Casing Wear Caused by Tooljoint Hardfacing

Abstract

Summary Casing wear caused by new tooljoint hardfacings, such as fine-mesh tungsten-carbide hardfacing and a hardfacing covered with a layer of relatively soft material, has been investigated in the laboratory. The tests were performed on a full-scale test facility with field conditions-forces, motions, and fluids-simulated as closely as possible. It was found that the major mechanisms responsible for casing wear by tooljoints are adhesive wear, abrasive wear, and ploughing. Wear mechanisms can be classified as mild, normal, and severe. A thorough understanding of these mechanisms could lead to measures for lessening casing wear. This can be achieved with (1) tooljoints that have a sufficiently large, smooth, round, and uniform surface and (2) an appropriate mud that has a sufficiently high content of soft solid particles, such as barites, to form a layer in the tooljoint/casing particles, such as barites, to form a layer in the tooljoint/casing contact area so that metal-to-metal contact is avoided and small, hard mud particles are embedded. Introduction Three parameters are important in investigating casing wear. Contact Force. The first important parameter is the contact force between the drillstring and the casing. The results of a mathematical analysis of the contact force between the drillstring and casing presented by Bradley and Fontenot show that below a dogleg severity (DLS) of 2/10 m [6/100 ft], the contact between drillpipe and casing is restricted primarily to the tooljoints. No significant contact force develops between the drillpipe body and the casing. At a DLS above 2/10 m [6/100 ft], in addition to the tooljoint contact force, a large contact force between the pipe body and the casing develops. For a buildup section with a DLS of 1/10 m [3/100 ft] and a local tension of 1000 kN [225 kips] in the 127-mm [5-in.] -OD drillpipe, a contact force of 15 kN [3.4 kips] is to be expected between the tooljoints and the casing (Fig. 1). The most severe casing wear can be expected near doglegs at shallow depths when deep, deviated wells are drilled because of the pronounced curvature and simultaneous high tension in the drillstring. Wear Track Length. The wear track length is a measure of the duration of contact between passing tooljoints and a point on the casing. It is defined as the cumulative distance that the tooljoints slide over a certain point of the casing. The wear track length definition (see Appendix) includes such parameters as time, rotational speed, rate of penetration, and contact length of the tooljoint surface. A major advantage of presenting casing wear as a function of the wear track length is that it enables results of laboratory wear tests to be related to casing wear in the field. Wear Mechanism. Casing wear is the result of a complicated system of three components: casing, tooljoint, and mud. Each component may influence the wear process both positively and negatively. For a proper qualitative discussion of the wear process, the following wear mechanisms are defined.Adhesive wear is the transfer of material from one surface to another during relative motion, caused by a process of solid-phase welding (galling). process of solid-phase welding (galling).Abrasive wear is the removal of material caused by hard tooljoint protuberances (two-body abrasion) or by hard particles in the mud (three-body abrasion).Ploughing is the formation of grooves by plastic deformation of the casing by hard tooljoint protuberances or hard cuttings without deformation of wear debris.