煤层气井同心管气体射流排采特征分析

Drainage Characteristics of Concentric Tube Gas Jet in CBM Wells

为解决煤层气井中有杆泵排采工艺存在的埋泵、卡泵及偏磨等问题,实现产气中后期降低井底流压、释放解吸气的目的,探索煤层气井中后期低产液量情况下的排采工艺至关重要。基于可压缩性的气体介质,考虑射流泵排采工艺的技术特点,分析了煤层气井井下同心管柱气体射流泵排液原理,计算管柱环空内气携液条件,建立了同心管气体射流泵喷嘴、喉管和扩散管等流动模型,考虑气体射流的音障,分析了不同喷喉组合下的携液特征。研究结果表明:初始注入气量、压力一定的情况下,喷嘴直径越小,射流速度越大,携液量越多;但喷嘴直径越小,所需的注入气量越小,气体的流速易超过临界流速导致流体特性发生改变;当产液量为3 m^(3)/d,注气压力为6 MPa,喷嘴直径为3.50、4.02、5.00 mm时,所需的注入气量分别为9338、11225、15023 m^(3)/d。气体射流泵排液机理、气体射流特征的研究结果可为同心管气体射流泵工艺设计和现场试验提供理论依据。

During drainage by sucker rod pump in coalbed methane(CBM)wells,the pump may be buried,stuck and eccentrically worn.For reducing the bottomhole flowing pressure and releasing the desorbed gas in the mid-late stage of gas production,it is crucial to explore the drainage process for obtaining lower bottom hole pressure with low liquid production in the mid-late stage of CBM well production.Based on compressible gas medium,and considering the characteristics of drainage by jet pump,the drainage principle of downhole concentric tube gas jet pump in CBM well was analyzed,the liquid carrying conditions of gas in the annulus of string was calculated,and the flow model of the nozzle,throat and diffusion bell of the concentric tube gas jet pump was built.Taking into account the sonic barrier of gas jet,the liquid carrying characteristics under different nozzle-throat combinations were analyzed.The results show that,given initial gas injection volume and pressure,the smaller the nozzle diameter,the greater the jet velocity,and the more liquid is carried.Otherwise,the smaller the nozzle diameter,the smaller the gas injection volume is needed,the more easily the gas flow rate exceeds the critical flow velocity,thus leading to a change in fluid behavior.When the liquid production is 3 m^(3)/d,the gas injection pressure is 6 MPa.When the nozzle diameter is 3.50,4.02 and 5.00 mm,the required gas injection volume is 9338,11225 and 15023 m^(3)/d respectively.The findings on the drainage mechanism and characteristics of gas jet pump provide theoretical basis for the technical design and field test of concentric tube gas jet pump.