This article describes the results of a theoretical and numerical study of the problem of a sharp drop in production rates of horizontal wells in one of Western Siberia fields represented by reservoirs consisting of poorly consolidated rock and saturated with heavy oil. The under gas cap oil rim is developed by depletion by horizontal wells. The reduction of the production rates is analyzed from different perspectives among which the most appropriate idea is elastoplastic deformation of the rock with its subsequent destruction in the bottomhole zone. Based on the analysis of the stress-strain state and theoretically justified rock ultimate strength, it is shown that actual bottomhole pressures lead to brittle fracture of the rock and, as a result of related processes, to particles movement and sharp increase in the skin factor.

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