Numerical simulation of casing deformation during volume fracturing of horizontal shale gas wells

Casing deformation problems occur frequently during volume fracturing of shale gas wells. It is crucial to assess the casing deformation near heel (the beginning of the horizontal wellbore trajectory) where natural faults exist among 84.6% of the deformation positions in some shale plays. Based on the source mechanism, the fault slip distance was derived from microseismic inversion. Then, three-dimensional physical and finite element models were established using a stage finite element model procedure to investigate the influences of fault slippage on casing stress and displacement. Other factors such as pump rate, fracturing pressure, formation anisotropy, formation elastic modulus, cement sheath elastic modulus, casing size, and cement sheath thickness were taken into consideration. The results show that (1) fault slippage is an important mechanism of casing deformation near heel. The casing has a higher risk of deformation under the conditions of fault slippage combined with high external stress and internal pressure. (2) Under different slip distances, the casing Mises stress dramatically increases with an increase in the cement sheath and formation elastic moduli and the ratio of the casing diameter to the thickness Dout/t. (3) The high pump rate, high inner casing pressure, large formation anisotropy, and small cement sheath thickness can aggrandize casing stress. The calculated casing displacements using this model obtained good agreement when compared to the deformations of the actual wells. Countermeasures such as avoiding natural fault areas, using cement with an elastic modulus smaller than 10 GPa, using a higher grade and smaller diameter casing with a value of Dout/t smaller than 12.7 can be effective to prevent casing deformation during multi-fracturing operation.