Many oil and gas fields are currently at a late stage of development, while most of them are being developed using flooding. These fields are characterized by the decreasing oil and liquid flow rates and accelerating water-cut. During the development process, the majority of oil reserves are extracted not using methods of production enhancement. Though, oil reserves within undeveloped areas are a valuable source for recovery. To involve residual reserves in active development, it is necessary to make a reasonable justification and a choice of the most effective geological and technical measures that take into account various geological field and well reservoir characteristics. Residual oil reserves at the late stage of development are classified as hard-to-recover and are mainly concentrated in areas not covered by flooding laterally and vertically. They belong to various categories that differ in the geological and technological characteristics. In this regard, it is necessary to plan various geological and technical measures taking into account the structure of residual reserves and patterns of their distribution. Studies of complex oil and gas fields were performed and a detailed analysis of the geological and physical characteristics, parameters of reservoir heterogeneity along with operational, geological and commercial assessment of reserves development were conducted.

At a late stage of development of any oilfield, there are big number of factors that affect recovery factor. One of them is related to presence of isolated zones, that were caused by combination of poor reservoir and oil properties of a rock. To solve the given problem variety of workover operations and enhance oil recovery (EOR) methods can be appled for the complex reservoirs such as Tevlinsko-Russinskoe oilfield. The number of particular studies were presented by reviewing of field data, construction of heterogeneity zones, revision of workover operations and selection of EOR methods. It has obtained that the reservoir has the lenticular structure, consists from 9 different facies and presented by 4 classes of heterogeneity. The immiscible gas injections of Nitrogen were selected as the most suitable EOR method for the given oilfield. Application of different composition of brine water was reccomended for wettability alteration.