The article discusses the results of the study of the causes of complete and catastrophic lost circulation when drilling exploration and production wells in the reef structures of the Kama-Kinel downfold system. The features of the distribution of zones of mud losses of different intensities in the drowned the Upper Frasnian-Famennian-Tournaisian reef system are analyzed using the example of the Blagodarovsky uplift of the Kuleshovskoye field. It is shown that total and catastrophic lost circulation is mainly confined to the zone of the reef-front apron, which consists of psephitic and weak carbonate rocks. Drilling in a reef-front apron can be accompanied by drilling tool failures with loss of circulation and a significant decrease in the static level of the flush fluid. Partial losses occur, as a rule, in the zone of the carbonate core of the reef, its frontal and rear zones. The absence of losses or insignificant loss of circulation of the drilling fluid is characteristic of the depression (backreef) zone of the organogenic rocks. It was noted that it is important to determine the contours and amplitudes of the drowned reef, the boundaries of its facies zones. This enables to use standard S-shaped profiles when designing wells, bypassing possible intervals of complete and catastrophic lost circulation. At the same time, it allows to make the necessary technical and technological decisions for timely prevention and effective elimination of complications when drilling wells in organogenic massifs. It is shown that a detailed analysis of geological and geophysical information, taking into account the technical and technological data on previously drilled wells, makes it possible to predict zones of possible complications and develop effective measures to prevent complications at the design stage in order to increase the efficiency, reliability and safety of well construction and reduce the cost of drilling works at the fields of Rosneft Oil Company.
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