Rock wettability and its transformation in the process of formation and development of oil deposits is a crucial factor influencing fluid content and many aspects of reservoir performance, especially during water flooding and application of enhanced oil recovery techniques. The concept of saturation of the complex carbonate reservoir with the variable wettability is presented. This concept is applied to the Kizelovsky horizon of Tuymazinskoye field. The results of analysis of the geophysical characteristics show that the cross section is divided into three geological units with significantly different values of the electrical resistivity. By means of joint analysis of core examination and geophysical well logging it is demonstrated that zones of the low-resistivity geological unit are chiefly characterized by hydrophilic type of rock wettability, while limestone of the high-resistivity geological unit – by hydrophobic. Oil saturation model is designed with the assistance of field data and core data based on the capillary gravitational equilibrium concept taking into account variable rock wettability. The results of the relative permeability experiments together with information about the initial water cut of well production, in correspondence with geological unit and perforation depth, allow to identify initial saturation distribution in reservoir.
The presented approach to the creating of the saturation model enables to consider previously ignored factors that affect the efficiency of the reservoir pressure maintenance system and recovery of reserves of particular areas, such as the electrical resistivity of the geological units, the presence of bridges between them, etc. In particular, the identification of the thick lower unit with high water saturation makes it possible to explain the reasons of watering rates of the production wells. Development of detailed reservoir simulation model, that takes into account the presence of three geological units with the variable wettability, helps to clarify the distribution of oil reserves, adequately perform history matching, predict well performance and improve the recovery of reserves.
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