| The SS low-permeability reservoir is a block-bottomed water anticlinal reservoir with low porosity and low permeability.In the past 20 years,the development methods have been changed from natural energy development to water flooding development.The current contradiction is the rapid decline of oil production,the high rate of water content and the poor efficiency of the water flooding in production.In order to solve the oil field production problems,the gas injection will be taken as the main way to improve the oil recovery in the field after referring to the domestic and international experience of gas injection development in low-permeability reservoirs.Due to the special geographical location of the reservoir,Nitrogen is the only option after the analysis of the available gas resources.Based on the characteristics of nitrogen and the mechanism of nitrogen flooding,the feasibility of nitrogen injection is carried out.After using the numerical simulation technique to study the gas injection mechanism in well group of experimental area and optimize the injection-production parameters,the best program to guide the development of the entire field is chosen.The main research contents and achievements are as follows:1.Through the numerical simulation research of gas injection flooding mechanism,the main parameters of the formation fluid,such as the saturated pressure,single degassing gas-oil ratio,ground density,formation density and so on have been obtained,which can reflect the changes of the physical properties of the formation fluid.The miscible state still can't be achieved by injecting nitrogen to the crude oil reservoir at 60MPa in the minimum miscible pressure experiment.2.The long core flooding experimental fitting shows that the increment of oil recovery obtained by simulating the gas injection is increased by 8.50%to 16.16%and the average value is 11.15%.The minimum increment of oil recovery is minimal by continuous injection of nitrogen.However,the degree of recovery is greater and the oil displacement effect is better with the slug size and the slug size ratio decreasing,during the nitrogen/water alternating drive.3.The model of gas injection is establishing and the historical fitting is carried out.The continuous water flooding development in the next 20 years is predicted by the model.The increased oil production is 4.80×104t and the average degree of recovery is 4.57%,which reflect the extremely poor developing effect and it is impossible to effectively drive the remaining oil in the formation.The distribution of residual oil is mainly related to the completeness of injection-production pattern,the continuity of the reservoir distribution and the interlayer heterogeneity.4.The nitrogen parameters are optimized by studying the established model,and it is determined that the gas ratio should be controlled at 2500m3/m3,the gas injection layers are the S1+S2+S3,the pressure level is maintained at about 20MPa,the alternate cycle length of the N2/water alternate injection is 1 month:1 month and the slug number is 48,the single well injection rate is 14000m3/d.According to the design of the recommended program in the next 20 years,the increased oil production is 9.78x104t and the increased degree of recovery is 9.32%.Comparing to the continuous water injection program which increases oil production by 4.86×104t and the degree of recovery to 4.75%,the oil exchange ratio of 23.59?t oil/104 m3 nitrogen?can be calculated.5.By comparison of the different time periods of the profile in the mechanism model,the main layers of S1,S2 and S3 are developed with Nitrogen injection.Due to the gravity separation,the residual oil in the upper part of the reservoir is flooded by injected gas,which improves the upper oil flow environment,the effect of S1 and S2 layer displacement is obvious,and the residual oil saturation is low,so the more residual oil can be droved out.S3 layer is relatively weak,and the effect of the displacement is slightly worse. |
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