Analysis of production well construction features in permafrost conditions

The purpose of the research is to present the analysis results of construction and operation features of oil and gas wells in permafrost conditions. Considering the materials relevant to this topic, the method of comparative analysis is applied. The object of the research is technologies and equipment used for high-quality and fast construction of deep wells in permafrost conditions. As a result, complications and accidents occur due to the collapse of well walls, occurrence of caverns, stuck pipes, casing deformation and subsidence of wellhead equipment. It is noticed that the most unstable rocks in permafrost sections are located in the range of 0–200 m. The effect of temperature on thermal interaction intensity in the ”flushing agent – wellhead – drill (production) string” system is analyzed. It is found out that maintaining negative temperature of well walls is the main way to prevent complications while drilling in permafrost. The use of thermally insulating casing pipes to form a well conductor string is considered. The rotary method is specified as the main drilling method in the cryolithozone while auger without flushing is relevant for drilling for shaft direction. It is determined that drilling with the use of drilling fluids has to solve the problem of solution freezing prevention when flushing has been stopped continuously. Taking into account that the permafrost zone, as a rule, consists of loose, unstable rocks, the great importance is given to the duration of drilling under the conductor, which should last not more than 3 days. During this time, almost no complications occur when using high-quality clay mud with the temperature in the range from 0.5 to 2.5 °C. The real possibility of using a drilling fluid with a negative temperature as a cleaning agent, as well as the same with a positive temperature, but with the use of additional special technologies, is investigated. The use of drilling fluids with a negative temperature is recornized to be not economical. It is specified that prevention of permafrost rock thawing requires not only pre-cooling of the circulating solution but also selection of increased values of the rotation frequency and axial load on the bottomhole with a simultaneous change in the amount of fluid supplied to the well when designing the drilling mode. Consideration is given to the implementation options of the drilling technology based on increasing mineralization degree of drilling fluids. A conclusion is drawn that “well – rock” system is in isotonic equilibrium if the mineralization degree of drilling fluids and pore water of permafrost rocks is the same.

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