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Рекомендовано членом редакційної колегії д-ром геол. наук, проф. С.А. Вижвою) Currently, cluster-analysis or automatic classification problems are widely used in various fields, in particular economics, sociology, medicine, geology, and other sectors, where there are sets of arbitrary kinds of objects to be automatically divided into groups of similar objects based on their "similarities-differences" features. In recent years, these methods have been widely used in data analysis problems. Conventional methods of cluster-analysis suggest a clear partition of the original set into subsets, in which each point is included only in one cluster after the partition. However, it is well known that such a restriction is not always true. It is often necessary to make such kind of partition, which allows determining the degree of membership of each object for each set. In this case it is advisable to use fuzzy clusteranalysis methods. Problems in this formulation arouse interest of specialists dealing with geology, geophysics, oil-and gaswell drilling and oil and gas production. One of the most important results of the study of lost circulation zones is determination of the coefficient of lost circulation intensity. Purpose. Estimation of drilling mud lost circulation during drilling and emerging risks. Methodology. The solution of the problems posed in the work was carried out using methods known from mathematical statistics and the theory of fuzzy sets. The technique of processing the results, as well as fuzzy cluster analysis, was used for that purpose. Findings. As a result of the research, 5 classes were obtained, each of which characterizes the rate of mud lost circulation, expressed by linguistic variables. On the basis of this, fuzzy models are constructed, expressing the relationship between the indices of petrophysical properties and the volume of the absorbed solution. Originality. A method based on fuzzy cluster analysis has been developed, which makes it possible to predict drilling mud lost circulation of different rate at an early stage during drilling. Practical value. The obtained results allow making decisions on prevention of lost circulation and timely liquidation of their consequences.

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Purpose. Determination of the required concentration of polymer solution, providing the maximum well insulation effect, depending on the filtration characteristics of the medium. Methodology. The research was conducted by experimental method using the methodology of planning the experiment. The theory of rational planning was used by varying two variables on five levels. Media permeability and polymer solution concentration were used as variables. Methods of mathematical statistics were used in processing the results. Findings. Experimental studies made it possible to build a model expressing the dependence of the residual resistance factor on the permeability of the medium and the concentration of the polymer solution. By further statistical processing the connection between permeability and necessary concentration of polymer solution was obtained. Originality. The experimental studies have substantiated the possibility of evaluating the influence of medium permeability and concentration of polymer solution on the residual resistance factor and determined its dependence on the concentration of polymer solution and medium permeability. The main point of polymer solutions application is justification and choice of the reagent concentration. The concentration should be selected in such a way that it provides the maximum value of the residual resistance factor and the viscosity of the solution necessary to level out permeability heterogeneity of the environment to some extent. To achieve this, a relationship has been obtained that allows determining the concentration of the polymer solution at a given permeability of the environment that provides the maximum residual resistance factor. Practical value. The conducted experimental studies allow developing the ideas about the regularities of water manifestations. The results of the research allow selecting purposefully both the formulations of composite systems and the technology of their application to improve the efficiency of oil production and to limit water inflows in specific geological and physical conditions.