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  6. Downhole Fluid Analysis and Asphaltene Onset Pressure (AOP)

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- [Instructor] Okay, we have a new way to measure asphaltene onset pressure. So in the past, people would take pressurized bottles and run in and acquire samples, and then take the samples to the lab in the pressurized bottles and measure asphaltene onset. You know why they used pressurized bottles? To prevent flocculation of asphaltenes in the bottles before the samples are measured. The view is, in the industry, that the asphaltene onset might be irreversible in some cases, particularly if the asphaltenes form a mat at the bottom of the sample bottle, they're not going back into solution. So consequently, if you have a asphaltene deposition in the bottle, the sample bottle, prior to measurement, it is not a reliable measurement. Well, so that's why the industry took great expense to deliver pressurized bottles all over the place, all over the world, and gave up sample volume to use the pressurized sample bottles to prevent flocculation. But the industry forgot something. Asphaltenes flocculate at low pressure and at low temperature. What happens to the bottles when they come out of hole? They cool off, and it is very likely that in cases where you have an important high AOP, that you cross the phase envelope when you go to low temperature. So the samples flocculate. So this big solution of delivering pressurized bottles everywhere in the world at the expense of giving... Well, it's more expense plus giving up sample volume. It only solves half the problem, it doesn't solve the temperature problem. So how do you get around this? Well the answer is fairly simple once you see it. This is invented by Adrian Dianoux here in Houston. He said, well, you take the wireline formation testing tool, you fill the sample in there, and as you pull out of hole, you're dropping the pressure, there's pressure, you're dropping the pressure as you pull out of hole. There's temperature, you're dropping the temperature as you pull out of hole, and you monitor whether or not you get scattering. So this is color of the oil, and there's no change in the color. No change in optical density, and the density is absorption of light plus scattering of light, and then when we hit the AOP, there's the scattering. You get the light scattering at AOP that's exactly the same measurement that the laboratories perform to see AOP. It's a very robust measurement for AOP. Then in our case, the sample in the wireline case, the sample is not stirred, so the asphaltenes fall down out of the optical view, and the color drops and drops and drops. Why, because the asphaltenes are falling out of solution. By the time we hit bubble point, which is just over here somewhere, we have 85% of the asphaltenes dropping out of the soil. So this AOP can be as high as 20,000 psi or higher. We have the ability to measure AOP up to reservoir pressure and reservoir temperature with this technique. We can reproduce the measurement because you can go back and get another sample if you think something has happened wrong, and run the whole thing again. You can run it as many times as you like. It's being done something like 70% of what WFT runs in the Gulf of Mexico are using this technique right now. It's a no-brainer. So the concept is this. You measure the AOP by DFA. We know for sure that the sample never had flocculation of asphaltenes prior to the measurement of the AOP. Then you take the sample to the lab, they restore what they can. Now the samples are in pressurized bottles. This is in the flowline, so not pressurized. You can see the pressure drop. So you can inhibit some of this deposition by having pressurized bottles, but you are gonna cool the sample. Take it to the lab and have the lab reproduce this AOP at this temperature. If they get the right AOP at this pressure and temperature, that lab sample is valid, and that sample can be used to characterize the phase envelope of the asphaltenes. However, if they don't get this AOP. If their AOP is at a lesser pressure and temperature, or smaller in one or both parameters, then they probably had some irreversible asphaltene dropout. We have never seen a case where the labs reported a higher AOP than the DFA AOP. So we think we are seeing some drop in the AOP due to irreversible dropout, and you will know the magnitude of that by comparing the down hole AOP versus lab AOP. So it's the most reliable AOP measurement, the DFA AOP, and it also provides a way to calibrate the lab sample.