Abstract
The increasing complexities of wellbore geometry imply an increasing potential of damage resulting from casing wear downhole. Much work has been performed to quantify and estimate wear in casing; however, the results of such predictions have been mixed. While the locations of critical wear areas along the casing string have been predicted fairly accurately, quantifying the actual amount of wear itself has been a challenge. The existing casing wear models were developed only for static wear conditions, where there is constant rotational contact between the tool joints and the inner wall of the casing. However, drilling activities and operating conditions have advanced rapidly, leading to several other downhole wear mechanisms contributing to casing wear.
The casing wear predicted using these static models cannot correctly estimate the actual volume of the wear, thus resulting in several different studies being conducted to improve the accuracy. This study is aimed at exploring and modeling one of the contributing factors to casing wear—wear caused by vibrational impacts of the drillstring on the inner wall of the casing. Impact wear in casing strings is observed at certain rotational speeds when the drillstring experiences resonating frequencies that lead to dynamic vibrations. These high-frequency vibrations generate an impact force acting on the inner casing wall as a result of the drillstring contact and thus influence the casing wear.
An analytical model to estimate and quantify the wear caused by drillstring impact is presented as a more comprehensive casing wear model. Various impact force models were considered in this study to better understand this phenomenon; namely, the parabolic model, the elliptical model, the cosine model, and the positively skewed force distribution model.
The different impact force distribution models were then applied to estimate downhole casing wear caused by impacts, and the estimates were compared to the linear static wear. As previously mentioned, static wear occurs because of continuous contact of the casing and the drillstring along a certain portion of the wellbore. Impact wear occurs over and above static wear, and the combined effect of both provides an improved prediction of the actual wear in casings downhole. This paper presents a field example to estimate the casing wear caused by impacts using a simulated case study. Estimation of the depth of the wear groove using the new casing wear model was alsoperformed using field drilling parameters and is presented as well.
