true, but, whatever background for this is what we need to do is we need to take this very good piece of interpretation, which is provided by Thomas-Striever, this very good piece. We should use this with other data that we have. For example, as we get Edward's, we can run Edward's log. We have coal information. We have to use those to tie our calibration and increase the confidence in our results. If we do that, then the study doesn't become an interpreter-dependent study. Then, two interpreters will come articles consistent results using the Thomas-Striever method. That's why I want to stress more on this slide here the end point selection is very crucial. And please, who all would like to view this method in their fields, please spend a lot of time in end point selection. Please, do a lot of hydrations because they give very different results with the different end points. Do a lot of hydrations, test it with your core measurements, test it with any other logs that you have, which can help you to narrow down your observations and decrease the uncertainty in your interpretation. After going through numerous hydrations, I concluded that the GR sand point for my study is 45, for shale is 115, the porosity of the sand fraction, I had to put into the model to run, and this had been obtained from core data. Again, when we look at core data, for example when we look at core porosities, we have to be very careful how we QC them. The QC of the core data is very crucial. In this study, some of the core porosity were even more than 36%. We all know from theory, that we cannot have any porosity more than 36%. The bed sorting of completely spherical balls will not give you porosity of more than 36%. What happens, during the measurements, we have to be careful about how the core plug has undergone heating or dryig. So, sometimes, they do conventional drying methods, and if you have sensitive glades, it is easy to have some kind of errors introduced in the core measurements. The way the plugs have been treated. So, when you see such high porosities, what we need to do is take them out of core evaluation. This is what I have done in this approach, and looking at the best possible QC values, I have used porosity of the sand fraction as 34%. Porosity of the shale has been used here as 20%, and the resistivity of the shale is 1 ohm.