Casing Wear: The Effect of Contact Pressure

Summary One of the major factors affecting casing wear is the magnitude of the loading between the drillstring and the casing. This paper describes a new test machine and experimental procedure used to study the effect of contact loads on casing wear. The results of extensive testing show that contact pressure instead of contact load is the appropriate parameter controlling the wear rate of the casing. An experimental relationship is found between contact pressure and casing wear rate for nonhard-faced tool joints in a laboratory prepared drilling mud. Knowing this, a computer program is devised to model accumulated casing wear during drilling. A hypothetical example is given estimating the additional feet that can safely be drilled past a dogleg without catastrophic wear. The paper also shows the relative effects of tool joint size and drilling penetration rate on casing wear. Additional factors penetration rate on casing wear. Additional factors such as the addition of steel wear protectors in the center of the drillpipe also are explored. Conclusions are drawn as to the limitations of simple casing wear guidelines based on maximum allowable contact load. It also is concluded that the addition of steel wear protectors does not increase casing wear. Introduction It is well known that casing wear can result in significant losses if allowed to progress to failure. As holes are drilled to greater depths, the length of time available for wear increases. The longer drillstrings used have greater tension and therefore larger forces between the tool joints and the casing. Also, highly deviated directional wells increase the contact loads resulting in increased casing wear. Obviously, it is important to understand casing wear more completely for today's drilling. One approach to controlling casing wear to acceptable values was suggested by Lubinski. A study of wells drilled before 1960 showed that when the tool-joint-to-casing loads were below 2,000 lbf (8.9 kN), the holes were trouble-free. This led to the concept of controlling the maximum side load to minimize casing wear. More recently, Bradley presented a means of estimating casing wear based on presented a means of estimating casing wear based on an experimentally determined wear rate. The wear rate was approximated as varying linearly with contact force. Whether these or other methods of estimating casing wear are used, it is clear that the magnitude of the contact load is a very important parameter. It is the objective of this paper to investigate the effect of loading more fully. Once this is done, other parameters such as the tool joint diameter, drilling parameters such as the tool joint diameter, drilling rate, and the addition of steel wear protectors can be analyzed. Background Casing wear is a complex process involving many variables: temperature, mud type, percent abrasives in the mud, tool joint hardfacing, revolutions per minute (second), tool joint diameter, contact load, and many others. In the face of such complexity, it easily is understood why so little is known about casing wear. Even so, several fine papers have been written on the subject. Some of these are listed in the references. Many papers are concerned with the effect of tool joint hardfacing on casing wear. Two of the papers looked at the drilling mud type effect. Only Bradley's work has studied the variation of casing wear with contact load. JPT P. 2382