Abstract: In view of casing damage during hydraulic fracturing in Zhaotong shale gas demonstration area, a numerical model of casing-cement sheath-formation combination was established based on casing deformation characteristics and theoretical analysis to study the mechanism of increasing casing external load by temperature effect, fracture network-induced stress, microfracture, formation slip and cementing quality. The study shows that the hydraulic fracturing leads to non-uniform change of the stress field in the Earths crust in the fracture reconstruction area, and greatly increases the stress field in the Earths crust and the difference between the maximum horizontal and vertical crustal stress in the heel and middle areas of horizontal wells; the induced stresses of fracture network, temperature effect, microfracture, formation slip and cementing quality defects will increase the external load of casing, among which the induced stresses generated by hydraulic fracturing has the most significant effect on increasing the external load of the casing, which may be the main reason for the high probability of damage to the casing in the heel and middle areas of horizontal wells. This may be the main reason for the high probability of casing damage at the heel end and central area of horizontal wells. Due to the high construction pressure and high stress difference, the weak surfaces such as stratigraphy and natural fractures in Zhaotong area are prone to shear slip in the process of hydraulic fracturing, and the casing deformation and force are the largest when the angle of the weak surface and the wellbore is 45°. The study results provide guidance for casing strength design and casing deformation prevention in horizontal wells of shale gas fields.

Key words: shale gas, hydraulic fracturing, casing external load, casing damage, stress field, Zhaotong shale gas demonstration area