Abstract
Casing wear by drillstring results in a thinner portion of casing wall and a reduction on casing burst strength (the ability to hold internal pressure). How to estimate the reduced casing burst strength on such a "crescent-worn" casing has been an important issue in oil and gas industry, as it is directly related to how to safely design casing strings. A common approach is to estimate the reduction of casing burst strength of such worn casing from API burst strength equation with a linear reduction by the remaining wall thickness or the wear percentage, equivalent to a "uniform-worn" casing model, despite a question on whether such a linear reduction of casing burst strength is over-conservative and may result in a higher casing cost.
This paper presents a further study on hoop stress and deformation of such a "crescent-worn" casing and discusses the reduction of casing burst strength. The hoop stress in the thinner portion of such a "crescent-worn" casing is found close to that from a "uniform-worn" casing, when local bending in the thinner portion of "crescent-worn" casing is ignored. The reduction of burst strength of casing worn by drillstring may still be estimated from casing yield or rupture burst strength with a linear reduction by wear percentage for sweet service well conditions, while more and non-linear reduction of burst strength of casing worn by drillstring may be needed for sour service well conditions, to prevent an "earlier" casing burst on sulfide stress cracking.
