Abstract: Given the high difficulties in formation damage prevention, low efficiency of well clean-up and high risks of well control of well completion and workover of horizontal openhole gas wells, a novel mechanical reservoir isolation valve was developed. This mechanical ball valve with repeatable switching is opened and closed by a specialized switching tool. The isolation system composed of the reservoir isolation valve and suspension packer forbids both the downward flow of fluids above the ball valve and upward flow of fluids below the ball valve and gas in the reservoir. The ball valve sealing performance, switch operation and ball valve re-entry sealing performance of the reservoir isolation valve were investigated via laboratory experiments, which validates the safety and performance reliability of the invention. Field testing demonstrated that the presented tool, with reasonable structures and reliable performance, can achieve zero leakage of working fluids and deliver high efficiency of well clean-up and cost-effectiveness of operations. This invention provides a safe and effective technical approach for life-cycle formation damage prevention in oil and gas wells.