Oil/Gas/oil well design and operation
I have an interest in two wells in the northern edge of the Eagle Ford (Zavala County). These wells have ~5000' horiz and 18 stage frack. Both have low gas/oil ratio but are still on natural lift. After about a year of operation, the operator is shutting the wells down for a few days/week then opening them up for a few days/week. Sort of like burping. The gas(10-40 mcf/day) is being flared (no pipeline available). Each well is averaging about 15 mcf/day and 85 bbl/day but varies from zero to 350 bbl/day
Is this "burping" normal? Are they trying to get what they can before putting the wells on pump?
Also, I believe the wells were completed with gas lift valves/mandrels(?). There is no compressor that I can see on site, is there a way to use the gas lift valves without a compressor?
Is there any significance to casing pressures in the range of 40psi or 90 psi? FTP ranges from 160 to 360 psi over a months time.
Any help you can give to help me understand what is going on with these wells is greatly appreciated.
Are they trying to get what they can before putting the wells on pump?
Jack, that is exactly what they are trying to do.
As you apparently realize, it is the gas pressure that pushes the oil to the surface, high gas/oil ratio = gas well, low gas/oil ratio = oil well but it must have something that pushes the product to the surface.
While the gas is pushing the oil to the surface the gas, being lighter than the oil, will also move up through the oil, flowing to the surface faster than the oil. As this continues the gas/oil ratio changes and the well slowly starts making more gas and if this is allowed to continue the tubing will fill with gas. The gas at the wellhead will try to compress as the gas at the formation tries to flow and an equilibrium will be reached where the well will "kill" itself.
Operators will shut the well in to let it stabilize and the wellhead pressure to change. When they re-open it, I think you will notice a large flare that then diminishes. This is the gas that is trapped right at the wellhead. The operator is flaring the gas because it is now so cheap that it is un-economical to lay a gas line. This is the condition that existed in the '50's and early '60's. As this gas/oil ratio changes with flow they cannot let too much gas escape because that would be losing the medium that drives the oil to the surface. There is an entire engineering desipline studying the effects of formation pressure drawdown. There is a field near Ranger, Texas that still has several hundred thousand barrels of oil that is un-recoverable because the operators in the 1920s were concerned with maximum production and all of the gas was flared off before the oil was recovered.
A well can be pumped that has a small amount of gas but not too much. How much depends upon the field and it sounds like they intend to pump it but as long as the natural production exceeds pumping it would be defeatist to do it.
Gas lift valves don't need a compressor at the surface. They are intended to prevent the opposite of a well loading up with gas. A high oil/gas ratio wells tubing can fill with so much oil that the hydrostatic pressure of the oil is greater than the formation pressure and the well again will 'kill" itself. The tubing is installed with one or more Gas Lift Mandrels near the bottom of the vertical section. this mandrel has a tiny hole in it and the Gas Lift Valve is installed in the Pocket of the mandrel. Some formation gas is allowed into the casing and this is introduced into the tubing to assist in pushing the oil to the surface.
Most of the gas lift equipment I have put in wells were those that were expected to make water as well as oil. Water being heavier than both will quickly load the tubing and stop the flow. The casing pressure will change as the gas lift valves do their thing.
Here is a link to gas lift operation http://en.wikipedia.org/wiki/Gas_lift