Contact Pressure Threshold: An Important New Aspect of Casing Wear

Abstract

Drilled wellbores have evolved from nearly vertical, shallow holes to tortuous, deep, directionally drilled wells. As wellbore geometries have increased in complexity, so has the potential of damage resulting from casing wear. It is not unusual to routinely install ditch magnets in the returned drilling fluid circuit to catch the iron filings created by tool joint wear against the casing or riser interior wall. While most "straight" holes attempt to control inclination, azimuth control is often times neglected, potentially creating a tortuous path. In directionally drilled holes, including horizontal and multi-lateral wells, the drill string tension holds the rotating tool joints against the inner wall of the casing for extended periods. This results in the generation of crescent shaped wear grooves (key seating) in the inner wall of the casing, often dangerously weakening casing or riser strings making them more susceptible to burst or collapse. Some investigators have sought to quantify this damage process. Computer models describe, measure, and predict damage from casing or riser wear. From these on-going studies, advances in technology continue to minimize casing and riser wear. More than 475 8-hour casing and riser wear tests have been conducted, producing the largest casing/riser wear database known. Analysis of this database led to the development of the contact pressure threshold concept, consistently demonstrating its validity. When a rotating tool joint impinges against the inner wall of a casing or riser, a crescent shaped groove is worn into the inner wall. The volume worn away from the casing or riser wall is proportional to the frictional work done on the inner wall by the tool joint. This is mathematically presented in the equation: Equation (1) Experimentally determined over an 8-hour testing period, the Casing Wear Factor, WF, is then defined as the ratio of friction factor to specific energy, as shown in equation 2, Equation (2) and the sliding distance is defined as shown in equation 3, Equation (3) If the length of a joint of drill pipe is Ldp, and the length of a tool joint is Ltj, then Equation (4) As an example: If Ldp = 30 ft., and Ltj = 14 inches, f = 0.039. Thus, the volume of casing wall removed per foot in time t hours is given by equation 5. Equation (5) To establish a benchmark test for comparison of casing wear factors, a set of standard dimensions and conditions for conducting the wear testing was established. These dimensions and conditions are shown in Table 1. Since discussions of the testing program and the significance of the measured wear characteristics are presented in other publications, they will not be repeated here.