Rapidly opening the wellhead valves on a high pressure well flowing into a low pressure collection system is a good way to ruin a well when the following two criteria are met:
• The wellhead choke is large.
• The well has been shut-in for a while.
The surge of gas flow resulting from following this procedure may, depending on the producing formation, suck sand out of the formation, through the casing perforations and into the tubing. To determine if sand is indeed covering the perforations, a weighted wire line is lowered through the tubing through a device called a “lubricator”. When the wire line loses tension, the operating personnel at the surface surmise that the weight has “tagged bottoms”. This tagged depth is compared to the well’s completion record to determine if any or all of the casing perforations are submerged in sand. If more than 20% to 30% of the perforations are covered, it is a good idea to wash the well out with a “coil tubing unit”.
The cost to tag bottoms with a wire line unit is only a few thousand dollars. Washing a well clear of sand with a coil tubing unit can cost ten times as much. A coil of tubing — perhaps 10,000 feet long, is lowered into the well. Water and high pressure nitrogen are employed to force the sand out of the bottom of the well and up through the annular space between the tubing and the outside of the coil tubing. It is not uncommon to see gas flow triple, after a well has been relieved of it’s load of accumulated sand.
Prior to placing a compressor on a partially depleted well, it is a good idea to obtain at least a qualitative idea of difference between the shut-in and the flowing wellhead pressure. If this difference is large, then it is far better to check for sand in the tubing than to blindly install the wellhead compressor. Certainly, if sand is covering the casing perforations, it is a waste of time and money to install a wellhead compressor.
Of course, the presence of sand in a relatively young well is indicative of a sloppy operation at some previous occasion. This is especially true if the material being pulled into the tubing is frac sand rather than formation sand. There is no sense pumping frac sand into a formation and then crushing the sand and sucking it out of the formation by over-rapid natural gas production.
The sun, having burned the last trace of moisture from the already parched hills, dipped below the horizon. Mr. Howlaway stared out the window at the reddening sky. “What about the black cow. Is the cow still relevant”.
“Of course. The cow is part of the story too”, I explained. “Men have been shot for leaving gas field gates open. Driving cattle back onto a lease is always relevant to troubleshooting gas production. As for the black cow, when it saw that I meant business; when it understood that I wasn’t leaving until it went back through the gate; it just naturally marched back onto the lease. It was only a matter of time and determination. Sure, the cow is part of the story too”, I concluded.