Research And Application Of Simultaneous Optimization Method Of Both Time And Injection-production Rate For Water-flooding Reservoir

Nowadays the oil resources in China are very scarce with the expanding demand gap for the crude oil.Most of the exploration process for oil field reservoirs in China have entered into high water cut period,represented by Daqing oilfields and Shenli oilfields.The oil production in the reservoir is declining and the cost of water displacement is increasing.The conventional adjustment and control methods for water injection,such as injection rates optimization and Bang-Bang type regulation,etc,have been unable to meet the increasing demands for practical engineering.It is of great social and economic value to reasonably allocate water injection distribution,optimize and make appropriate oil wells' operating strategies,improve the producing degree of reservoir,and ameliorate the exploitation effect of water-flooding oilfields.In order to further improve the water displacement efficiency in each stage of oilfield development,this thesis proposes the simultaneous optimization of both segmented time and injection-production rate,and the numerical simulations have been carried out to study the optimization efficiency of the proposed simultaneous optimization method for water-flooding reservoir.By establishing conceptual model and calculating realistic reservoir model in Shenli Oilfield,it validates the significant effect on improving cumulative oil production and net present value(NPV)for the simultaneous optimization method.This thesis applies the proposed simultaneous optimization method to the study of the oil production optimization efficiency of Bang-Bang control.The Bang-Bang control's optimization efficiency at various stages of production process and its certain influencing factors are studied for water flooding reservoir,where the simultaneous optimization method with both segmented time and injection rate is solved as the reference solution for Bang-Bang control.Certain influencing factors have been taken into consideration which include the boundary effect,oil-water displacement characteristics,the geometry features of wells' location,such as well's density,distance ratio and angle.The specific contents and conclusions are summarized as follows:The simultaneous optimization of both segmented time and injection-production rate can further improve the cumulative oil production and NPV for the reservoir at various stages.It also enhances the efficiency of water displacement and achieves the exploitation effect of increasing and stabilizing oil production.Under the condition of fixed total water injection rate,the production process is divided into several segments.To consider the correlations between the time intervals of the segment and the wells'injection rate or production rate distributions,the segmented time of each segment and each well's water injection or production rate are optimized simultaneously to maximize the cumulative oil production in the complete production process.Compared with other conventional optimization schemes,such as injection rate optimization scheme,segmented oil production optimization scheme,cumulative oil production optimization scheme and Bang-Bang regulation scheme,etc,the simultaneous optimization method is based on global optimization method which take the segment time of each production process and each well's injection or production rate in each production process as correlative control variables.If both the number of segmented production process and total water injection rate are fixed,the solutions of the simultaneous optimization of both segmented time and injection rate is the optimal strategies for segmented water injection optimization problem.The simultaneous optimization method can not only achieve increasing effect on exploitation by improving the cumulative oil production and NPV at various stages of production process,especially at high water cut period,but also insure the effect of stabilizing production and increase continuous development efficiency by making the remaining oil saturation distribution more uniform.The simultaneous optimization of both segmented time and injection-production rate can insure exploitation effect in circumstances of fewer number of production process divisions.The proposed method can significantly reduce the cost of regulation and equipment maintenance in the practical application of the engineering,in the meanwhile greatly decrease the calculation time of the optimization method,with relatively higher calculation efficiency.The results show the proposed method with 2-segment division and 4-segment division for production process have almost the same improvements of NPV.The simultaneous optimization method can achieve similar stimulation effect on oil production and water-flooding efficiency with fewer production process divisions.For engineering practice,the simultaneous optimization method can reduce the frequency of adjusting the working parameters of oil wells,simplify the decision-making of oilfield optimization scheme and the operation procedures in oilfields exploitation prominently,which effectively reduce control cost and maximize the economic benefits of the whole oilfield system.Based on the optimal solutions of NPV with the simultaneous optimization method of both segmented time and injection rate,the optimization efficiency of Bang-Bang control at various stages of production process is studied.Simulation results show that in early stage,Bang-Bang control keeps the optimal regulation strategies,which are same with the solution of simultaneous optimization method.In middle stage,Bang-Bang control may deviate from the reference solution,accompanied with oil production efficiency decreasing.During high water cut period,there still exist oil production difference between Bang-Bang control and simultaneous optimization method.Based on the optimal solutions of water injection rates with the simultaneous optimization method of both segmented time and injection rate,certain factors have significant influences upon the optimization efficiency of Bang-Bang control,which include boundary effect,oil-water displacement characteristics and the geometry features of well's location.The influencing rules of these factors on Bang-Bang control have been summarized below.Firstly,for the small-block oilfield influenced by boundary effect,there exist the critical distance ratio between wells and Bang-Bang control's efficiency will decline when distance ratio is larger than critical value.Secondly,for the large-block oilfields that are less affected by boundary effect,in the case of piston-like displacement,the efficiency of Bang-Bang control will not be affected by distance ratio and angle.For non-piston-like displacement,there are both critical distance ratio and critical angle where Bang-Bang control's efficiency will decline when distance ratio is less than the critical ratio value or angle is larger than the critical angle value.In summary,this thesis has proposed a simultaneous optimization method of both segmented time and injection-production rate,and studied the optimization efficiency and displacement effect of the proposed method in optimization problem of oil production for water-flooding reservoir.The application of simultaneous optimization method to the study of Bang-Bang control's optimization efficiency is illustrated.Calculation results show that the simultaneous optimization method can further improve the cumulative oil production and NPV at various stage of oilfields development,enhance the efficiency of water displacement and achieve the exploitation effect of increasing and stabilizing oil production,which have important reference value for practical reservoir exploitation.The optimized solutions of simultaneous optimization method are also applied to the studies on Bang-Bang control's optimization efficiency.In early stage,Bang-Bang control keeps the same optimal regulation strategies with simultaneous optimization method.In middle stage and high water cut period,Bang-Bang control may deviate from the reference solution,accompanied with oil production efficiency decreasing.Certain factors have significant influences upon the optimization efficiency of Bang-Bang control,including boundary effect,oil-water displacement characteristics and the geometry features of well's location,which have a very important influence on the study of the production laws of Bang-Bang control in practical engineering.