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Permalink Reply by Les Bamburg on June 13, 2009 at 3:58

Robert, the reservoir pressure and shut-in tubing pressure are primarily a function of depth. The deeper the formation, the higher the pressure - but you will read references to over-pressured formations. The normal pressure gradient is 0.433 psi pressure per foot of depth. So a formation at 10,000' would have an initial reservoir pressure of 4,333 psi. But an over-pressured formation such as the Haynesville Shale may have a gradient of 0.92 psi/ft which means at 10,000' the formation would have an initial pressure of 9,200 psi. The shut-in tubing (or casing) pressure is measured at the wellhead and will be slightly lower due to the weight of natural gas in the well.

The flowing tubing (or casing) pressure at the surface will be lower due to the pressure loss thru the formation, perforations and tubing. This pressure is measured at the surface upstream of the choke and is reported with the flow rate as part of the well potential test. The flowing wellhead pressure of Haynesville Shale wells has varied from +/- 7000 psi down to 2000 psi as typical values.

Initially downstream of the wellhead choke the production facilities and pipeline will have an operating pressure of 1000-1200 psi so pressures have to be safely reduced to this level across the choke. Also, this downstream piping will have a maximum allowable operating pressure or MAOP (ie 1440 psi) and the production facilities will include safety devices to guard against the pressures exceeding this level. There are state and federal regualtions that govern the determination of the MAOP and the required safety devices.

Choking a well is also necessary to guard against high erosional velocities and pressure drops downhole that could damage the formation, perforations or equipment.

As a formation depletes the reservoir and flowing pressures will decrease over time and the flow rate will decline. As this happens the operator will adjust the choke in an attempt to keep the flow rate up. Eventually the flowing wellhead pressure will be at the same level as the production facility operating pressure and the operator will need to install gas compression (or switch to a low pressure gathering line) in order to keep the well flowing. Operators have estimated 2-3 years before Haynesville Shale wells will require compression.

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Permalink Reply by red rider on June 13, 2009 at 17:52

Les, if the operators are estimating 2 to 3 years before shale well will require compression, would it not mean that the rate of flow is steady for 2 to 3 years too?
How is it that a well can deplete as much as 90% within a year, when the pressure can be expected to last 2 to 3 years before compression? is there a formula the operators use?

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Permalink Reply by Les Bamburg on June 14, 2009 at 3:57

Robert, as described above a well's initial flowing pressure can be as high as 7000 psi. This flowing pressure declines over time toward the 1200 psi causing the flow rate to decline also. Check the graphs on Pages 14 & 15 of the following presentation:

http://www.encana.com/investors/presentationsevents/pdfs/20090527-h...

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Permalink Reply by red rider on June 14, 2009 at 17:45

OK i see what your saying about pressure coming down as rate comes down. starting to understand this stuff, as a newbie to oil and gas. Thank you. I just thought that rate of flow would stay constant with pressure controlled.

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Permalink Reply by Lee on June 14, 2009 at 16:56

Thank you for taking the time to explain this-- very helpful.

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Permalink Reply by P.G. on June 13, 2009 at 4:42

Why couldn't they use that pressure to drive portable turbines to produce electricity?

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Permalink Reply by Checkmateking on June 13, 2009 at 5:34

I have asked myself this question a million times.

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Permalink Reply by Max on June 13, 2009 at 10:25

Using high pressure turbines would not be cost effective. When the HP gas enters the primary stage of the turbine through special nozzels, it expands, this expansion causes the turbine to rotate. As the gas expands, the pressure of the gas decreases. The secondary stage of a high pressure turbine continues to increase in size and the gas decreases in pressure as it is directed through the HP side of the turbine. On most turbines the gas is then directed to a 'Low Pressure Turbine' (LP), usually operating at less than 20 psi, but on a much larger scale. This last amount of pressure is exhausted out the end bells of the LP turbine and at that point it is under a vacuum. You would have to recompress the gas (using energy) to pump it into a pipeline system. Any electricty generated plus an extra amount would have to be used to recompress the gas. A small portable HP/LP turbine/generator with an output of 0.25 MW would weigh in around 50 tons and cost over 80 million dollars. You could drive a turbo fan type generator with the high pressure gas, but you would lose net energy with this type of application also. Cost per KWH would be cheaper by using diesel generators.
max

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Permalink Reply by P.G. on June 13, 2009 at 11:09

Perhaps another type of mechanical means. The fact is that high pressure is moving in one direction just like a river moves or compressed stream of air. Not taking advantage of it seems like a waste.

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Permalink Reply by Max on June 14, 2009 at 5:13

Yes, the energy potential stored in underground sources such as NG resorvers is wasted at this time. And yes, a system could be developed to capture this source of energy, the only problem, the recapture cost would take years if possible at all, and most investers would choose a quicker return on their money. A flowing river requires a dam being built to create a reservoir (potential engery storage) to maintain a flow volume/pressure to insure a steady source to drive water turbines. Today's cost to build a small to medium reservoir is around one to two billion dollars, (proposed Marvin Nickles reservoir, NE Texas, 1.2 billion). Even with this amount of dollars spent, reservoirs of this size are not cost effective to install hydro-electric plants. The bottom line is cost. Oil, NG, and coal are still the cheapest form of potential engery that is available to produce electricty and fuel our transportation systems. Untill the day when we can use the sun as a direct source of energy cheaply, all other sources will only be utilized according to cost per unit. I agree with you that it is a waste of energy to have this amount of pressure from a NG well or water flowing down a river and not use it in some way. But, on the other hand, I hope nobody invents a cheap source before I can get a well on my place.
max

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Permalink Reply by The_Baron on June 15, 2009 at 3:25

A flowing river has the constant force of gravity to continue its movement downhill. A natural gas well will drop in pressure over time. Compressor stations for nat. gas are expensive and loud. the fewer needed the better. Also, as pressure in nat gas changes, other problems could arise such as condensate, tempeture changes etc. as pressure decreases, so does the tempeture, problems could arise from the extreme coolinng effect this would have on equipment (ever put your hand on the side of a propane tank as it empties???)

This is an interesting line of thought but attention might be better used to invent a perpetual motion machine.

Although it should be noted that underground storage of gas is a concept of a way to store energy from unreliable sources like wind. When there is a surplus, gas is pumped underground or into tanks, when the energy is in demand the gas is released through turbines to produce electicity.

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Permalink Reply by Max on June 15, 2009 at 15:14

Yes, a perpetural motion machine would be nice to invent, but there is still one little problem, E=mc2. The best way to use NG is to burn it.

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