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Water breakthrough in horizontal wells is now the main factor restricting the stable production of the Tainan Gas Field in the Qaidam Basin. In view of this problem, the distribution characteristics of irreducible water saturation were investigated by using the nuclear magnetic resonance logging interpretation technology. And combined with the production situations of the horizontal wells in Tainan Gas Field in the initial stage of their production, the characteristic parameters of the reservoir which produced the intrastratal water as soon as it was put into production were determined. Then, the factors influencing the production of irreducible water were studied by means of gas drive water core experiments, and the factors influencing the sealing ability of the interbeds were researched by conducting mudstone breakdown tests. What's more, the effects on the bottomhole pressure by the length of horizontal section at different daily gas productions were investigated through numerical simulation. Finally, the prevention & control countermeasures for water breakthrough in horizontal wells were proposed. And the following research results were obtained. First, the horizontal well which drills into the reservoir with mobile water saturation higher than 7.2% and gas saturation lower than 63.5% produces formation water in its initial stage of production. Second, the lower the shale content is and the greater the production pressure difference is, the more favorable it is for the production of irreducible water. The production of irreducible water in the reservoirs with stronger areal heterogeneity is a continuous process. Third, the sealing capacity of the interbed increases with the decrease of its vertical permeability and water saturation and with the increase of its shale content and thickness. Fourth, the breakthrough pressure of type I mudstone (shale content > 90%) is about 4 MPa, that of type II mudstone (80%