The structure and composition of oil reserves deteriorate in the course of a mature oil field development due to geological and technological factors. The geological factors include vertical and lateral macro-and micro-heterogeneity of the reservoirs under development. At the same time, macro-heterogeneity leads to the formation of bypassed residual oil in low- permeability and stagnant layers, zones and lenses that are not exposed to stimulation, while micro-heterogeneity contributes to the formation of residual reserves of capillary oil and oil films. Technological factors include impossibility to fully account for reservoir heterogeneity when substantiating well spacing, pressure gradient, or flooding technology. The key problem here is the lack of clear understanding of the distribution pattern, quantity and deterioration of the quality of residual oil in water-flooded reservoirs, which makes it challenging to reasonably select a technology for effective oil extraction.

Despite the improvements constantly made to the production logging, hydrodynamic and interpretive, seismic, lithofacies and other methods of exploration and residual reserves modeling, it is often impossible to obtain timely reliable information about the percentage of reserves depletion and their coverage with the development activities. This paper proposes another method — optical — that, in combination with well logging and hydrodynamic surveys as well as geological and hydrodynamic simulation, has been tested to assess the quantity and spatial pattern of residual oil reserves of the Kynovian (Do) and Pashian (D1) horizons at the experimental site of the Romashkinskoye oil field (Devonian formation). From the theoretical and methodological standpoint, this method is underlain by the results of in vitro studies of core columns and involves the selection and research of optical properties of oil wellhead samples, mathematical processing and correlation of the obtained optical and field data. The obtained estimate of residual reserves is compared with data derived from geological simulation, the most promising automated method for identifying, geometrizing and calculating residual recoverable oil reserves (RROR).

The proposed method offers such advantages as ease of use, promptness, reliability, functionality and cost-effectiveness. It uses actual data on cumulative well production and takes into account the current reservoir and fluid porosity and permeability (P&P), which provides the possibility to obtain more precise estimates of the RROR quantity and spatial pattern.

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