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  2. Bhagyam Reservoir, India Case Study

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- Okay, so let's see what happens in a reservoir where conventional reservoir under conventional production five stack reservoirs and there's oil-water contact. This particular reservoir's at 300 meters depth, so we could dig it up with shovels. Yes, it's gotta be undergoing biodegradation. This is a project with Cairn Energy. So, here we're gonna measure, as a function of depth, true vertical depth, the coloration of the fluid. We do that with a wireline formation tester. So downhole fluid analysis, and you can see that there's a substantial break in the gradient curve. So, here we're watching the color of the oil, what you would see with your eyeballs if you could look at samples of the oils. The color's increasing dramatically as you approach the oil-water contact. But, away from the oil-water contact, up-structure, there's a mild gradient. So, what's going on? If you look at the GCs, what you can see in the GCs, in this part of the column is that the n-alkanes are very, very big. So, the biodegradation levels are very modest at this point. You can see all these n-alkanes quite easily, but if you look at the GCs of the oils as you approach the oil-water contact, this undifferentiated mass of components is starting to get big relative to the alkanes. And then when you get really close to the oil-water contact, the only n-alkanes present are in the OBM contamination; there's no n-alkane in the oil itself. Why are these samples contaminated and these samples are not? Because these are low-viscosity and these are high-viscosity. The asphaltene content is varying here over this range, a factor of eight, and the viscosity range is much bigger than that. Okay, so what we can see is we have a lot of biodegradation of the oils near the oil-water contact. And it's diminished as we move away from the oil-water contact and at some point here, there's really no indication of biodegradation. We have a depletion of the n-alkanes near the oil-water contact and the alkanes have to diffuse to the oil-water contact to get consumed, so there's the microbes eating the n-alkanes. You see the big mouth, eating the n-alkanes. And that depletes the n-alkanes here, but up here diffusion hasn't had time to deplete the n-alkanes up here. So, this is kinda the profile that we would get for n-alkanes in this, matching the GCs that we see, so all these would have a full complement of GCs up here, full complement of n-alkanes observable in the gas chromatographs. But down here, we're seeing a diminishing content of n-alkanes as we approach the oil-water contact. There are papers, predominately for example, from Steve Water and IM Head showing that, for severe biodegradation, the oil volume can be reduced by a factor of three times, or two-thirds of the volume of oil can be consumed by the bugs. The last third cannot. How does this data- how can we analyze this data that we have? Now, I'm showing data from the five stack sands basically overlaying. So, towards the crest of the field, the asphaltene gradient is rather mild, not a very big gradient. And it's matching the equilibrium model from the Flory-Huggins equations state using a two nanometer asphaltene particle size as expected for black oils. Okay, we have a lot of papers on that; I'm not gonna go into that issue right now. But, as you get towards the base, the equilibrium model starts to diverge from the data more and more and more and by the time you get to the oil-water contact, the divergence of the equilibrium model from the actual data is three times. Well, that's the number, that's the factor of enrichment of asphaltenes for severe biodegradation. And the oils here have no alkanes; it's rather severe biodegradation. So, the concept then is that we need to incorporate a diffusive gradient of alkanes going to the oil-water contact and it's diffusion with a cliff, you could say. Once the alkanes get to the oil-water contact, the bugs eat them and there's no back flux. So we add a diffusive term and that is time-dependent to the equilibrium model. And equilibrium models have no time dependence. So now, with diffusion, we have time involved. The next question is, how long has this been going on? Well, you know the lengths. You have a rough idea of what diffusion constants of alkanes in oils should look like so the number we get for the model is about 50 million years. So there's a lot of play in that because the diffusion constant of alkanes or some oil components through the porous media is not known that well, but we have a pretty good idea. So, anyway, 50 million years and how does that match from a concept of the oil itself? The petroleum system context of the reservoir, I should say. So, if you look at a simple two-dimensional petroleum system model. So this is the time axis, how many years ago. And this is depth. So you can see a subsidence then oil generation; the source rock heated up at this point and generated oil. That's all this green stuff. And then further subsidence, more oil generation and then once you get uplift, that terminates oil generation by-and-large. Subsidence again. So, anyway, the point being, we started charging the reservoir in the Lower Eocene, more or less, right here, and that's 50 million years or so. So, in other words, the diffusive model does have play in the parameter, but it's roughly