Calculation Of CO₂ Injection Wellbore Parameters In Tahe Oilfield

Due to the special geological structure,Tahe Oilfield carbonate fractured-vuggy reservoir has strong heterogeneity and the fracture is the main reservoir and flow space.Because of this complex geological structure,it's a big challenge for developing Tahe Oilfield,especially in later stage of secondary oil recovery,the effect of water injection is becoming worse,the moisture content increases and the oil well productivity decreases.Especially because of the special slot element structure in Tahe Oilfield fractured-vuggy reservoir,a large amount of"attic oil" between wells has not been used,for this part of the remaining oil which can't be spreaded by the injected water,we can consider CO2 injection technology:on the one hand,using the gravitational differentiation,the injected CO2 rises to the high place and occupies the high position of reservoir space,forms "gas cap".It can complement formation energy,push the crude oil flow down,so as to improve the development effect;on the other hand,by using the special physical properties of C02,it can contact with crude oil in miscible or immiscible phase,the viscosity of crude oil is reduced and the mobility ratio is improved,so the crude oil is more easily extracted.There are two main directions of research on carbon dioxide gas injection recovery technology:the gas injection mechanism in reservoir engineering and the gas injection process in oil production engineering.The gas injection process firstly realizes the downhole injection allocation(parameter)submitted by reservoir engineering.In the gas injection process,the analysis and calculation of parameters from the wellhead to the downhole is a daily work of gas injection well production control.Therefore,the following work is carried out in this paper.(1)On the basis of Ramey's classical wellbore heat transfer model,the injection well temperature model was established,and the wellbore pressure model was established according to the fluid continuity equation,the momentum conservation equation and the energy conservation equation.(2)The relevant thermophysical parameters in the model,such as friction coefficient,dimensionless heat transfer function and wellbore total heat transfer coefficient,are derived and calculated.Meanwhile,in order to improve the accuracy of the model,the calculation methods of various physical parameters of CO2 were optimized and calculated.(3)The temperature and pressure coupling model was programmed and solved,and was verified by a actual CO2 injection well.By comparing with the wellbore data measured on site,it was found that the model had a good fitting effect and could accurately predict the wellbore temperature and pressure distribution.(4)The sensitivity of wellbore and bottom hole temperature and pressure is analyzed by control variable method.The results show that the injection velocity has the greatest influence on the wellbore temperature,followed by the wellhead injection temperature.For wellbore pressure,the injection pressure at the wellhead has the most obvious influence,followed by the injection speed,which has obvious influence near the bottom of the well.(5)Choose A block in Tahe Oilfield as the research object,from the perspective of reservoir engineering,through the relative permeability curve and the formation water absorption data computing inhale index to determine formation inhale capacity,and according to the statistics experience and relative permeability curve method to determine the relationship between the daily injection rate and the the bottom hole pressure,according to the scope of reasonable pressure difference get the best injection rate for each Well;Taking S-5 well as an example,at the best injection rate,the relationship between bottomhole pressure and wellhead injection pressure was calculated based on the established wellbore temperature and pressure model,and the optimal wellhead injection parameters of CO2 injection Well were finally obtained.Through theoretical analysis and example calculation,this paper achieved indirect bottomhole pressure monitoring,clarifies the determination method of injection parameters at the well head,and provides theoretical basis and analytical means for the production management of gas injection Wells in Tahe Oilfield.