Drainage Characteristics of Concentric Tube Gas Jet in CBM Wells
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 pres-sure 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 diame-ter,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 m3/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 9 338,11 225 and 15 023 m3/d respectively.The findings on the drainage mechanism and characteristics of gas jet pump provide the-oretical basis for the technical design and field test of concentric tube gas jet pump.
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