Well logging techniques, which are used to estimate oil saturation in reservoirs characterized by a complex pore space structure and non-hydrophilic properties, are not accurate enough and overestimate the initial oil saturation value. In order to increase the reliability of hydrocarbon reserves calculation, a direct method of estimating the residual water saturation can be applied based on the analysis of the core sampled in a well drilled with all-oil drilling fluid (Order of the Ministry of Natural Resources and Environment of the Russian Federation No. 564 dated 28.12.2015). This method is not only the most reliable, but also the only one, according to which the decrease of the initial oil saturation coefficient of non-hydrophilic reservoirs from currently accepted value (90-95%) for the hydrocarbon reserves calculation (RC) to the estimated value (75-80%) can be proved to the experts of the State Commission for Reserves of Commercial Minerals (GKZ). There are no ready engineering solutions for drilling fluids for the mining and geological conditions of the Timan-Pechora petroleum province (TPPP) fields.

The purpose of this work is the development and application of a oil-based drilling fluid formulation with a water content of not more than 2.5% to provide the core sampling in prospecting and exploration wells of TPPP. It is planned that the use of this drilling fluid formulation, developed taking into account the best Russian and world practices, will allow the sampling of an isolated core material from productive formations with maintaining its initial fluid saturation. It will provide a possibility to determine the oil and gas saturation and residual water saturation of reservoirs without errors in the results.

Based on the results of analytical and research studies, all-oil drilling fluid (AODF) formulations have been developed for the conditions of three TPPP fields. Aqueous phase in these formulations is completely absent. The technological parameters of the developed fluids correspond to the geological and technical conditions of the construction of prospecting and evaluation wells in the fields under consideration and make it possible to ensure trouble-free drilling.

Based on the results of the core filtration studies, it is established that the application of the AODF has practically no effect on the change in the fluid saturation of the core samples during simulation of the coring. It was also shown that application of the AODF did not lead to a deterioration in the reservoir properties of the productive formations.

The developed AODF formulations are characterized by resistance to contaminating solid impurities and formation water and meet the requirements of industrial safety in relation to the conditions of the fields under consideration.

The technology for the AODF preparation and application in field conditions has been developed, and it is implemented using a standard set of drilling rig equipment. To ensure the possibility of AODF re-use, a technology has been developed to use all-oil drilling fluid for prepare the conventional oil-based drilling fluid (water-in-oil emulsion), which can be used in the following drilling of production wells of complex profiles.

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