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The frequent occurrence of casing failure has significantly affected the development of shale gas in the Sichuan Basin. Based on engineering and geology analyses, natural fracture/fault slip leads to casing shear deformation during hydraulic fracturing. Hence, natural fracture/fault slip and casing‐cement sheath‐formation assembly finite element models were established to quantitatively examine the characteristics of natural fracture/fault slip and mechanisms of casing shear deformation. Many factors, such as formation elastic property, in situ stress, natural fracture/fault geometry property, fluid pressure, casing elastic property, cement sheath elastic property, and well trajectory, were considered. The simulated results were in good agreement with the field data. The results indicated that natural fracture/fault slip leads to a sharp increase in casing deformation and stress in the adjacent region. Furthermore, factor sensitivity analysis results showed that a higher fluid pressure, larger fracture length, larger fracture dip angle, higher in situ stress difference, lower formation Young's modulus, and shorter distance from the wellbore to the center of the slip surface lead to severe casing stress and deformation. The countermeasures for preventing casing shear deformation involve decreasing the fracturing treatment pressure, reducing the crossing angle between the wellbore and natural fracture/fault, maintaining a safe distance from the wellbore to the large fracture/fault center, and adopting “balanced stress” fracturing operation. Hence, this study proposes a reasonable method to evaluate casing shear damage under hydraulic fracturing.

In this study, new kinds of hydrogen-bonded (H-bonded) chiral molecular switches (CMS) composed of azobenzene moieties as photo-responsive parts and H-bonded complexes as thermo-responsive parts were developed. Photo- and thermally-reversible switching behavior of cholesteric liquid crystals (Ch-LCs) based on the H-bonded CMS was investigated in detail. The results demonstrate that H-bonded CMS undergo a trans–cis photoisomerization under UV light, which results in the decrease of helical twisting power (HTP). Meanwhile, the modulation of the intermolecular forces between proton donors and acceptors with temperature has a positive influence on the change in HTP. Reversible blue, green, and red reflections of the self-organized helical superstructure could be achieved under UV/Vis light as well as with temperature change. According to the geometry optimization based on Gaussian 03 calculations at the B3LYP/6-31G(d) level, the molecular aspect ratio could be considered to be one of the important factors influencing the HTP of H-bonded CMS. This dynamic tuning of the self-organized helical superstructure based on dual and selective molecular mechanism opens up a way to achieve new kinds of LC photonic materials.

A novel series of photoresponsive chiral switches are fabricated by a facile hydrogen-bonded (H-bonded) assembly method, in which the binaphthyl azobenzene molecule is used as the proton acceptor, and binaphthyl acids with opposite chiral configuration are proton donors. We find that the helical twisted power of H-bonded chiral switches and the helical handedness of induced chiral nematic liquid crystals (N*-LCs) are mainly determined by the terminal flexible chain length in proton donors of binaphthyl acids. Controlling the lengths of the terminal flexible chain leads to different photoswitching behaviors by light irradiation, such as a helical inversion in the N*-LCs and a phase transition from N*-LCs to nematic LCs. This is mainly because of chiral counteraction and intensity attenuation of opposite chiral configurations between the proton acceptor and proton donor during UV-vis irradiation. Additionally, the thermal switching behavior of N*-LCs doped with H-bonded chiral switches is also demonstrated, and the related tuning mechanism may be attributed to the H-bonded effect and the changes in a dihedral angle of the binaphthyl rings. This facile assembly approach provides a new way for the fabrication of functional chiral switches for photonic applications.