Stable-production period prediction method and application in high-rank coalbed methane wells：A case study of Fanzhuang-Zhengzhuang Block in southern Qinshui Basin

The prediction of stable-production period of Coalbed Methane (CBM) wells is of great significance to the prediction of reasonable stable gas rate and optimization of drainage strategy. Based on mass production data of different well types in Fanzhuang-Zhengzhuang Block, southern Qinshui Basin, the basic concepts of stable-production stage and period of CBM wells are clarified, and the empirical formula for the prediction of stable-production period is proposed and its influencing factors are analyzed. The results show that the CBM well keeps stable production by continuously reducing the bottom-hole-flowing pressure JHH, and the stable-production period is the time required for the bottom-hole-flowing pressure at the beginning of the stable-production stage decreases to the pressure of gas collection pipeline. The empirical formula proposed in this paper can effectively characterize the relationship between cumulative stable-production period and BHFP in the stable-production stage of vertical wells, L-shaped screen horizontal wells and L-shaped casing fractured horizontal wells. The final stable-production period calculation formula based on the empirical formula can accurately predict the stable-production period of each well type, with the error margin of −8.30%-8.03%. Moreover, the stable-production period is affected by many factors. First, the greater the stable-production BHFP loss coefficient, the shorter the stable-production period, and the BHFP loss coefficient in the stable-production stage is inversely proportional to the desorption pressure, while directly proportional to the BHFP loss coefficient in the production-increasing stage. Therefore, keeping the BHFP loss coefficient in the production-increasing stage below 0.006 5 d⁻¹ is conducive to long-term stable production. Second, the higher the BHFP at the beginning of stable-production stage, the longer the stable-production period, and thus a high BHFP should be maintained to stabilize even stimulate production. Third, for different wells, increasing the stable gas rate may not shorten the stable-production period, which needs to be supplemented by a reasonable stable gas rate. The calculation method for stable-production period can also be applied in the prediction of cumulative gas production under different stable gas rates, so as to provide a basis for determining the optimal stable gas rate.