| Large-scale sand fracturing is the key technology for effective utilization of shale gas.In the practice of shale reservoir fracturing,it is found that the wellbore is in gas-liquid two-phase flow for a long time after fracturing and the fracture is easy to produce sand.At present,the study of proppant reflux in shale gas wells is mainly focused on single-phase gas flow and control technology.The gas-liquid two-phase flow state in fractures and proppant reflux mechanism are basically blank.Therefore,around the mechanism of gas-liquid two-phase flow proppant reflux in shale gas wells,the pressure state of proppant in fracture,the pressure distribution of gas-liquid two-phase flow in wellbore and the dynamic characteristics of wellhead chock are systematically studied in this paper.With pressure as a link,the coupling model of fracture-wellbore-chock is established to control fluid pressure and reasonable production speed by changing the size of surface chock,so as to prevent the backflow of proppant.Firstly,this paper assumes that the crack end is a supporting arch formed by the consolidation of proppant.By considering the effects of capillary force,crack closure pressure and gas-liquid drag force on the stability of the supporting arch,combining with the failure criterion of the supporting arch and the high-speed non-Darcy flow equation of fluid in the crack,a gas-liquid two-phase proppant reflux model is established.The classical "startup of sand" model and experimental data are used to verify the model,and the influencing factors of critical velocity are analyzed.Secondly,the pressure drops calculation models of horizontal,inclined and vertical sections of gas-liquid two-phase flow in wellbore are studied.The pressure drops along the inclined section is calculated accurately by using the improved position iteration method combined with the idea of micro-element,and compared with the field data.Finally,Gilbert,Ros and Achong classical chock flow models are used to fit and analyze the flow behavior of two-phase chock in shale gas wellhead,and a three-part coupling model of fracture-wellbore-chock is established based on the pressure and flow continuity equation.Through the relevant research,the following conclusions and understandings are obtained:(1)By considering the effects of capillary force,crack closure pressure and gas-liquid drag force on the stability of proppant,a proppant reflux model for gas-liquid two-phase flow in cracks was established and the critical reflux velocity was calculated.The results show that the critical velocity of diamond arrangement is the highest,followed by square arrangement and the lowest;the stability of supporting cracks is proportional to the critical velocity;the critical velocity increases with the increase of closing pressure and particle diameter;the stability of supporting arch decreases with the increase of temperature,water saturation,production pressure difference,viscosity and surface tension after gel breaking of fracturing fluid;and the stability of supporting arch is the main factor.Influenced by proppant particle diameter,proppant arrangement,water saturation,differential pressure,closure pressure and fracturing fluid viscosity,it is less affected by fracture width,temperature and surface tension.(2)By optimizing the classical pressure drops model of gas-liquid two-phase flow in shale gas wellbore,a pressure drops calculation model of gas-liquid two-phase flow in shale gas fracturing horizontal wellbore is established according to the characteristics of typical horizontal,inclined and vertical two-phase flow.Based on the mechanism of gas-liquid two-phase flow and the idea of micro-element,the pressure drops of gas-liquid two-phase flow and the pressure drops of vertical section are calculated more accurately by position iteration method than those of predecessors.The calculation results show that the pressure drops of gas-liquid two-phase flow can reach several or even more megapa in the whole shale gas wellbore,which has great influence on the optimization results of shale gas backflow parameters and can not be ignored;the pressure drops of gas-liquid two-phase flow in shale gas wellbore mainly concentrates in the vertical section,accounting for 70%of the total pressure drop;the potential energy pressure drops accounts for the largest proportion,accounting for about 90%of the total pressure drops in the vertical section,while the friction resistance.Pressure drops accounts for about 10%.Well bore pressure drops increases with the increase of tubing roughness,gas production and fluid production,and decreases with the increase of tubing diameter.(3)On the basis of Gilbert,Ros and Achong chock flow equation,combined with well H13-5 flowback and production data,a new model of gas-liquid two-phase chock flow in Changning area is established by using two-dimensional multiple least squares method.With pressure as a link,a coupling model of fracture-wellbore-chock in shale gas wells is established,and the optimization program of chock size for preventing proppant reflux is programmed and calculated.In the process of back drainage after fracture closure,given gas-liquid ratio and gas production,the maximum critical production and optimal chock size for preventing sand production in gas wells can be calculated effectively.The results show that the critical chock size decreases with the increase of wellhead oil pressure,fracture width and fracturing fluid viscosity,and the critical gas production decreases accordingly.The critical gas production increases with the increase of proppant particle diameter,closing pressure and gas-liquid ratio,and the size of wellhead oil chock varies greatly with wellhead oil pressure,gas-liquid ratio and closing pressure.Viscosity changes slightly. |
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