Research On Transient Characteristics Of Steam Injection Wellbore Heated

With the increase of the world’s demand for oil and the exhaustion of conventional oil resources,the effective development and utilization of heavy oil reservoirs has been paid more and more attention.Steam injection thermal recovery plays an important role in the development of heavy oil reservoirs.The purpose of enhancing oil recovery is to increase reservoir temperature,reduce viscosity of heavy oil and improve fluidity by injecting high temperature and high pressure steam into the reservoir.In the process of steam injection development,with the heat exchange between steam and formation low temperature environment and the influence of wellbore pipe string’s own heat capacity,steam dryness,pressure and temperature and other parameters must change with time,and the quality of instantaneous steam is crucial for hot washing of wax-bearing Wells and thermal stress of steam injection pipe string.Therefore,it is of great significance to accurately predict the transient thermal parameters of wellbore for efficient development of heavy oil reservoirs.In this paper,based on the analysis of heat transfer in the process of steam injection,the equivalent calculation method of the heat capacity of tubing,annulus and casing based on the arithmetic mean method is proposed,and the unsteady heat transfer model of the wellbore under cylindrical coordinates is established.The model is solved by the method of separation of variables,and the analytical solution of the temperature distribution equation of the borehole wall is obtained.Based on the momentum analysis of steam in the process of wellbore flow,the steam pressure calculation model and the steam dryness calculation model are established through energy analysis.Combined with the temperature distribution model of shaft wall,the calculation model of steam pressure and the calculation model of steam dryness,the transient flow and heat transfer coupling model of wellbore was established.MATLAB software was used to program and solve the model,and the accuracy of the model was verified by field test data.Will set up the transient model and the traditional model of comparison and analysis,the results show that when the steam injection time more than 7 days,in this paper,the model and the traditional model has good accuracy,but at the beginning of the steam injection(especially within 24 hours),the traditional model with heat transfer in the steady state heat transfer formula,calculation results and the actual situation gap is larger;The model in this paper considers the wellbore heat transfer as unsteady heat transfer,and the calculated results are closer to the actual situation in the field,which proves that the model in this paper has better adaptability.At the same time,the flow and heat transfer process of steam in the wellbore were simulated,and the variation rules of parameters such as temperature,pressure and dryness were obtained.The calculation results show that:with the increase of steam injection time,the time required for steam parameters to reach stability at different depths of the wellbore is different,and the deeper the depth,the longer the time required.At the same depth,the time required for each parameter of steam to reach stability is different.The time required for pressure and temperature to reach stability is shorter,while the time required for dryness to reach stability is longer.Under the change of single variable of wellhead steam injection parameter,steam injection dryness has a great influence on the time required for bottomhole steam parameter to reach stability,while the influence of steam injection pressure and flow rate is small.The formation thermal diffusion coefficient has great influence on the bottom hole steam dryness,but little influence on the pressure and temperature.In oilfield production,waxy oil Wells often need to be cleaned by high temperature hot water or steam,but the hot washing time is often short.Considering the influence of the thickness and heat capacity of the wax layer,and taking the melting heat of the wax as the criterion to judge the end of the well cleaning,the transient heat transfer model of the wellhole of the wax oil well was established and simulated.The simulation results show that the transient model can define the reasonable time of paraffin removal and well cleaning.Under the given conditions,it is recommended to clean the well for 3.37 hours at 4 t/h flow rate and 80 ~oC for a50mm wax deposit well.The time required to reach the wax washing temperature in the whole well section and the water temperature at the tubing outlet decrease with the increase of the water injection flow rate.The total well washing time decreases gradually,but the water injection required increases gradually.With the increase of water injection temperature,the time required for the whole well section to reach the wax washing temperature decreases gradually,and the water temperature at the tubing outlet increases gradually,so the total well washing time and the required water injection decrease gradually.The economic analysis of hot washing shows that the most economical way of well washing should adopt smaller flow rate and lower water injection temperature.