Neat well cement experience significant strength retrogression at high temperatures above 110 °C, especially at approximately 150 °C. To reveal the mechanism of performance degradation and guide the preparation of high-performance cement, we investigate the hydration process, mechanical behavior, and fracture process for well cement at the temperature of 150 °C based on molecular dynamics simulations and experiments. From triaxial pressure tests and Brazilian splitting tests, the strength, elastic modulus, and Poisson’s ratio of well cement decrease drastically with temperature increases from 80 °C to 150 °C. According to XRD, TG/DTG/DSC, and SEM, the hydration degree is insufficient, and larger pores exist in the microstructures. As the main binding phase of well cement, the mechanism of calcium silicate hydrates (C-S-H) influenced by curing temperatures is investigated through molecular dynamics simulations. C-S-H with calcium/silicon ratios (C/S) of 1.1 and 1.8 are simulated in the aqueous and solid states to investigate precipitation and mechanical behaviors. By reducing the C/S ratio to 1.1, the strength rebounds to a certain extent, and the adequacy of the hydration degree improved. It is found from the polymerization process that the increasing temperature promotes the polymerization rate, which is higher with C/S = 1.8 than that of 1.1. However, an increase in the C/S ratio will lead to a decrease in bridging oxygen content, thus a lower polymerization degree. The fracture simulations of C-S-H gels at different temperatures indicate that the failure of the C-S-H structure is mainly attributed to the disassembling of the calcium oxygen layers. With a higher temperature, there are fewer Ca-O bonds breaking, thus less strain energy consumed, resulting in worse performance. The elasticity of C-S-H, including Young’s and shear moduli, also exhibits certain degradations at a higher temperature. The elastic behavior of C-S-H with a low C/S ratio is generally higher than the high C/S.

In cementing operation, the “cement-–rock” interface bonding ability is an important factor affecting the long-term sealing ability of cement sheath, and the evolution mechanism of the interface sealing ability needs to be further studied. However, due to the particularity of the interface transition area, it is difficult to comprehensively analyze the macroscopic acoustic properties, mechanical properties and sealing evolution ability of the interface due to the lack of precise quantitative testing equipment. To solve the above problems, a cement-sheath cementing-interface sealing capability evolution evaluation device was developed, and a laboratory acoustic response accuracy experiment was carried out to form a quantitative evaluation method of cement–formation interface capability. The developed apparatus withstood pressures up to 30 MPa and temperatures up to 150 °C during basic performance testing. The ultrasonic probes were calibrated using standard cylindrical steel specimens, with P- and S-wave measurement accuracies exceeding 95%. Preliminary cement stone–sandstone acoustic calibration was subsequently performed at 17 °C and 9.8 MPa for 9 days, corresponding to the conditions of the selected field interval. Comparison of the normalized laboratory acoustic response with representative field VDL data showed an agreement of approximately 86.8%. These results provide preliminary validation of the basic operational and acoustic measurement capabilities of the developed platform; however, cement stone–rock interfacial behavior under a broader range of temperature and pressure conditions remains to be systematically validated.