Casing Wear: Laboratory Measurements and Field Predictions

Abstract

Summary Casing wear caused by rotation of nonhardbanded tool joints has been investigated with a full-sized test machine. Three grades of casing were used during this study: K55, N80, and P110. Tests were conducted in both water-based and oil-based muds. Contact forces between the tool joint and casing were either 1,000 or 2,000 lbf [4.4 or 8.9 kN]. Both the casing wear and friction forces were measured during the tests. A linear wear-efficiency model was used to correlate the data. This model relates casing-metal removal to the amount of energy dissipated as friction in the wear process. The wear-efficiency model fits the data well and is easily used for field wear predictions. The data from this study indicate that high-strength casing steels wear more rapidly than K55 and that wear in oil muds is possibly more rapid than in water muds. Partial hydrodynamic lubrication occurred during many tests: friction measurements and the wear-efficiency model were used to account for its effects. Introduction Casing wear, though historically not considered a problem to the drilling industry, has had and will continue to problem to the drilling industry, has had and will continue to have an impact on both well design and drilling operations. The petroleum industry probably spends tens of millions of dollars per year on extra well thickness to allow for wear. A better understanding of the basic wear process would help allocate this money in the most efficient process would help allocate this money in the most efficient manner. With this in mind, an experimental program was devised to measure casing wear in a full-scale test machine. This paper presents those measurements and conclusions drawn therefrom. This is not the final work on casing wear. New concepts and thought-provoking results are included. but there are still unanswered questions. We hope this paper stimulates further work in this complex area. Experimental Program Earlier studies of casing wear have concluded that most wear is caused by drillpipe rotation rather than reciprocation. Bradley and Fontenot reached this conclusion after they studied wear caused by rotation and examined casing samples recovered from the field. Fontenot and McEver studied the effect of pipe reciprocation on casing wear and concluded that it was less important than rotation. Thus this study's aim was to study casing wear caused solely by drillpipe rotation. Furthermore, it was assumed that only the tool joints, not the drillpipe body, contact the casing. An experimental apparatus was designed and constructed under the assumption that casing wear is caused mainly by tool-joint rotation. Fig. 1 is a schematic of the casing-wear test machine. This apparatus could operate at between 15 and 120 rev/min and at up to 3,000-lbf [13.5-kN] side force on the tool joint. The depth of the wear scar and the friction force were continuously measured and recorded. Because there was no axial motion in our test machine, all the wear was concentrated in one short section of the casing sample. Casing samples of any size or grade could be bolted to the casing-sample carrier arm. This study tested 9%-in. [24-cm] casing in Grades K55, N80, and P110. Each test started with a new casing sample; all grade samples were cut from the same joint of casing. The physical and chemical properties of the casings tested in this study are shown in Tables 1 and 2. The tool joints were 6%-in. [16-cm] -OD standard API tool joints without hardbanding or tong marks. The surface roughness of the tool joints varied during the tests but was usually in the range of 6.4×10–6 to 128 × 10–6 in. [1.6 to 32 mu m].