As a consumable resource,petroleum is the main energy and chemical material in the world today.It plays an inestimable role in promoting the development of national economy,ensuring social progress and safeguarding national defense security.It is particularly important to accurately describe the development of oil reservoirs and improve the efficiency of petroleum resource collection.Injection water development is the main way of oil field exploitation in China.As injected water continuously erodes the pore structure and the connected channel between injection and production Wells,the reservoir will appear uneven displacement in both plane and vertical,and the static reservoir connectivity can no longer reflect the formation properties accurately.At the same time,it also causes ineffective circulation of injected water between production and injection Wells,which reduces oil recovery efficiency.In order to understand the reservoir condition,evaluate the reservoir and determine the direction of future development adjustment,the interwell tracer monitoring technology can be used to monitor and collect the tracer production in the well group by injecting the tracer into the injection well,and obtain the data of the breakthrough time,peak time and concentration of the tracer in the well group.The connectivity characteristics between injection well and production well,whether there are high permeability bands and channels,can be qualitatively determined,and the flow channel length between injection well and production well,water flooding advance speed and water flooding sweep volume and other parameters can be quantitatively calculated.In this paper,the tracer flow channel between injection Wells and production Wells is equivalent to a flow tube.According to the one-dimensional convection-diffusion equation,the mathematical model of single-peak and multi-peak tracer output is established.The model is applied to a well group system in northern Shaanxi by Matlab programming,and the tracer output curve is inverted by genetic algorithm.The spread volume of the injection tracer and the length of the flow passage between injection-production Wells were obtained.The cross-sectional area,equivalent radius of the flow passage and average flow velocity of the flow passage between injection-production Wells were obtained through the secondary calculation.The experimental results were compared with those calculated by the semi-analytical method.Specific work is as follows:(1)Study on mathematical model of interwell tracer.A single fracture,a single pipeline,a single karst cave between Wells and a fissure or a fissure or pipeline connected in series with a karst cave type fracture-cave structure are equivalent to a flow channel with equivalent radius r and length l,which has basically the same physical characteristics as the original fracture-cave body on a macro level.The seepage and diffusion mechanism of the tracer is analyzed.The convection-diffusion equation of tracer migration in one-dimensional flow channels with arbitrary section shape was derived,and the mathematical model of heterogeneous tracer with Lorentz coefficient,Dykstra-Parsons coefficient and heterogeneous coefficient was introduced.(2)Establish the interwell tracer monitoring model.The single-peak tracer output curve model and multi-peak tracer output curve model were established based on the basic tracer concentration model under the constant concentration boundary of the smooth circular tube.The mathematical modeling tool Matlab was used as the platform for compilation.The input parameters were tracer concentration c(t),average tracer retention time,water injection allocation coefficient and tracer slug injection volume.The output curve of the tracer was fitted by genetic algorithm,and the sweep volume and the length of the flow passage between the tracer injection and production Wells were shown.(3)Application of the interwell tracer monitoring model in Yan ’an reservoir well group: A well group system in northern Shaanxi was taken as the experimental object.Through the analysis of the regional geological condition,the tracer was selected as the tracer with low background concentration and good stability(NH4)2HPO4,and the tracer solution was configured according to the empirical formula of Brigham-Smith.The well group was monitored for two months,and effective tracer samples were collected from 3 of the 6 affected Wells.Two parameters of average tracer retention time and water injection allocation coefficient were obtained by preprocessing the test samples,and the data were input into the program.The flow channel connectivity between injection-production Wells and reservoir homogeneity were evaluated by analyzing the output data.(4)Compared the output results of the program with the interpretation results of the semi-analytical method,the results show that the average error of the interwell sweep volume of the three Wells is 9.2% compared with the calculation results of the semi-analytical method.The maximum relative error is 16.8% in well A,and the minimum is 3.3% in well D.The error may come from the local minimum value or overfitting phenomenon when using genetic algorithm to fit the tracer output curve.The tracer interpretation method requires few parameters,and can be used to calculate the basic parameters in the actual oilfield application.By comparing with the fitting results of the semi-analytical method,the method of inter-well monitoring interpretation model is feasible,and can complete the description of inter-well connectivity,whether there are cracks and faults,etc. |