Identification of remaining oil reserves at the late stage of development of the Gilbert field using integrated geophysical data

The completeness of oil recovery under elastic water-drive conditions depends on numerous factors, including the geological structure of the reservoir, the properties of the oil-bearing formations, the interaction between the production zone and the peripheral area, the current reservoir pressure relative to the initial level, and the extent to which the productive horizons are swept by waterflooding throughout their thickness and areal distribution. The main objective of this study is to evaluate the remaining oil reserves in the field and to develop technologies for their efficient recovery. The degree of reserve depletion was assessed through a comprehensive analysis of all available data, enabling the identification of the oil–water contact (OWC) front movement and the current energy state of the reservoir. The assessment of recovery completeness was carried out using the results of field–geophysical surveys, the characteristics of oil-displacement by water, and data from hydrodynamic modelling. Geophysical monitoring was performed for each well individually to track the OWC position and identify water-swept zones of the productive reservoir. The Pulsed Neutron–Neutron Logging (PNNL) method was employed for real-time monitoring of oil–water interface movement during field development. It was established that the remaining recoverable reserves (RRR) account for 32.5% of the initial recoverable reserves (IRR). The current oil recovery factor (ORF) is 0.507. The field is currently at the fourth stage of development, characterized by a high water cut (94.8%) and a low annual oil-production rate (1.71–2.32% of the IRR).