[Gorm]

Just a noobish thought of mine, but what is replacing the oil when it is pumped up from the ground or seabottom?

Under the ocean, I guess water puors in and fill the void, but in deserts and so? Do the ground itself become lower in places like Saudi Arabia? What do happen?

[Tanada]

Just about every layer of rock has mater in its structure. The water comes up with the oil, that is what they mean when the experts talk about 'water cut', how much of the liquid coming out is oil and how much is water. Even in the desert deep rocks are often filled with water in the same pore spaces that hold the oil. In some of the fields in the North Sea under water the pressure from the weight of the sea actually compressed the rock a little bit as the oil was being pumped out, and the deeper in the ground or under the water a reservoir is the more likely this is to happen.

[ROCKMAN]

T has it right on. Here’s a little more detail. Let’s use a sandstone reservoir for brevity. Imagine holding a handful of beach sand. While it may feel solid about 30% of the volume in your hand is air: the sands grains have spaces between them: pores. A lot of pores (hence good “porosity”) can be 30%+. Few pores low porosity or “tite” can be 10%.

No reservoir contains 100% oil in its pores. A typical oil reservoir may have 40% of the pores space filled with oil and the balance with water. And yet will produce 100% oil (zero water cut). A long explanation I’ll skip. Two basic types of reservoir drives. Water drive: think of oil floating on the top of vinegar: the oil floats above the water: suck the oil out and the water level rises. Such reservoirs tend to have fairly constant pressure. As the % of oil in the reservoir decreases as it's produced the water begins to be produced also. Pull enough water out and the well becomes non-commercial when it “waters out”.

And pressure depletion drive: think of a shakened up warm soda bottle. Punch a hole in the bottom and the fluid shoots out hard. But as the pressure is relieved the flow slows down. Lose enough pressure and the flow stops completely. Then you have to suck the rest of the fluid out. Thus those big pump jacks you see going up and down pulling out the oil. And thus the problem with most fractured shale reservoirs: the pore volume containing producible oil can be a very small amount of the rock: just a few %. But the fractures are extremely permeable and produce that oil very fast. Making it worse is that most are pressure depletion drive and lose pressure very quickly: hence the high decline rates.

Gets more complicated when you look at limestone reservoirs but a lot of similarities too.

[Newfie]

Not oil, water.

http://water.usgs.gov/edu/earthgwlandsubside.html

landsubsidence.jpg

[Keith_McClary]

Water drive: think of oil floating on the top of vinegar: the oil floats above the water: suck the oil out and the water level rises. Such reservoirs tend to have fairly constant pressure. As the % of oil in the reservoir decreases as it's produced the water begins to be produced also. Pull enough water out and the well becomes non-commercial when it “waters out”.

And pressure depletion drive: think of a shakened up warm soda bottle. Punch a hole in the bottom and the fluid shoots out hard. But as the pressure is relieved the flow slows down. Lose enough pressure and the flow stops completely.

I hate to quibble with experts, but sometimes they leave out bits that they take for granted.

I think the water level rise is due to (slight) expansion of the much larger volume of water below the oil as the pressure is relieved:

Wikipedia: in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.

[Gorm]

Thanks for info, now I can focus on more relevant thougths

[dissident]

And pressure depletion drive: think of a shakened up warm soda bottle. Punch a hole in the bottom and the fluid shoots out hard. But as the pressure is relieved the flow slows down. Lose enough pressure and the flow stops completely. Then you have to suck the rest of the fluid out. Thus those big pump jacks you see going up and down pulling out the oil. And thus the problem with most fractured shale reservoirs: the pore volume containing producible oil can be a very small amount of the rock: just a few %. But the fractures are extremely permeable and produce that oil very fast. Making it worse is that most are pressure depletion drive and lose pressure very quickly: hence the high decline rates.

To think that there is a discussion about the production curves from conventional and fracked reservoirs. It is physically impossible for them to have the same production profile (ceteris paribus). The story of fracked oil is the story of the thin tail in the distribution.

[efarmer]

In the oil bearing rock, it would be water, in the oil using folks it is probably hope and fear.

[ROCKMAN]

Keith - true and a lot more detail to go into. But despite our much above avg IQ here I try to avoid getting into it too deep. As you say even though the expansion factor of water isn't seen until you get down a small fraction of 1% when billions of millions of gallons of water expanding it does add up quick.

I also avoid send folks to links on the detailsince that woould take days of trying to understand thousands of pages of details. In the end I rather be less then 100% correct and make the general point a easily understood as possible.

[Pops]

In the oil bearing rock, it would be water, in the oil using folks it is probably hope and fear.

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