Because of the high cost associated with tubing replacement, rodwear tubing leaks are one of the most costly types of subsurface failures on sucker rod pumped wells. However, many rodwear tubing 1eaks can be prevented by properly identifying the cause of tubing leaks and taking the appropriate corrective measures.
While severe hole deviation is most often blamed for rodwear tubing leaks, case studies show that most rodwear leaks are caused by accelerated corrosion which is the result of the removal of a protective corrosion scale from the inside of the tubing. For this reason, the key to economically eliminating rodwear tubing leaks is correctly identifying the problem as either mechanical wear or corrosion wear and then implementing the appropriate corrective measures.
Four case histories, involving a total of nearly 100 problem wells, are presented to illustrate the methods used to differentiate between corrosion-wear and mechanical wear. In these fields, tubing leaks were reduced by as much as 80% by properly identifying the cause of the leaks.
Approximately 100 wells in five (5) different fields were investigated to determine the cause of their excessive downhole wear. Many of the subject wells were experiencing tubing leaks as often as once every six months. Without exception, every problem well had been labeled as a "crooked hole". Upon further investigation, it was found that only one well out of the group had a problem that could be attributed solely to a crooked hole. That well had a 25° average deviation from vertical and still had a projected life without tubing leaks of three years. From this investigation, it was determined that other factors, most importantly corrosion, have a much greater effect on downhole tubing wear than well deviation. Even mild corrosion, when combined with wear can result in a tubing life as short as two months. For this reason, it is important that corrosion-wear leaks be identified and treated as such.
Mechanical wear is the removal of metal due to the frictional rubbing of the sucker rod string upon the tubing. The softer of the two metals should suffer the most weight loss. With nearly every combination of tubing and couplings, the tubing is the softer metal. (Table 1) However, from the field studies, it can be shown that on wells with J-55 grade tubing, T-grade sucker rod couplings, and a mechanical wear problem, the sucker rod coupling will usually fail first. This can be attributed to the fact that the tubing is wearing over the full length of the stroke where the sucker rod coupling is only wearing over its short 4-inch length.
An effective method for reducing downhole mechanical wear is the use of soft rod guides to prevent the rod coupling from contacting the tubing wall. Rod rotators can also extend the life of rod guides and rod boxes by distributing the box or guide wear more evenly.
