Theoretical Model And Scheme Optimization Of Intermittent Heat Injection In Middle And Late Stage Of Steam Flooding

The development and utilization of heavy oil resources play an important role in the petroleum industry.Steam flooding is a relatively matured and widely used heavy oil production technology.However,the phenomena such as steam override and channeling often occur in the middle and late stage of steam flooding,which leads to uneven steam suction profile and ineffective steam circulation in the reservoir.These phenomena affect the production effect and need to be dealt with by measures.Field tests and numerical simulation results showed that the technology of intermittent heat injection is an effective method to improve steam displacement effect,but further study is needed to understand its theoretical basis and implementation scheme.Based on this,the theory of intermittent heat injection in the middle and late stage of steam flooding and the implementation scheme were studied.The main tasks accomplished and the main conclusions obtained are as follows:The research on decreasing heat injection and its implementation mode in the middle and late stage of steam flooding were carried out.In the early stage of steam flooding,a high constant rate of heat injection is usually adopted,while in the middle and late stage,it is generally necessary to adjust the heat injection scheme to improve the efficiency of steam thermal utilization and the economic benefit of steam flooding.Based on the research results of Neuman et al.and combined with the analysis of the heat compositions of the injected steam and their contributions to the growth of the steam zone,the reasonable constant heat(steam)injection rate equation in the early stage of steam flooding and the decreasing heat(steam)rate equation in the middle and late stage of steam flooding were derived.The specific operation of decreasing heat injection can be selected from the potential modes of continuous decreasing heat injection,step decreasing heat injection and intermittent heat injection.The mode of intermittent heat injection has the advantages of improving the steam flooding effect.A theoretical study on intermittent heat injection in the middle and late stage of steam flooding was carried out.Maintaining the stability of the steam zone in reservoir is the constraint for the method of intermittent heat injection steam flooding.According to the energy conservation law,during the period of stopping heat injection,the heat released by the steam zone due to the temperature decrease during the period of stopping heat injection is equal to the sum of the heat loss at the upper and lower boundaries of the steam zone and the heat consumed by the steam zone growth.With this,it was proved that the stability of steam zone in the reservoir after stopping heat injection during steam flooding can last for a certain time,so the technology of intermittent heat injection is theoretically feasible.It was proved theoretically that the volume of steam zone in the middle and late stage of steam flooding is only related to the total amount of heat injection,but not to the heat injection process.This shows that the intermittent heat injection can be adopted to implement the decreasing heat injection in the middle and late stage of steam flooding.Combined with the volume expression of steam zone at the middle and late stage of steam flooding,the equation of time length of stopping heat injection was derived for the intermittent heat injeciton cycles at the middle and late stage of steam flooding.Based on the principle that the total heat injected in the intermittent heat injection mode should be equal to total heat needed to be injected by the decreasing heat injection law at the middle and late stage of steam flooding,the equation of heat injection rate at the injection stage in the intermittent heat injection cycles was derived.According to the theoretical equations obtained,computer program was developed to calculate the parameters of intermittent heat injection.Combining with the properties of the field test well group and operation parameters in A block of Liaohe oilfield,the theoretical calculations were carried out.The calculation results show that the volume of steam zone increases gradually with the increase of steam flooding time,and the stopping heat injection time of intermittent inj ection cycle increases gradually.If intermittent heat injection method is implemented during the 3-6 year period,the average stopping time calculated is 43.98d,and the periodic heat injection rate is 1.5 times of the constant inj ection rate used in the early stage of steam flooding.For the intermittent heat injection test in A block,the stopping time is 30-40d,and the periodic heat injection rate is 1.2-1.5 times of the average rate of the early stage continuous steam injection.The field test parameters are in good agreement with the theoretical calculation parameters.Physical simulation experiments of intermittent heat injection steam flooding were carried out to compare the effects of steam flooding modes of continuous constant heat injection,continuous decreasing heat injeciton and intermittent heat injection.The results showed that the recovery degree of the continuous decreasing heat injection steam flooding is lower than that of the continuous constant heat injection steam flooding.But the cumulative oil-to-steam ratio of the continuous decreasing heat injection is higher than that of the continuous constant heat injection,which indicates that the steam utilization efficiency of the continuous decreasing heat injection is higher than that of the continuous constant heat injection steam flooding.With the same amount of heat injected,the recovery degree of intermittent heat injection is 2.24%higher than that of continuous decreasing heat injection,and the cumulative oil-to-steam ratio is 0.012 higher than that of the continuous decreasing heat injection,which indicates that the effect of intermittent heat injection is better than that of the contious decreasing heat injection.After stopping heat injection for the time period calculated with the obtained theoretical equation,the volume of the steam zone did not shrink sharply.With the same amount of heat injected,the steam zones of intermittent heat injection and continuous decreasing heat injection have basically the same size.These experimental results proved that the theoretical model of intermittent heat injection in the middle and late stage of steam fl ooding established is correct.Numerical simulation results show that with the same amount of heat injected,the recovery degrees of the intermittent heat injection schemes by alternating the well rows perpendicular to each other,by alternating well rows parallel to each other and by alternating the wells in the whole block are all higher than that of the continuous constant heat injection scheme,with a value of 0.44%,0.54%and 0.73%respectively.The recovery effect of the intermittent heat injection becomes better with the increase of intermittent influence degree.The whole block intermittent heat injection may cause the oil field the actual production problem,such as insufficient liquid supply.With comprehensive consideration of recovery effect and actual situation of oil field,the parallel alternate intermittent heat injection is recommended as the scheme to implement the intermittent heat injection in the middle and late stage of steam flooding.