Wear Analysis And Safety Assessment For Down Hole Casings Of Directional Wells

In directional wells，due to the effect of the angle of deviation and the weight of the drillstirng, the wear of oil well casing was very serious. The reduction of the resistance tocollapse and burst strength of worn casing directly inlfuence the formation testing and wellcompletion operation. The correct analysis and prediction of casing wear degree, and thecalculation of the residual strength of casing wear were great significance to safedevelopment of directional wells. According to the special conditions of the directional wells,this paper combines with analysis of directional wells casing wear degree, evaluates safetydownhole casing, so as to provides the suggestions for the oil and the completion afterwards.Based on the basic theory of tribology of directional wells The paper analyzed the maininfluencing factors and wear mechanism for down hole casing, established reasonable casingwear predictive model which combined with directional well contact force calculated underthe practical condition, the wear efficiency measured by experiment, and quantitative analysisof the degree of wear of casing. According to the directional well casing wear in differentworking conditions underground, the ring block type drill-casing wear test machine wasused to do the experiment of dirll pipe and casing wear test, which can simulate variousworking conditions during dirlling operation, including temperature, pressure, rotation speedof the drill pipe, casing wear time and types of drilling mud. the experiment was conducted todetermine the wear efficiency, frictional coefficient and wear between different steel gradecasing and tool joint. Meanwhile the tendency of wear eiffciency and firctional coefifcientrate was analyzed. The wear regularity of different conditions was summed up. The degree ofcasing wear can be accurately predicted and calculated by the wear model in the process ofdirlling. By using the wear casing stress distributio’ns bipolar coordinates and ANSYS numericalmethod, the residual collapse strength and burst strength of worn casing which have thedifferent size, different casing steel grade，and different wear depth were calculated Theauthor establish a new finite element model and analysis the inlfuence of non-uniform in-situstress on the residual strength of worn casing.According to the idea and method of analysis, optimization of drilling parameters andreasonable matching of casing, drill pipe, and mud can reduce the wear degree of directionalwell casing. The paper makes safety assessment after the completion of the drilling of wells,and provides a reference for the subsequent oil ’well testing and completion parametersselection.