Drilling Oil Wells

[Ron]

I have come across the following paragraph on drilling of oil wells in a book:

 

Natural gas is found above petroleum, trapped between the rock cap and the petroleum layer. With the help of giant rigs, a hole is drilled in the earth's crust. When the hole reaches the rock cap, the natural gas comes out first with a great pressure. After that, petroleum starts flowing out due to the pressure of natural gas. When the pressure falls further, the petroleum is pumped out by electric motors.

 

What does the line petroleum starts flowing out due to the pressure of natural gas mean here? After the natural gas comes out with great pressure, petroleum beneath the natural gas flows out but what does due to the pressure of natural gas mean here? Natural gas has already come out (after which petroleum starts flowing out); so how does pressure of natural gas come into the picture after the natural gas has already been released?

 

Thanks,

 

Ron

[insane_alien]

not all the natural gas would have come out.

 

and besides, just because they've penetrated the rock cap doesn't mean they've stopped drilling. they'll keep going until the hit the oil and they'll let the pressure of the natural gas above the oil push it up the pipe until it becomes too low and then they'll switch to pumps.

[Ron]

Thanks mate for your prompt reply but sorry to say I couldn't follow it. How will the pressure of the natural gas above the oil push the oil up even if all the natural gas would not have come out? Won't the whole of the natural gas come out first after which only the petroleum will flow out?

 

Thanks,

 

Ron

[insane_alien]

nope. you seem to underestimate how much gas is down there. the answer is, lots. if you just opened up the pipe to the gas itself, it'd take decades for it to all come out.

 

what they do once they breach the top of the field is to continue drilling down deeper so they can put the end of the pipe below the surface of the oil in the field. this means the gas presses down on the oild and pushes it up the pipe.

 

the way to think of it is like using a drinking straw. in this case, the pressure caused by the gas is provided by the atmosphere and the oil is your orange juice(or whatever your favourite drink is).

 

you put the end of the straw well below the surface of the liquid and then suck it up. only in the case of oil wells no sucking is necessary to start as the reduced pressure comes from the atmosphere instead of your lungs in the analogy i gave.

 

i hope this made a bit more sense.

Edited September 18, 2010 by insane_alien

[Ron]

OK....so is it like this? When natural gas gets released, the pressure decreases. At this point, the atmospheric pressure which is greater than the pressure above the layer of petroleum forces petroleum to flow out. Am I correct?

 

Thanks,

 

Ron

[insane_alien]

no, you've got it backwards.

 

they don't wait for the natural gas to be released, they use the natural gas to push the oil out because the natural gas is at a much higher pressure than the atmosphere.

[Ron]

You mean to say that the natural gas is at such a high pressure that it forces petroleum to flow out? If that is the case, does that mean that the natural gas & petroleum come out simultaneously?

 

You have also said that they don't wait for the natural gas to be released but in the book I am reading, it is stated that the natural gas comes out first with a great pressure.

 

Thanks,

 

Ron

[insane_alien]

depends on if you are drilling for gas or drilling for oil, or both.

 

what the drillers want will determine the order they take off material in.

 

now, in the example i'm giving the oil is extracted first, the gas does not simultaneously come up with the oil(appart from some dissolved gasses so the oil is a bit like a fizzy drink as its coming out of the well) but there isn't an appreciable gas fraction usually.

[Mr Skeptic]

The key is that most of the gas does not escape, so that the well still has lots of pressure by the time they get the pipe down to where the oil is. Then the pressure differential (more pressure in the well, less in the atmosphere) pushes the oil up the pipe. The drinking straw was a good example, the difference is that with the drinking straw you reduce the pressure above rather than increase the pressure below.

 

You could play around with a straw and a balloon filled partly with water and partly with air to get an idea of how it works.

[Ophiolite]

Get a new text book. If it says the natural gas comes out first then it is badly written.

 

The well is drilled into the reservoir and (unless it is a horizontal well) a short distance beyond the reservoir. It is then cased off i.e. pipe is run into the well bore and cemented in place. Logging information has identified the water, oil and gas zones within the reservoir. Holes are perforated in the casing opposite the oil zone. Tubing is run within the casing to conduct the oil to surface.

 

In some cases, rather than perforating the casing, a slotted liner will be run, through which the oil will flow. In other instances, where the reservoir rock is robust, an open hole completion will be carried out.

 

All oil produced will contain some amount of gas in solution and some water. At surface there is equipment to separate these components from the oil. Often the gas is flared (burned) off at the location.

 

As to new textbooks, get yourself a copy of A Primer of Oil Well Drilling. (If that's the book you do have I'll go out and hang myself.)