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  5. Density Stacking Versus Lateral Sweep in Trap Filling

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- [Dr. Mullins] Here, this is a case study with Hess. That last one was with London and Norway. This is a case study with Hess. We have a reservoir with several wells, and I'm depicting Well A and Well B basically at the same height, and we have a secondary charge coming up the flank of the interface of the salt and the formation, and that's a typical charge plane for reservoirs in the Gulf of Mexico. And that flank appears to be charging with gas or condensate. and here I'm plotting the GOR measured, now all in laboratory. For Well A it's 5,000 scuffs per barrel, And well B at the same height is half that. So, here, I have totally different densities of fluids at the same height. There would be an immediate potential explanation that I have a fault, or ceiling barrier between Well A and Well B, but is that the case? Here is the data from Downhole on the asphaltenes. There is wells from most of the reservoir, lining up with the FHZ, and here is the flank. So what we can see is well A in the flank that we just looked at on the last view graph has very low asphaltene content. In addition, it has very high GOR. So is it a separate compartment? Well, you have to worry about what the RFG processes are, you can't just assert that because you have different fluids, at different densities, at the same height in these two wells, that therefore, it's not connected. So let's take a look at well B, a little more expanded view, and here is the DFA data acquired, and here is the GOR. The heavy line is obliterating the actual data But there are data points associated with each one of these DFA stations and what you can see is that the DFA GOR is increasing in the high perm sands, which are shown here. The GOR is surging across the field, so we have a very high GOR here, and we have the forerunner of that high GOR coming across all the way to Well B. In addition, the mud gas isotope is showing that we have this charge coming into Well B as well. The only way to have that charge get across that field is no barrier. So the prediction was made by the operator that the Well A and B are connected in spite of this very different GOR. What happened in production, it was amazing. Here is a cartoon of the crest of the field of basically, again, salt. So the rest of the field is sloping out of the page towards you, sloping downwards. Here is the depiction of a Pleistocene condensate charge coming into this black oil reservoir, where the bulk of the charge made it to Well A and just the forerunner of that charge made it to Well B. After production, for two months, the condensate bubble that was here, surrounding Well A, is gone, and now Well A is producing the same black oil that's being produced elsewhere in the field. In addition, there is pressure interference from Well A and Well B. They are definitely connected in production. The fluids are not density stacked. This is a very novel observation, because most of the standard literature would say the fluids must density stack. They don't. So, there it is, we have lateral sweep and trap filling in a macroscopic picture here, we had it in a microscopic picture with the lindane example, showing how the asphaltenes deposit in core separately for a lateral sweep, then a vertical sweep. So this is a very novel understanding of reservoirs, but now that we know to look for a lateral sweep near the charge point, we see this all the time. Mother Nature does a lot of repetition when she finds these different RFG scenarios. And now we're gonna look at what else can happen when you have condensate charge in the asphaltene. The asphaltenes can become unstable, so you have to worry about asphaltene onset pressure