Abstract: During the process of developing carbonate reservoirs in nature reserves using "from the outer to the inner" extended reach cluster horizontal wells, such challenges as limited well site area, prone formation leakage and collapse, difficult trajectory control, and large friction and torque are generally faced, resulting in low efficiency and long cycle in directional drilling operations. To address these issues, on the basis of combined well-seismic technology and four-pressure profile analysis, straight-line cluster wells and a four-stage wellbore structure with high-level sidetracking were designed to effectively avoid or isolate complex formations. To solve the leakage and collapse, a tailored deformation or rigid particle sealing formula compatible with oil-based drilling fluid was selected, which increased the indoor bearing capacity tenfold. Aiming at the difficulty in trajectory control, a seven-segment profile design and rotary steering speed-up process were adopted, achieving a 100% target accuracy within a radius no more than 30 meters. To improve the cutting carrying efficiency, the low-viscosity and high-yield-point performance of drilling fluid was optimized, and "large and small wellbore rule" was utilized to quantitatively adjust the rotary speed and six-speed readings, reducing the frictional torque by 48%. Ultimately, an optimal and fast drilling technology system centered on improving directional drilling efficiency and reducing friction and resistance was formed. This technology was applied in 12 wells, shortening the average drilling cycle by 26 days. The actual drilling data show that the horizontal displacement of Ø149.2 mm borehoe achieved 3,473.5 m, and the depth reached 9,432.6 m, which present the leading onshore horizontal well at home.