Study On Optimization Of Process Parameters For Gas Injection Huff And Puff Reproduction Technology Of Low-pressure Gas Wells

Gas wells in the old gas fields of southwest oil and gas fields have long been in a corrosive environment with acidic gas and liquids,which has caused serious damage to gas well tubing and it has been difficult to maintain production.The tubing replacement operation requires conventional well killing.The low pressure and good permeability of some old well formations results in the well fluid being liable to leak,causing bottom hole reservoir contamination and even difficulty in completing well killing.After the use of temporary plugging agent killing process,the problem of well killing has been solved.However,after the killing operation,the liquid produced by the temporary plugging agent gel is trapped in the bottom-hole reservoir to cause plugging.When the well is resumed production,the formation gas cannot rely on the formation energy to break through the pollution belt at the bottom of the well.Due to the restrictions on wellbore structure,plugging characteristics and production efficiency,it is difficult to apply the conventional production process.In response to the difficulties in the production of low-pressure gas wells,a gas-injection,reciprocating,production process was proposed.The re-production process is to displace the liquid in the formation near the bottom of the well by means of surface gas injection until the injection of natural gas breaks through the closure of the liquid phase,and when the gas is connected to the formation gas and has sufficient connectivity,the gas injection is immediately stopped and the well is opened..After the well was opened,the high-pressure gas phase was released with gas expansion energy and formation pressure,and the gas-liquid two-phase flow was pushed into the inflow wellbore to achieve fluid recovery of the gas well.The process relieves the liquid blockage by gas injection,restores the gas phase permeability of the plugged area,and provides a flow path for the formation natural gas to flow into the wellbore.The process is effective but inefficient after it is put into use.The analysis of the principle of the re-production process is known.When the connectivity meets the production recovery of the gas well,the well can be resumed.The connectivity at this time is the critical condition for the gas well to resume production.If the gas injection time is insufficient,the gas-phase connectivity is not good enough,and the critical conditions for gas well production resume are not reached.After the gas injection is stopped,the gas-phase channels may be closed and occupied by the liquid phase to form secondary plugs,resulting in the failure of gas well production.If the gas injection time is too long,gas phase connectivity is increased,but resources are wasted and the cost of production restart is increased.Therefore,the gas injection time is an important factor in controlling the gas phase connectivity.Combined with the analysis of the process flow,the gas injection pressure and gas injection volume can control the gas injection time.At the same time,the gas injection time is also related to the stress of the fluid in the wellbore and stratum,indicating that some factors in the wellbore and formation will affect the gas injection time..By comparison,the most direct and effective method to improve the production efficiency of low-pressure gas wells is to optimize the process parameters of the gas injection and recovery production process,namely the gas injection pressure and gas injection volume.To this end,the research on optimization of process parameters for gas injection and resumption production is very important for high-efficiency production of low-pressure gas wells and potential tapping of old wells.This study combines the process flow and process principle of the gas recovery and recovery production process,determines the process parameters,establishes the physical model,mathematical model and optimization model;and uses practical examples of gas well data.The main work and achievements of this research are as follows:(1)Starting from the principle of the reproduction process and combined with the reproduction process,the influence of various parameters on the productivity of gas wells is analyzed.Using AHP to analyze the weights of influential factors,it's determined that the gas injection pressure,gas injection volume,fluid viscosity,pore radius,porosity,reservoir thickness,formation pressure,and wall roughness are the main influencing factors.According to the difficulty of actual construction,adjusting parameters to make sure of gas injection pressure and gas injection volume as optimization parameters is carried out.(2)Through analyzing,the parameters that characterize the connectivity of injected natural gas and formation natural gas with fluid saturation are determined.In combination with the process flow and the identified major influencing factors,a physical model of the reproduction process is established.(3)According to the physical model,the mathematical model related to the main influencing factors and the gas injection distance is established.The Fluent simulation is used to simulate the process of reproduction,and the linear regression is performed on the simulated data,resulting in that the mathematical relationship between the gas injection distance and the time of reproduction is obtained.Mathematical relations are brought into the mathematical model to obtain an optimized model of the reproduction technology.(4)The optimized model was used to optimize the process parameters of the well S1 in the construction well.The gas injection pressure was 15 MPa,the gas injection volume was 2.1×10~4m~3/d,the actual gas injection pressure was 13.5 MPa,the gas injection volume was 2.6×10~4m~3/d,and the final gas well production resumed successfully.The error between actual reproduction time and forecast time is 10.7%.