matching the petroleum system model, saying that the reservoir started biodegrading as soon as it was charged in the Lower Eocene. And then again, charge stopped in the Upper Oligocene. Okay, that model makes sense, so what that allows us to do, is to predict the asphaltene gradients throughout each of the five stacked sands near and away from well bore throughout the entire reservoir using a very simple model. And that's really the objective. So, with the asphaltene content then, you can use a simply parameterized model to give you the viscosity, which have in our paper. We have papers in all these case studies. So, now we're going to look at a situation that's a little different. Those samples I didn't get from India, so we had to rely on the standard PBT analysis. Here, we're going to get the samples, so I wanted to mention a little bit more complexity. So, we're gonna look at situations where we have severe biodegradation. Peters-Moldowan six. And, in that case, the microbes convert hopanes to the corresponding 25-norHopanes. So, these sound like big words, but it's not that big a deal, really. So, there are a lot of carbons in this compound. This carbon, right here, is the 25th carbon in the standard labeling. And, if the bugs start eating the stuff, for them, this is like shoe leather. They don't like to eat this; they're gonna eat the n-alkanes first, the isoalkanes, the isoprenoids, they're gonna eat everything else they can get their hands on or a lot of the other stuff they can eat before they're really gonna do much consumption of this and when they do work on this, they just remove this as one of their degradation products. They just remove that one methyl group, eat the one methyl group, and these are called 25-norHopanes, in this case, this particular hopane, because the 25-nor means that that's missing that methyl group. So, we can then look at the relationship between, in this case, Ts versus the 25-nor-Ts, the standard hopanes versus their corresponding 25-nor-hopane to see if we have Peters-Moldowan six or more in the biodegradation extent. Okay, and here's Tm, another closely-related hopane. And, I will mention, in addition, these two because the ratio of Ts to Tm tells you something about the thermal maturity of the oil if the oil itself is generated at low temperature, then you have more of this metastable form. That's why I like to think of the m as metastable form of this particular hopane. But if it's at high temperature, then you give the compound enough energy to go from the metastable to the stable configuration and you get more TS. And the s, again, the way I think of it, stands for stable. So we can use that, I think we will show examples of that where we can check the oil thermal maturity using the standard technique of Ts and Tm evaluation.

- Okay, so let's see what happens in a reservoir where conventional reservoir under conventional production five stack reservoirs and there's oil-water contact. This particular reservoir's at 300 meters depth, so we could dig it up with shovels. Yes, it's gotta be undergoing biodegradation. This is a project with Cairn Energy. So, here we're gonna measure, as a function of depth, true vertical depth, the coloration of the fluid. We do that with a wireline formation tester. So downhole fluid analysis, and you can see that there's a substantial break in the gradient curve. So, here we're watching the color of the oil, what you would see with your eyeballs if you could look at samples of the oils. The color's increasing dramatically as you approach the oil-water contact. But, away from the oil-water contact, up-structure, there's a mild gradient. So, what's going on? If you look at the GCs, what you can see in the GCs, in this part of the column is that the n-alkanes are very, very big. So, the biodegradation levels are very modest at this point. You can see all these n-alkanes quite easily, but if you look at the GCs of the oils as you approach the oil-water contact, this undifferentiated mass of components is starting to get big relative to the alkanes. And then when you get really close to the oil-water contact, the only n-alkanes present are in the OBM contamination; there's no n-alkane in the oil itself. Why are these samples contaminated and these samples are not? Because these are low-viscosity and these are high-viscosity. The asphaltene content is varying here over this range, a factor of eight, and the viscosity range is much bigger than that. Okay, so what we can see is we have a lot of biodegradation of the oils near the oil-water contact. And it's diminished as we move away from the oil-water contact and at some point here, there's really no indication of biodegradation. We have a depletion of the n-alkanes near the oil-water contact and the alkanes have to diffuse to the oil-water contact to get consumed, so there's the microbes eating the n-alkanes. You see the big mouth, eating the n-alkanes. And that depletes the n-alkanes here, but up here diffusion hasn't had time to deplete the n-alkanes up here. So, this is kinda the profile that we would get for n-alkanes in this, matching the GCs that we see, so all these would have a full complement of GCs up here, full complement of n-alkanes observable in the gas chromatographs. But down here, we're seeing a diminishing content of n-alkanes as we approach the oil-water contact. There are papers, predominately for example, from Steve Water and IM Head showing that, for