Temperature And Pressure Prediction Of Nitrogen-water Mixed Injection Wellbore In Tahe Oilfield

The oil-water interface has been continuously rising and the water injection failure well has been increased gradually after long-term waterflooding in Tahe Oilfield carbonate fracture-cavity reservoir.In the process of waterflooding,the residual oil in the top part of the roof is formed due to the special slot structure of tahe oilfield,which is called "attic oil".To solve the problem that the top oil is difficult to extract,the Tahe oilfild formed the nitrogen-water injection technology.This technology uses gravity difference,water and gas separation to advance,which can obtain the longitudinal sweeping coefficient that can not be obtained by the single flooding phase.Not only maintain the pressure of the reservoir,but also reduce the injection pressure at the wellhead and In field application,good results have been achieved for improving oil recovery.However,in the process of mixture injection of nitrogen and water,the two phase flow temperature and pressure coupled calculation theory is relatively few.In view of this problem,this paper carried out the following research.(1)The physical properties of nitrogen,such as density,viscosity,thermal conductivity and heat capacity under different pressure and temperature were calculated.It was concluded that the physical properties of nitrogen were greatly influenced by the temperature and pressure and could not be considered as constant during the injection process;(2)The wellbore pressures of the nitrogen-water mixed injection well under the three traditional pressure models of Beggs-Brill model,Hasan&Kabir model and Mukherjee&Brill model are calculated.It is concluded that the traditional pressure model has large calculation error,and the pressure calculation result is not stable when the gas-water ratio of injected fluid changes;(3)Based on the theory of gas-liquid two-phase flow,the injection wellbore pressure calculation model was improved.The air bubbles in the liquid slug part of the slug flow are taken into account in the calculation of the gas content of the entire slug flow,and then according to the continuity principle,the pressure drop calculation in the annular flow is converted into a pressure drop calculation at the air core.The small droplets carried in the air core are considered in the calculation of the air core density.The improved pressure model prediction results show that the error between the new pressure model calculation results and the gas-water mixed injection pressure Data in the Tahe Oilfield is within 10%;(4)Based on the consideration of two-phase flow heat transfer,a wellbore heat transfer model was established and the main parameters of the model were solved.Finally the temperature model was solved by programming.The influence of gas and liquid flow velocity on the convection heat transfer coefficient was analyzed.The results shows that the apparent velocity of liquid phase has a greater effect on the heat transfer coefficient of two-phase flow than the apparent flow velocity of gas;(5)Based on the established model,a program for the calculation of the temperature and pressure of the nitrogen-water mixed wellbore was prepared,and the actual test data of two wells in Tahe Oilfield was used to verify the example.The influence of wellhead injection conditions on the bottomhole temperature and pressure was analyzed,and it was concluded that different injection gas-water ratios had different degrees of reduction in wellhead injection pressure;wellhead injection temperature had a greater influence on the vicinity of the wellhead,but almost no effect on the deep well temperature,and the injection temperature has little effect on bottom hole pressure;the injection pressure significantly affects the bottomhole pressure,but has almost no effect on the bottomhole temperature;the greater the injection velocity,the larger the bottomhole pressure and the lower the temperature due to the high flow rate does not allow the fluid to fully exchange heat with the formation;The validity of the wellbore temperature and pressure prediction model is illustrated by Tahe Oilfield's cases.The establishment of the injection well temperature and pressure prediction model provides a theoretical basis for the optimization of nitrogen-water injection parameters and a reasonable working system in Tahe oilfield.