To address the unclear mechanical response mechanisms of the cement sheath in deep oil and gas well cementing under complex temperature-pressure conditions，this study systematically investigated the coupled effects of curing temperature，curing pressure，testing temperature，and confining pressure on the mechanical properties of Class G oil-well cement. Specimens were prepared at different curing temperatures ranging from 25°C to 140°C and curing pressures of the atmospheric pressure and 21.7 MPa. Triaxial compression tests were then conducted at different testing temperatures from 25°C to 140°C and confining pressures from 0 to 20 MPa to analyze the evolution of stress-strain curves，peak stresses，and elastic moduli. The results show that increasing curing temperature reduces the peak stress and elastic modulus of the set cement，while high-pressure curing significantly inhibits pore development and improves structural compactness. Increasing testing temperature helps enhance the mechanical performance of cement at the testing temperature close to its curing temperature. Increasing confining pressure improves the plasticity and residual strength of cement，and the cement exhibits staged evolutionary characteristics under different coupled conditions. The findings of this study provide a theoretical basis and experimental support for the design of cement slurry systems and the integrity evaluation of cement sheaths in deep formations.

The narrow annulus in slimhole horizontal wells leads to significant differences in Cuttings transport，compared to conventional horizontal wells. To investigate the cutting transport mechanisms in slimhole horizontal wells，a CFD-based numerical model accounting for drill string eccentricity was developed for solid-liquid two-phase flow in the annulus. This study analyzed the effects of key factors，including flow rates，drill pipe rotation speeds，well inclination angles，and drilling fluid properties，on cutting transport in slimhole horizontal wells. The results showed that increasing the drill pipe rotation speed enhances the tangential and axial velocities of the annular fluid and expands the "viscous coupling" region. This facilitates the upward movement of cuttings from the lower side to the upper side of the annulus，thereby improving transport efficiency. Critical thresholds for "rotation speeds" and "flow rates" were identified in highly inclined sections，where cutting transport becomes the most challenging with the two parameters both below the thresholds，and in horizontal sections，where transport becomes more difficult with the two parameters both exceeding the thresholds. Increasing the drilling fluid density enhances the buoyancy acting on cuttings and reduces deposition. The effects of drilling fluid rheological parameters on wellbore cleanup exhibit non-linear trends，with an optimal range existing under different flow rates and rotation speeds. The findings of this research provide theoretical support for optimizing hydraulic parameters in slimhole horizontal wells and preventing issues like drill string sticking.