February 12th, 2006 at 9:56 pm
Perforating
Logging and perforating operations are often performed by the same people. While logging involves lowering some sort of sensor package down a wellbore, perforating involves lowering a “perforating gun” into the hole for the purpose of punching holes in the casing at a desired depth.
A perforating gun is a long tube that holds shaped charges. Originally borrowed from tank-killer technology, these shaped charges explode and pierce holes through metal casing and cement to reach the rock on the outside. The charges are detonated with an electric current that travels down the line and ignites primer cord, which then ignites the charges.
A logging/perforating engineer’s most common confirmation of detonation is when he rests a hand on the cable hanging the gun in the hole in the ground. When the charges detonate, he can feel the cable kick back. While a perforating company is on location, no radio equipment is allowed to be on. Primer cord is particularly sensitive to the keying of a two-way radio. Some fun with electromagnetic induction, I believe.
Several options exist when perforating a well. By choosing the right type of gun, the engineer can control how close the holes are in the casing. Too many holes and the casing is destroyed; too few and there aren’t enough holes for petroleum to come out. Most configurations I have seen range from four to six shots per foot.
Different explosives can be used in the shaped charges; most I’ve seen use HMX and RDX. Temperature is the primary factor in choosing an explosive; higher temperatures require more stable compounds. More powerful explosives can also reach deeper into the rock, which can be useful if previous treatments have plugged up the rock around the well.
A customer can also choose shot phasing. Phasing is the angle from one explosive to the next as one looks along the wellbore. Imagine holding a paper towel tube vertically with a compass on top. If you wanted to make holes with 90-degree phasing, you could punch a hole at North, then move down a couple of inches and punch a hole East. Further on down you’d punch South, then West, then North again. A bad phasing angle can made for more difficult stimulation treatments. The most popular phasing is 60 degrees, suitable when time isn’t taken to learn the mechanical forces at work downhole.
The latest technology in perforation involves not only selecting shot strength, shot density, and shot phasing, but also shot direction. When a perforating gun is suspended with a cable, it can twist as the cable stretches. One of our newer services, Tubing Conveyed Perforating, allows the customer to choose the direction that his perforations can be shot. This is a big help in planning stimulation treatments for the well.
My co-worker Arabinda has shown interest in sending some information on production surveys. The next post will either be about that or about stimulation, Halliburton Energy Services’ other monster moneymaker.
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