The sweep efficiency enhancement technologies applied in Russia were analyzed. It was shown that the most common sweep efficiency enhancement techniques are sidetracking, non-stationary flooding and different flow-diverting technologies, based on chemicals injection, restricted by volume. In some cases, hydraulic fracturing can be considered also as a means to improve the sweep efficiency by treatment. The case histories of all treatment techniques specified in the paper are presented and trends of their development in the future are identified.
The problem of enhanced oil recovery is particularly pointed in Russia today: for the last 25–30 years the tendency is from slow to steady decline of oil recovery factors in the fields. Within last 5–6 years the oil recovery factor in Russia has become more or less stable and can be estimated for Russian fields as 34–36 %37.
The decrease of oil recovery factor in the country can be explained by the change of the so the called "structure of oil reserves", specifically by increase of the share of oil reserves and production volumes from regions with more complex terrain and climatic conditions and fields with more complex geological conditions: low productive formations and deepseated accumulations. For the last decades the share of Western Siberian fields has been significantly increased in the total oil production, while production share of European part and Urals area has dropped, i.e. developed regions with good infrastructure (map with main Russian hydrocarbon basins is in Appendix A). Adverse climatic and geological conditions stipulated the development of fields with less dense well spacing and consolidation of the large number of productive formations in single production targets. All this inevitably results in a sweep efficiency and oil recovery factor decrease.
It is necessary to mention that in the former Soviet Union a lot of attention has been paid to oil recovery problems since the end of 70-s (the review of EOR used in the former Soviet Union and Russia has been presented in the paper 37). Unfortunately, the economic shocks during the period of the well-known events in the end of 90-s caused the catastrophic reduction of EOR application. The oil industry restructuring seriously affected the scale of EOR operations. Start-up shops and newly-made oil companies needed to develop and get a profit rapidly and couldn't support expensive EOR projects. Unfortunately, for the last ten years there was no official statistics on EOR application and efficiency available in Russia. According to different estimates 8,52, the number of projects with actual EOR implemented, such as gas injection, WAG, thermal EOR techniques has significantly reduced, and incremental oil production due to their application is insignificant in the overall oil production (Fig.1). It is significant that chemical flooding came to naught also during last 20 years and chemical EOR constitutes practically the so called "Flow Diverting Technologies" or "Flow Deviating Technologies" (FDT). These technologies are based on injection of insignificant volume of special agents into injectors for "redistribution" of injectivity profiles between layers, "smoothing" water displacement front in heterogeneous formation and as a result the enhancement of sweep efficiency. Together with flow diverting technologies other techniques in Russia are applied to improve the sweep efficiency which is not really correctly referred to EOR techniques. In some cases these technologies may include massive hydraulic fracturing, sidetracking and some other sweep efficiency improvement techniques which in the common world practice are referred to the reservoir and water management. Total incremental oil production due to traditional EOR doesn't exceed 2.8–3.0 MMtons while FDT and other techniques of formation stimulation and bottomhole zone treatments bring about 44–45 MMtons (Fig.1). Part of this oil production related to well stimulation while the other one was connected with the sweep efficiency enhancement.