Abstract: After two rounds’ exploration in a certain deep low porosity, low permeability-extra low permeability gas reservoir, high-yield wells are achieved, but no large-scale breakthrough is realized during the late exploration. In the encountered deep layers of friable hard mudstone and volcanic rock, the wellbore stability is poor and complex downhole accidents happen, e.g. collapse, drop and leakage. The well test and fracturing stimulation results are not satisfactory and it is difficult to discover and liberate oil and gas reservoirs. It was proposed to carry out window sidetracking in old wells and ensure the design idea of integration engineering of well drilling and completion while paying attention to reservoir stimulation for reservoir protection and efficient development as much as possible. Then, the sand production possibility of the reservoir and the critical sand production pressure difference were predicted based on the logging data, and combined with the well completion risk evaluation and the reservoir stimulation requirements, it is determined that the composite well completion mode is better suitable for sidetracked holes. Tri-pressure profile prediction was conducted on the strata where sidetracked holes were located. It is revealed that the formation collapse pressure is higher than the pore pressure, so underbalanced drilling tends to result in wellbore collapse. Finally, the idea of only being used for reservoir in the target layers was put forward, it was demonstrated that the liquid-phase accurate managed pressure drilling is superior to the underbalanced air drilling, and the liquid-phase drilling fluid system with the property of reservoir protection was prepared. As for reservoir stimulation, it was recommended to carry out open-hole commingled fracturing in the lower reservoirs. The postfrac productivity prediction reaches the target of stimulation.