How Does Cement Curing Affect PVC Pipe?
A friend called the other night to catch up and chew the fat about things in general. I asked him if he was busy, etc., and he told me that he had lost a well that day. Not good, but it happens for all sorts of reasons. He had set a 4-inch PVC well in a 7-inch hole. The contract called for bottom to top cement. He set his pipe on bottom and ran an outside tremie line to cement. When he came back the next morning, the casing had collapsed about 8 feet downhole, so he was going to move over and redrill. I told him to hold up. I suggested that he go do some installations or service calls for the rest of the day, and that I’d be there in the morning.
When I got there, sure enough the casing was collapsed, but not crushed. It looked like it had just melted in on itself. I asked him about the formation at that depth. He remembered drilling through about 3 feet of very fine, flowing sand that took forever to circulate out, even at that shallow depth. A clue! Obviously, he had a large washout in the hole. No telling how big it was, but it was big enough to cause the problem.
Portland cement sets by a chemical reaction. This releases heat. If you have enough of it, it releases a lot of heat. In this case, enough for the casing to soften and collapse. An extreme example of this would be when they built Hoover Dam. They ran cooling water through pipes in the dam for years to cool it! Apparently, the cement was thick enough in that area to heat up above the temperature that would plasticize the casing my friend used.
Instead of redrilling the well, we got the backhoe and dug it up. It was only 8 feet. We had to break the cement off the casing, but when we got below the collapse, we cut it off and glued on a new piece. Problem solved.
Observations and lessons learned: There are things you can do to avoid the heat of hydration problems. First, make sure the casing is completely full of water or mud. Cap the casing, and have a provision to monitor the pressure and a pump in if need be. The casing will not collapse if the internal pressure is equal to the hydrostatic load of the cement. Also, don’t just pressure the casing up to a high number and call it good. The heat of hydration may soften the casing enough for it to balloon in the hole. Not good. Second, consider retarded cement — cement with chemical additives to slow the chemical reaction that sets it and slows the heat of hydration. You will need to give it extra time to set, but this is better than redrilling.
A good way to tell is to take some samples in a clean paper cup while you are cementing. Set a couple in a bucket of water and a couple in the air. When you get back, take a look at them. If they are still semi-liquid or very soft, wait a little longer. When they have set up to your satisfaction, you can proceed operations. Sometimes, WOC (waiting on cement) time is a good time to do routine maintenance and other things that you might have put off because you are so busy. In the oilfield, we generally slip and cut the drill line during this time, and do pump maintenance or other things we had been putting off. That way you are not shutting down the “critical path” to do these things.
I know this is a little short this month, but today is my mother’s 90th birthday. We are going to celebrate, as much as a wheelchair-bound little old lady can stand. She never figured she would last this long, but I did. Her parents and grandparents lived to almost 100! Love ya, Ma!
Keep it turning to the right!
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.