Geo Mechanical Analysis of Casing Failure in Bedded Rock Salt Formation

Abstract: There are many bedded rock salt resources in China which are serviced as the hosts of underground natural gas storages. For the relatively thin nature of bedded rock salt and the local presence of other sedimentary rock formations, the design and safety evaluation of well completion casing take greater challenges to the engineers than that of casing in other type formations, i.e., limestone and carbonatite. The 2D and 3D geomechanical models of casing-cement sheath-rock salt are established in the paper based on the field data to obtain the creep loads in casing and find the main reasons causing casing failure. In addition, the effects of non-salt layer dip angle, friction factor between salt and non-salt layers and non-salt layer thickness, etc., on the stresses and deformations of casing are studied. The comprehensive results show hoop creep loads are the main reasons causing casing failure rather than radial creep loads. The cement sheath can improve the safety and optimize force state of casing even in perfect wellbore, which disagrees with Willson's view that the cement sheath can be neglected in a salt formation wellbore with high quality. The non-uniform factor of radial creep loads in cement sheath is slightly smaller than that of original in-situ stresses, while that of radial creep loads in casing is greatly smaller than it. The stresses and deformations of casing increase with increasing thickness of non-salt layer and decrease with the increase of friction factor. When the non-salt layer dip angle is with a value of 50 deg, the stresses and deformations achieve the max and subsequently decrease