Study On Temperature And Pressure Law Of Nitrogen Injection Wellbore In Tahe Oilfield

Carbonate reservoirs are widely distributed in China,with large caverns,dissolved pores and fractures as the main reservoir space,and the rock matrix part basically does not store oil and gas.For the secondary oil recovery stage in the Tahe Oilfield,the moisture content of crude oil was increased and the oil-well productivity was significantly reduced during the waterflood development.Based on the above reasons,the pilot test of nitrogen injection was started in Tahe Oilfield in 2012.The residual oil at the top of the reservoir structure was extracted by nitrogen injection,and achieved good results.However,the problem of high-pressure gas injection technology was highlighted during the gas injection process that limits the promotion of gas injection technology.In this paper,the author has carried out relevant research on the problem of difficult prediction of wellbore temperature and pressure in Tahe oil high pressure injection nitrogen technology.The main research work content:fitting out the new phase diagram of nitrogen.By comparing the equations of state,the Span-Lemmon(S-L)equation that can accurately calculate the nitrogen physical properties and thermodynamic parameters has been obtained.Through the study of the traditional unsteady model,it is pointed out that it is insufficient to consider the assumptions in the construction of the model.In this paper,according to the heat transfer theory constructs the boundary conditions and combines with the law of conservation of energy to establish an unsteady temperature model that considers both the heat conduction and the convection heat transfer coefficient.At the same time,By using the continuity equation and the equation of motion established a transient pressure drop model of the actual fluid motion.Through the finite-difference discrete mathematic model and using the "chasing method" to solve the numerical results,this model was compared with the classical E-model and the Z-model about the temperature calculation results.At the same time,according to the data in the literature analyzed the model for various influencing factors had yielded some conclusions and understanding:(1)The cubic equation of state has larger errors in nitrogen physical properties.The S-L equation with high accuracy should be used;(2)The newly established wellbore temperature model considering the heat transfer and convection heat transfer functions improves the shortcomings of the traditional model,and improves the prediction accuracy of the wellbore temperature,pressure and physical properties during gas injection;(3)Comparing the calculation results of the new model and the E-model and Z-model under different injection time conditions,it is found that the change law of wellbore temperature predicted by the new model with the E-model and the Z-model.and the predicted temperature is basically consistent.These results verified the rationality and accuracy of the new model;(4)When the injection temperature and displacement are constant,the higher the injection pressure at the wellhead,the higher the bottomhole pressure,but the relationship between wellbore temperature distribution and injection pressure is not significant;(5)When the injection temperature and pressure are constant,the increase of injection displacement and frictional resistance will increase,and the pressure at the bottom of the well will not change too much,which means that the pressure of the static gas column generated by the increased displacement and frictional effect will almost cancel each other;(6)As the injection time increases,the bottomhole temperature gradually decreases and tends to a stable value,the bottomhole pressure gradually increases and gradually tends to a stable value;in the radial direction with time increases,the wellbore radial(from the far to end)temperature is gradually reduced from the wellbore,and the disturbed formation distance is further and further away with time.It shows the characteristics of "temperature drop funnel",in which the greater temperature drop is closer to the well wall,and vice versa,"remote" is small;(7)In practice,considering to use liquid nitrogen,it is possible to sue a lower inject pressure and achieve a higher bottom injection pressure.