Oil fields with complex structure of productive strata, high water-cut and highly-viscous oil are predominant among oil reserves in the Republic of Tatarstan at the present time. For more than fifty years of oil production at the giant Romashkino oil field, significant experience has been accumulated both in the application of different oil recovery techniques and in the area of the investigation of phenomena affecting the efficiency of oil recovery. Studies related to improved oil recovery and scientifically-based approach to select the most efficient technology of oil recovery become more important every year worldwide. The mechanism of oil displacement is very complicated in the deposits with hard-to-recover oil due to many factors affecting oil production. Oil, gas and water are entrapped in the porous media of reservoir rock and form a complex multiphase system containing mineral solid state and saturating fluids of different compositions.
Formation of solid-state bitumens which are found in the Devonian and Carboniferous formations, is one of the reasons of the decrease in oil recovery in reservoirs with high permeability. Solid bitumens fill porous media and decrease porosity and permeability by hindering oil filtration. Study of the specific features of deposition of bitumen-bearing rocks allows us reveal the reasons why recoverable crude oils is transformed into practically unrecoverable ones.
Second major physico-chemical specific feature of oil deposits is uniform character of the surface properties and structure of porous media in reservoir rock. New thermodynamic conditions in a formation caused by the development of oil field lead to a change in the sorption characteristics of rock and fluids, interfacial tension and reservoir wettability.
Deep understanding of the phenomena related to retention of crude oil in reservoir rock is necessary for the efficient oil recovery from hard-to-recover oil deposits.
Complicated nature and poor-studied mechanism of formation are typical for the hard-to-recover oil deposits. That causes a need to evaluate different factors affecting formation of the chemical composition of crude oil in clay-containing reservoirs with low permeability, oilwater contact zones and highly-permeable strata with high water-cut at the late state of exploitation.
The methodological approach to study the transformation of oil composition due to interfacial interactions in productive oil strata is based on a hypothesis of I.Prigozhin(1). The hypothesis states that the deposit of hydrocarbons is an open organo-mineral system exchanging energy and substances with the environment, having an ability of the self-organization and obeying the laws of thermodynamics. Formation of the 3D-structure reveals itself in the regular distribution of fluids through the cross section and areal section and is detected by the outflow of substances and products of irreversible physico-chemical processes which are mostly bitumens and newly-formed minerals. A comprehensive investigation of the transformation of residual oil composition has been performed in this study. Residual oils are extracted from core samples taken from the natural oil-water contact zones, zones underwent longterm water-flooding, a near well-bore area which was flooded for 40 years with water polluted by oil products, a well-bore with abnormally low pressure, and the areas of oil fields near tectonic fractures.
