Abstract: Compared with traditional geothermal resources, deep geothermal resources have greater development potential. However, due to the higher bottomhole temperature of deep geothermal wells, the high temperature fluid in the development and exploitation causes the casing thermal expansion, which tends to cause casing yield deformation. At present, there are many challenges in theoretical research and technical practice for casing safety of deep geothermal wells. Therefore, according to the thick-walled cylinder theory and the basic theory of thermoelastic mechanics, the axial thermal stress of casing is reduced by applying thermal stress in advance, and the contact problem of formation-cement ring-casing after high temperature cement stone damage is considered. Based on the principle that the effective stress of casing is controlled within the minimum yield limit at the corresponding temperature, the safety factor of casing string is designed according to the strength of casing string, the safety performance of casing is evaluated, and the design method of casing preheating stress in deep geothermal wells is established to alleviate the problem of thermal damage of casing caused by high temperature. The analysis results show that the density of cement slurry has little effect on the damage factor, and the elastic modulus of cement ring plays a leading role in the damage factor. Under the water injection condition (water injection temperature at 65 ℃) preheat below 250 ℃ can meet the design conditions. Under the condition of geothermal production (geothermal temperature at 346 ℃), the safety design requirements is satisfied when preheat is above 200 ℃ to meet.

Key words: deep geothermal well, cement ring damage, casing, preheating stress, safety evaluation