Monday I needed to drain a produced water gathering system for maintenance.  This system is supplied by four gas wells with pump jacks on them.  The pump jacks discharge directly into the gas gathering system (so they can overcome any system pressure encountered).  Three wells flow together in a 2-inch Performance Pipe (spoolable composite) system that terminates at the fourth well and changes to 4-inch Performance Pipe and flows 1 mile into atmospheric tanks and has an elevation change of +200 ft.  

The four wells together flow 600 bbl/day of produced water (17.5 gpm).  This very small volume flow rate in a 4-inch very smooth pipe should develop less than 1 psi of pressure drop due to friction.  The elevation change should cause the head of the 4-inch to be around 85 psig.  Actual pressure at the head of the 4-inch was 420 psig.  The volume of the pipeline is just under 100 bbl.

At 9:00 am we started blowing the system down into an empty atmospheric tank through a 2-inch line.  This line bucked and rumbled for four hours with steadily decreasing pressure and at 13:00 we had recovered 75 bbl and the pressure was under 20 psig (minimum on the gauge we were using) and flow became very intermittent.  At that point we decided to break a flange on the 4-inch and see if we could flow the rest of the water into a prepared catchment.  When we got the top half of the bolts removed we had an ear-splitting noise of gas escaping and water shooting 50 ft in the air.  This went on intermittently for two more hours.

I calculated blowdown time for the line being gas full at 420 psig and got 46 minutes.  Drain time for liquid full was about an hour.  Anybody have a theory why the gas/water mixture blew hard for 6 hours?

The purpose of draining the system was to install a gas knockout. We installed it Tuesday morning and put the wells back on (with the gas knockout bypassed waiting for electrical connection).  By this afternoon the pressure in the system was back up to 420-430 psig.  Late this afternoon we put the gas knockout in service and it is shifting considerable gas to gas sales instead of to the 4-inch.  After 15 minutes in service the line pressure dropped to 350 psig.  Can't wait to see what it is in the morning.

First, why did you open it up without knowing whay there was 420 psi there when you thought it should only have been 80?

Sounds like you were moving a sand plug and then the sand broke loose???  In South Texas, we'd get those lines showing 0 psi, until the sand broke loose somewhere.  Super dangerous to open them up.  Before I got there an operator was killed when he opened up a sand trap valve and got no flow, until he decided to look inside the opened valve.

The line was built by a very small independent operator.  They don't have as-builts or profiles, hell they don't even have issued for construction drawings.  Picture a guy in a cowboy hat standing on a knoll pointing out a pipeline route to the track hoe.

I got the elevation difference between the blowdown point and the last block valve with a handheld gps.  From the blowdown point the elevation is increasing reasonably steadlily (probably 4-5 undulations that are at least 4 pipe diameters) for about 120 ft over 1/4 mile.  Then it drops 40 ft and goes under a river and immediately goes up anouther 100 ft very rapidly (over about 50 yards) and then undulates with the terrain for almost 3/4 mile (probably 10 dips that are greater than 4 times the pipe diameter) generally increasing another 40 ft over that distance.

Lots of opportunities for gas pockets, coal dams, and slime.

This line was never intended to have gas in it, but there was a lot of wishful thinking on the part of the field guys that "designed" it.  The first gas knockout has lowered the pressure by 70 psig (420 down to 350 psig) and is extracting about 25 MCF/d.  Trouble is that there are 20 other wells pumping into the system after the gas knockout (and if the gas doesn't go through the knockout it really doesn't matter how well it works).  I don't think we'll see hydrostatic pressure until we get more gas out.

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