Productivity prediction and production performances of deep coalbed methane horizontal wells throughout full life cycle

The deep coalbed methane (DCBM) reservoir is mainly composed of desorbed gas and meanwhile rich in free gas with the characteristics of high gas saturation. During the development of DCBM fields, changes of both reservoir pressure and gas saturation can result in production performances of DCBM wells, thereby influencing their productivities. Based on the Langmuir adsorption isotherm model, this work establishes a transient productivity model of gas-water two-phase and two-zone flows in DCBM fractured horizontal wells after consideration of the content of desorbed gas and free gas changing with the production time. Herein, gas well in Daning-Jixian Block on the Ordos Basin is used to validate this proposed model. Subsequently, comparisons of gas production rate, cumulative gas production and dynamic contribution rate including desorbed gas and free gas of Well JS-1 for 10 years in DCBM reservoirs are implemented, respectively. The results show that the daily gas production of DCBM wells increases rapidly at the early stage. After reaching the peak, it drops rapidly and then gradually declines slowly over the following two years. The daily gas production is composed of free gas and desorbed gas. At the early stage, the contribution rate of free gas is considerably high, while at the later stage, the contribution rate of desorbed gas is higher. The average shares of desorbed gas and free gas production are about 80% and 20% in the daily gas production throughout the entire period of 10 years, respectively. Furthermore, the cumulative gas production increases with the production time. The growth rate of cumulative gas production is faster at the early stage and slows down at the later stage. At the early stage, the contribution rate of cumulative free gas is relatively high in the cumulative gas production. After two years, the contribution rate of cumulative desorbed gas in the cumulative gas production exceeds that of cumulative free gas. Cumulative desorbed gas contribution is 63.54% and cumulative free gas contribution is 36.46% after 10 years. Hence, this model can be used for a judgement of well production performances and dynamic contribution rates including desorbed gas and free gas in DCBM fields throughout full life cycle. Besides, the method in this work has accurate, reliable and fast advantages, and provides theoretical and technical supports for efficient development of DCBM reservoirs.