Numerical investigation for different casing deformation reasons in Weiyuan-Changning shale gas field during multistage hydraulic fracturing

Abstract Casing deformation has been found to be a very significant issue in the development process of shale gas in China. The rate of casing deformation is 34.34% (34 wells of 101), and statistical data show that it decreased the test output of wells by approximately 30%. Based on investigation and analysis of the engineering and geological conditions of 25 wells, 12 of which have casing deformations, both physical models and finite element models were established. Furthermore, the effects of fracturing pressure, geostress, anisotropy, lithologic interface, temperature variation, and cement void were analyzed. The results show the following. a) When the casing deformation points appear far away from point A (the landing point, which is the starting point of the horizontal segment), the influence of the lithologic interface on casing stress plays a decisive role under the combined effect of internal and external pressures when the cement sheath is completed. b) Under the same conditions, but when the casing is in contact with the borehole wall near the bottom because of gravity, a narrow fluid retention space was formed, leading to cementing sheath voids. The effect of cement void pressure reduction is the main reason behind the casing deformation. c) When casing deformation points appear near point A, the activation of the weak plane in the beddings during fracturing causes the casing deformation. The stress of 14 casing deformation points was calculated according to actual engineering and geological conditions, and 78.6% of the results were consistent with field measurements.