Effect of Mud Composition on Wear and Friction of Casing and Tool Joints

Abstract

Summary Friction and wear between casing and tool joint have been measured on full-scale equipment. The results show that the most important mud property, with respect to lubricity and protection against wear, is it ability to form a film that separates the two steel surfaces. Results fro small-scale tests are shown to be nonrepresentative. Introduction During drilling of deviated wells, the torque and drag of the drillstring and wear of the casing may become very high. This can result in considerable operational problems and increased drilling costs. The torque of a drillstring is generally determined by three phenomena: the friction between the drillstring and the casing (cased hole), the friction between the drillstring and the borehole wall (open hole), and the bit. Drag is determined by the first two phenomena only. Casing wear, of course, will occur only phenomena only. Casing wear, of course, will occur only in cased holes. In this paper, we discuss only those processes that take place in cased holes-i.e., the friction between the place in cased holes-i.e., the friction between the drill-string and the casing and the wear of the casing. The effect of mud composition on these two processes has been examined. Wear and friction are the result of a complex tribological process that occurs in the contact area between the tool joint and the casing with mud as the intermediate medium. Parameters-e.g., contact load, surface roughness, hardness, geometry, and chemical composition of both the tool joint and the casing-and mud composition will determine what kind of wear mechanism occurs. The wear mechanisms I addressed here are those relevant to tool-joint/casing contact:adhesive wear is the transfer of material from one surface to another during relative motion as a result of solid-phase welding;two-body abrasion is the removal of casing material caused by hard tool-joint protuberances; andhree-body abrasion is the removal of casing or tool-joint material by particles present in the contact area. The wear process determines the condition of the casing and tool-joint surfaces, which will certainly affect the friction between the two. However, there is no general, unambiguous relationship between the two. High friction and low wear rate can occur simultaneously (e.g., in a brake shoe) as can the reverse (e.g., metal cutting with a cutting oil). This means that wear and friction have to be evaluated separately. The API method for evaluating the effect of mud composition on the friction between the casing and the tool joint uses the lubricity coefficient instrument, which is a small-scale laboratory test machine. No standard method is available for evaluating casing wear. Initially, both the API lubricity tester and several other small-scale test machines were used to measure friction and/or wear in steel/steel contact areas immersed in various muds. Wear and friction were measured later with full-scale equipment. The effects of mud density, type of weighting material, and the addition of several mud additives and salts, lubricants and diesel, and silt and sand were investigated in water-based muds. For comparison, several tests were also done with standard invert-oil-emulsion