A significant part of hydrocarbon deposits in Russia is in the late stage of development. The distribution of residual oil and gas reserves is determined by the properties of the holding rocks. Estimating of deposits’ residual gas saturation is an important scientific task. The allocation of zones with the maximum undeveloped gas reserves will allow to select areas in long-developed fields for the intensification of production in the most efficient way. To search for such “sweet” zones, it is necessary to determine the factors that provide the value of the residual gas saturation. The reliance of the value of trapped in pores, residual gas saturation on such rock properties as pore space structure and wettability is studied in this article. The influence of formation pressure value and behaviour on making up of residual gas saturation during field development is not accounted in this work. The study of a wide collection of core sampled from productive deposits of the Orenburg oil and gas condensate field, the Vuktylskoe oil and gas condensate field, oil and gas field of Orenburg region, and also three areas in the East Caucasian petroleum province confirmed that the value of structure-trapped oil and gas saturation of carbonate and terrigenous rocks is directly proportional to the ratio of pore diameters and channels connecting them. Herewith the angular coefficient of the regression equation for this relationship for carbonate rocks directly depends on the quantitative characteristics of the predominant (relative) wettability. The obtained relationships make it possible to predict the value of residual gas saturation based on knowledge about the pore space structure and the surface properties of rocks.

Основные «структурные элементы» геологических запасов (ресурсов) нефтегазоматеринских карбонатных толщ месторождений жидких углеводородов и газа:

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Research is based on conception about field as not only migrating fluids trap with formed pore volume and shielding cover but as self-assembling oil-gas source system on one or another oil-gas generation stage. In order to study oil-and-gas source carbonate rocks in detail a core studying laboratory investigations complex was developed. Core samples, salt exhausted but not extracted, were studied by petrophysic, geochemical, electron microscope and physicochemical methods. Complex treatment to investigations allowed to discover regularities of carbonate oil-and-gas source deposits' catagenetic transformations. These regularities were put into created hearth-clustered algorithm for rock properties modeling. A hearth-clustered conception taking into account oil-and-gas source rocks' transformation regularities would allow to differentially estimate and classify hydrocarbons reserves and also choose directed recovery technologies in hearths' pay zones which correspond to different organic matter and rock-forming matrix catagenetic transformation stages. Geological hard-to-recover oil reserves in gas-saturated zones of gas-condensate and oil-gas-condensate fields are traditionally not considered in estimation of reserves, nor in field-development program but they represent tremendous unaccounted and not realized resource base of Russian Federation in regions and fields with wholly developed infrastructure. They should be necessarily considered in design estimates as on early so on late field development stages.

Evaluation of rock wettability is an important task, since this parameter determines the distribution of water and oil in the reservoirs and their relative and phase permeability. The reliability of evaluation the wettability of rock samples depends on the drilling-in conditions during core sampling and core sample preparation methods. The investigation of the surface properties of the core from the Orenburg oil and gas condensate field showed that using of polymer-colloidal drilling mud leads to hydrophilization of the samples' surface. To obtain information on the actual wettability values of rock samples taken from wells drilled with polymer-colloidal drilling mud a method for estimating the relative (predominant) wettability of rocks based on petrophysical and lithological studies data is proposed. The authors suggest that the extraction of oil and gas source rock samples leads to irreversible changes in surface properties that cannot be restored. Keywords: selective wettability; relative wettability; predominant wettability; polymer-colloidal drilling mud; residual gas saturation; trapped gas saturation; pore space structure; extraction.