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View Diary: The 4 biggest oil fields in the world are in decline (197 comments)

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  •  yep 30 to 50% (none)
    Look at the BP presentation linked to in my diary above, it has fascinating graphs about the final recovery rate, depending on the complexity of the reservoir.

    In the long run, we're all dead (Keynes)
    Read more on the European Tribune - bringing dKos to Europe

    by Jerome a Paris on Thu Jan 26, 2006 at 11:49:19 AM PST

    [ Parent ]

      •  Oil reservoirs are not underground tanks (4.00)
        30-50% is typical for a carbonate reservoir like Ghawar.

        Think of the oil as filling the holes in a porous rock formation, like a sponge. The top of the reservoir is non-porous rock cap that won't let the oil through, and the bottom is an aquifer (water) that pushes up on the oil.

        As long as the reservoir pressure is high, the oil flows to the surface through wells without pumping. Hydrocarbon gasses (natural gas, butane, propane etc) are dissolved in the oil like the bubbles in soda. As the pressure in the reservoir drops, the drive pushing the oil to the surface decreases, and the gasses come out of solution and accumulate at the top of the reservoir (called a "gas cap").

        Once a gas cap forms, now the wells get more complicated, cause they'll start sucking gas into the bores instead of thicker, heavier oil ("gas coning"). These wells are generally shut and production moves to an expanding ring of wells where the reservoir column is more oil and less gas.

        Saudi Aramco adopted a technique of injecting water into the aquifer at the bottom to replace the oil that's pumped out. This maintains reservoir pressure, keeps the gas in solution and the wells at the top of the formation productive.

        After a while, the water layer reaches the wellbores and water is pushed to the surface with the oil ("water coning"). As you can imagine, as the water level goes up, more and more water comes out with the oil. The amount of water that comes out with the oil is called "water cut".

        The water is separated from the oil and reinjected into the aquifer. The water becomes a kind of conveyor belt, flushing oil out of the rock formation and into the wellbores and then getting pumped back into the ground. The energy required to do this is problematical and once the wells get a significant water cut, those wells are shut and new wells are drilled.

        Ghawar is also a very complicated reservoir. The permeability (how well fluid moves through the rock) is highly variable. Ghawar is shot through with "super-k zones", layers of particularly spongy rock that at first flowed vast quantities of oil into the wellbores and now are transporting the reinjected water very fast across the fields trapping productive layers of oil below injected water.

        Ghawar is also old. There is a significant gas cap and water coning problems all over the northern, productive part of the field (the southern part has very low permeability and we'll probably never get that oil out). Really exotic well technogies are being used, like smart completions and maximum reservoir contact wells. These wells are high-tech super straws that suck out the remaining mobile oil in pockets as fast as possible.

        When Ghawar drops off in production, it will be rapid and steep.

        •  what about re-completing old wells ? (none)
          and altering the physical layout of resovoirs with explosives ?

          after some time has passed in older wells, wont the pressure tend to come up again ?

          R we talking costs of extraction or the very possibilty of extraction ?

          Out of my cold dead hands

          by bluelaser2 on Thu Jan 26, 2006 at 02:40:32 PM PST

          [ Parent ]

          •  Permeability problems (4.00)
            I've heard of high pressure acid treatment of wellbores in gas fields to intentionally fracture the rock to artificially increase permeability, but I don't know if that's done for oil as well.

            The fracturing in oil reservoirs tends to be considered a problem, because it makes the flows through the reservoir less predictable -- the fractures can move water around, too.

            The low-permeability problems of the south end of Ghawar are both economic and physical. Wells drilled there have low rates of production and short lifetimes, because the oil isn't mobile. That means that the cost of the well is hard to recover, and the rate of production is throttled by the ability to drill more wells.

            What will happen in Ghawar is that eventually the incremental profit of one more barrel of oil will be less than the value of the gas in the gas cap. When this happens, the field will be "blown down" -- oil will be extracted and not replaced with water and the pressure will be allowed to drop to the bubble point, and the gasses will come out of solution and gas wells will be drilled.

            There will still be a huge amount of oil in Ghawar then, and secondary/tertiary recovery will start. That means pumps -- very low production rates and high costs. The pumpjacks that you see on oil wells in the US are on wells that recover a few tens or a hundred of barrels a day, not the 50,000 barrels a day of a primary recovery well in Ghawar today.

        •  thanks for that post (none)
          I have been able to learn most of the details of Saudi Arabia's oil field degredation, but I've never seen so much of an explanation within so few words

          it ain't exactly a nut shell, but that's the best "short" answer to why Saudi Arabia ain't gonna save the world, or george bush

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