The paper discusses the problem of structural phase transition in oil that takes place in the course of production of hydrocarbon reserves. This phenomenon is due to the fact that light hydrocarbon fractions are the first to be produced, and that the formation is cooled by the injected displacement agent in waterflooded development. Phase change of the high-molecular oil components including asphaltenes, resins, and paraffins, results in phase transition in oil, which, eventually, might end in loss of recoverable reserves. Because of presence of a large number of interacting organic compounds in oil, to predict phase transition is a challenging task; still, for successful reservoir development, structural-mechanical properties of oil that are controlled by diversity and interinfluence of oil system’s components shall be taken into consideration. The study encompassed a large number of oil fields in the Republic of Tatarstan. Analysis of distribution of hydrocarbon accumulations, pressure and temperature conditions, and reservoir properties made it possible to define main factors responsible for the in-situ phase state of oil. Based on the results of laboratory experiments and statistical analysis, phase transition threshold values of in-situ viscosity and cumulative concentration of asphaltenes, resins, and paraffins have been determined. A hypothesis of a geological environment whose temperature conditions restrain phase transition has been justified. It was found that the in-situ reservoir temperature has a controlling influence on the phase state of oil components. The effect of temperature conditions of a geological environment and oil composition on the potential risk of phase transition is shown. The known power-law and logarithmic dependencies were used to demonstrate a possible change of recovery factor resulting from the in-situ oil viscosity increase in the course of reserves production.
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