severe biodegradation, the oil volume can be reduced by a factor of three times, or two-thirds of the volume of oil can be consumed by the bugs. The last third cannot. How does this data- how can we analyze this data that we have? Now, I'm showing data from the five stack sands basically overlaying. So, towards the crest of the field, the asphaltene gradient is rather mild, not a very big gradient. And it's matching the equilibrium model from the Flory-Huggins equations state using a two nanometer asphaltene particle size as expected for black oils. Okay, we have a lot of papers on that; I'm not gonna go into that issue right now. But, as you get towards the base, the equilibrium model starts to diverge from the data more and more and more and by the time you get to the oil-water contact, the divergence of the equilibrium model from the actual data is three times. Well, that's the number, that's the factor of enrichment of asphaltenes for severe biodegradation. And the oils here have no alkanes; it's rather severe biodegradation. So, the concept then is that we need to incorporate a diffusive gradient of alkanes going to the oil-water contact and it's diffusion with a cliff, you could say. Once the alkanes get to the oil-water contact, the bugs eat them and there's no back flux. So we add a diffusive term and that is time-dependent to the equilibrium model. And equilibrium models have no time dependence. So now, with diffusion, we have time involved. The next question is, how long has this been going on? Well, you know the lengths. You have a rough idea of what diffusion constants of alkanes in oils should look like so the number we get for the model is about 50 million years. So there's a lot of play in that because the diffusion constant of alkanes or some oil components through the porous media is not known that well, but we have a pretty good idea. So, anyway, 50 million years and how does that match from a concept of the oil itself? The petroleum system context of the reservoir, I should say. So, if you look at a simple two-dimensional petroleum system model. So this is the time axis, how many years ago. And this is depth. So you can see a subsidence then oil generation; the source rock heated up at this point and generated oil. That's all this green stuff. And then further subsidence, more oil generation and then once you get uplift, that terminates oil generation by-and-large. Subsidence again. So, anyway, the point being, we started charging the reservoir in the Lower Eocene, more or less, right here, and that's 50 million years or so. So, in other words, the diffusive model does have play in the parameter, but it's roughly matching the petroleum system model, saying that the reservoir started biodegrading as soon as it was charged in the Lower Eocene. And then again, charge stopped in the Upper Oligocene. Okay, that model makes sense, so what that allows us to do, is to predict the asphaltene gradients throughout each of the five stacked sands near and away from well bore throughout the entire reservoir using a very simple model. And that's really the objective. So, with the asphaltene content then, you can use a simply parameterized model to give you the viscosity, which have in our paper. We have papers in all these case studies. So, now we're going to look at a situation that's a little different. Those samples I didn't get from India, so we had to rely on the standard PBT analysis. Here, we're going to get the samples, so I wanted to mention a little bit more complexity. So, we're gonna look at situations where we have severe biodegradation. Peters-Moldowan six. And, in that case, the microbes convert hopanes to the corresponding 25-norHopanes. So, these sound like big words, but it's not that big a deal, really. So, there are a lot of carbons in this compound. This carbon, right here, is the 25th carbon in the standard labeling. And, if the bugs start eating the stuff, for them, this is like shoe leather. They don't like to eat this; they're gonna eat the n-alkanes first, the isoalkanes, the isoprenoids, they're gonna eat everything else they can get their hands on or a lot of the other stuff they can eat before they're really gonna do much consumption of this and when they do work on this, they just remove this as one of their degradation products. They just remove that one methyl group, eat the one methyl group, and these are called 25-norHopanes, in this case, this particular hopane, because the 25-nor means that that's missing that methyl group. So, we can then look at the relationship between, in this case, Ts versus the 25-nor-Ts, the standard hopanes versus their corresponding 25-nor-hopane to see if we have Peters-Moldowan six or more in the biodegradation extent. Okay, and here's Tm, another closely-related hopane. And, I will mention, in addition, these two because the ratio of Ts to Tm tells you something about the thermal maturity of the oil if the oil itself is generated at low temperature, then you have more of this metastable form. That's why I like to think of the m as metastable form of this particular hopane. But if it's at high temperature, then you give the compound enough energy to go from the metastable to the stable configuration and you get more TS. And the s, again, the way I think of it, stands for stable. So we can use that, I think we will show examples of that where we can check the oil thermal maturity using the standard technique of Ts and Tm evaluation.