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Hydraulic fracturing technology can be used to jointly exploit unconventional natural gas such as coalbed methane and tight sandstone gas in coal-measure superimposed reservoirs for the enhancement of natural gas production. Hydraulic fracturing usually induces mixed fractures of I and II modes, but existing studies have not considered the influence of reservoir lithology on the stress intensity factor of I/II mixed fractures in coal-measure superimposed reservoirs. This paper develops an analytical stress model and a seepage-mechanical-damage numerical model for the vertical propagation of I/II mixed fractures in coal-measure superimposed reservoirs. The variation of stress intensity factor of I/II mixed fractures is analyzed when the fractures are close to the interface of different lithologic reservoirs and the effects of elastic modulus difference, stress state, fracturing fluid viscosity, shear and tensile failure modes on the vertical propagation of hydraulic fractures are investigated. Finally, the ratio of elastic modulus of adjacent reservoirs is proposed as an evaluation index for the fracture propagation through reservoir interface. These investigations revealed that hydraulic fracture propagation through the reservoir interface is a process of multi-physical interactions and is mainly controlled by the injection pressure and the elastic modulus ratio of adjacent reservoirs. A critical line is formed in the coordinates of elastic modulus ratio and injection pressure. A fracture can propagate through the reservoir interface when the combination of injection pressure and the elastic modulus ratio is in the breakthrough zone. These results can provide theoretical support for the site selection of horizontal wells in coal-measure gas exploitation.

To improve the durability of cement-based revetment materials serving in different positions relative to the water level, slag powder and polypropylene fibers were added into cement to prepare paste, mortar, and concrete. Based on three simulated experiments of high-humidity air, dry–wet cycles-coupled chloride erosion, and complete immersion-coupled chloride erosion, the half-year durability of cement-based revetment materials was investigated. An abundant amount of Ettringite containing chloride was formed in the pores of the cement, and its formation was accelerated by dry–wet cycles. Replacing 30% of cement by slag powder and adding 0.1 vol.% of polypropylene fibers helped concrete in the intertidal zone to obtain a compressive strength of 47.58 MPa after erosion, equal to 159% of the reference. Slag powder was found to induce cement to form Friedel’s salt and C-S-H with a more amorphous structure, increasing its chemical binding ability and physical adsorption ability to chloride ions, and reduce the chloride ions’ penetration depth of concrete from 22.5 to 12.6 mm. Polypropylene fibers controlled the direction of surface cracks to be perpendicular to the specimen’s sides. These findings lay a foundation for the design of high-durability cement-based revetment materials serving in costal environments.