Optimal Design And Process Technology Research Of Variable Diameter Gas Lift Plunger

In the middle and late stages of gas well production,gas well productivity begins to decline and bottom-hole fluid accumulation increases.The offshore gas well string uses safety valve to ensure the safety of gas well production.The inner diameter of the safety valve is 58.72 mm and the inner diameter of the tubing is 62 mm.Therefore,the traditional plunger cannot adapt to this combined pipe string with variable diameter.In view of the above problems,this paper designs a new variable diameter gas lift plunger tool based on the principle of turbulent sealing.The tool can maintain the reduced diameter state in the tubing to smoothly pass through the safety valve,and can quickly descend to shorten the shut-in time;maintain the expanded diameter state when ascending to reduce the leakage of fluid accumulation and the channeling of gas.It has the characteristics of simple structure,low manufacturing cost,convenient maintenance and high economic benefit.The main research results of this paper are as follows:（1）Structure design of variable diameter gas lift plunger.Firstly,the principle of plunger diameter reduction is analyzed,and three-dimensional modeling software is used to design the structure of the variable diameter plunger tool,and calculate the specific structure size.It is concluded that the maximum outer diameter of the plunger is 56 mm when the diameter is reduced,the maximum outer diameter of the plunger is 59 mm when the diameter is expanded,the total length of the plunger is 630 mm,and the weight is 5kg.Secondly,it is proved by experiments that when a certain axial force is applied,the plunger can achieve diameter expansion;when the axial force is removed,the plunger returns to the reduced diameter state.（2）Research on sealing performance of variable diameter gas lift plunger.Using the CFD method,the resistance coefficient,the axial pressure drop of the plunger and the leakage amount are used as evaluation indicators to establish the sealing structure of different plunger rectangular grooves.It is concluded that when the rectangular groove sealing structure adopts groove depth of 6 mm,groove width of 7.5 mm,and the number of grooves 9,the turbulent sealing effect is the most obvious;the depth of the safety valve is 204 m from the ground,and when the plunger reaches the ground in the expanded diameter state,The leakage amount is 1.04×10^-3 m³;the leakage amount when the plunger reaches the ground in the reduced diameter state is 1.98×10^-3 m³.When the diameter is expanded,the leakage is reduced by 48%,so the expansion of the plunger has a significant effect on reducing the leakage of the lifting liquid.（3）Research on the process mechanism of variable diameter plunger.Firstly,the specific process of plunger drainage and gas production is analyzed;secondly,it is calculated that the plunger needs 2.97MPa pressure at the bottom of the well to achieve diameter expansion,and the spring is selected;finally,the applicability of plunger gas lift technology is analyzed,and the plunger tool string passes In the case of highly deviated wells and horizontal wells,the change rate of well inclination is mainly considered.When the inner diameter of the tubing is62 mm,the outer diameter of the plunger is 59 mm,and the length of the tool string is 8 m,the change rate of the well inclination cannot exceed 15°/100 m.（4）Research on the influencing factors of plunger gas lift.The single-factor and multi-factor methods are used to analyze the influence of formation pressure,gas-liquid ratio and well inclination on gas wells.It is concluded that the gas-liquid ratio and formation pressure are important dynamic factors affecting the gas production of gas wells.With the increase of gas-liquid ratio and formation pressure,the gas production of gas wells increases continuously.（5）Optimal design of plunger gas lift process.According to the basic parameters of the CX-A9 gas well in the Can Xue oil and gas field in the East China Sea,and using the gas well process calculation model,the after-flow well opening time and well opening oil pressure were optimized.It is concluded that the optimal gas lift process scheme for this gas well is oil pressure3.5 MPa,daily plunger cycle times 14.56 times,well opening time 42 min,and periodic gas production 3318.64 m³.