Analysis of Casing Damage Mechanism of Carboniferous Salt Layer in Tarim Basin

Abstract

ABSTRACT: The Carboniferous strata in Tarim Basin, China developed salt beds. Casing damage occurred in the drilling process, which led to frequent sticking and even forced sidetracking, which seriously affected the development of oil and gas in this block and caused great economic losses. To solve the problem of casing damage in the Carboniferous salt layer, the mechanism of casing damage is analyzed in this paper. Based on the statistical analysis of geological and engineering data, the casing damage mechanism of bending and stretching deformation is deduced, and the above casing damage mechanism is confirmed by establishing the finite element model of casing strength checking. The effects of borehole enlargement rate, casing center degree and casing program on the bending and stretching yield of casing are also analyzed. On the whole, the casing damage risk is positively correlated with borehole enlargement rate and axial tension, and negatively correlated with arm of force and casing center degree. In the casing program design, the salt layer should be located at the bottom of a certain run, and the anhydrite should be destroyed by reaming multiple times in the salt layer to avoid the formation of "hard fulcrum" during cementing, which can effectively reduce the risk of casing damage. 1. INTRODUCTION There are a lot of oil and gas buried in salt layers around the world, such as the United States Gulf of Mexico, offshore Brazil, West Africa, and the North Sea (Greenhalgh et al., 2012; Chitale et al., 2014). Casing damage in the salt layer is a problem that has been discovered for a long time (Cheatham et al., 1964). This problem is still happening today, resulting in oil and gas wells being forced to sidetrack or even be abandoned, which not only wastes much money but also seriously restricts oil and gas production. Rock salt can creep and deform, and creep begins the moment it is drilled into the salt layer (Wang et al., 2016). Even after the cementing operation is carried out, the cementing quality is often difficult to guarantee, so the rock salt creep will still exert force on the casing (Willson et al., 2002). At present, the damage modes of casing in the salt layer are mainly extrusion deformation and bending deformation (Clegg et al., 1971; Taheri et al., 2020).