Deep drilling into the ice sheet at Vostok station, Antarctica, was started by specialists of the Leningrad Mining Institute (since 1991, St Petersburg State Mining Institute) in 1970. Five deep holes were cored: hole No. 1 to 952 m; hole No. 2 to 450.4 m; hole No. 3G (3G-1, 3G-2) to 2201.7 m; hole No. 4G (4G-1, 4G-2) to 2546.4 m; and hole No. 5G (5G-1) to 3650.2 m depth. Drilling of hole 5G-1 is not yet complete. The deep drilling at Vostok station has had successes and problems. All the deep holes at Vostok have undergone at least one offset drilling operation because of problems with lost drills. These deviations were made successfully using a thermal drilling technique. Several drilling records have been achieved at Vostok station. The deepest dry hole, No. 1 (952 m), was made during Soviet Antarctic Expedition (SAE) 17 in 1972. The deepest fluid-filled hole, No. 5G-1, made by a thermal drill (TBZS-132), reached 2755 m during SAE 38 in 1993. The deepest fluid-filled hole in ice, No. 5G-1, was drilled with a KEMS-132 electromechanical drill and was stopped above Vostok Subglacial Lake at 3650.2 m depth during Russian Antarctic Expedition (RAE) 51 in 2006.In the summer season of the 15th SAE, the first drilling shelter was constructed on two steel sleds (Fig. 3), providing a work area measuring 15 Â 2.9 Â 2.5 m. A 9.7 m round tower, measured from the top of the hole, was constructed in the center of the work area and wrapped in rubberized cloth for protection ( Fig. 4).

The paper reviews challenges associated with designing of equipment required to implement the selected technology of penetration into Subglacial Lake Vostok in Antarctica. It describes the design process of inertial mechanical reamer, which is required for one of the stages of the proposed penetration technology. The initial design of the reamer was developed, manufactured and tested in Borehole 5G during the seasonal shift period of the 62nd Russian Antarctic Expedition (December 2016 – January 2017). Based on the test results the reamer design was critically reviewed, enhanced and subsequently tested in Borehole 5G during the seasonal shift period of the 63rd Russian Antarctic Expedition (December 2017 – January 2018). Tests of the improved design of the mechanical reamer confirmed that the adopted design solutions are correct.

ABSTRACT. This paper documents the drilling of the 5G deep hole at the Russian Vostok station, Antarctica. The hole construction is described and the specifications of the drill and surface drilling equipment are given. The peculiarities of drilling at various depths are considered. Based on the extensive experimental data collected at Vostok station, the processes occurring at the hole bottom are investigated: ice breaking and cutting, bottom cleaning and chip transport, and accumulation of the chips in the screen. The main factors affecting ice-drilling efficiency are the coarseness of ice crystals and the ice temperature. When ice crystal size exceeds 10 mm the cutting efficiency sharply decreases, and when ice temperature exceeds -5°C the chip transport from the hole bottom and the chip density in the screen are reduced. The drill advance then becomes irregular, slows down and may even be terminated in some cases. Optimal values of the basic drilling parameters (e.g. cutter head rotation rate and drilling fluid flow rate in the circulation system) have been deduced empirically. The designs of the cutter head and circulation system including screens have been significantly modified, allowing the drilling process to proceed at a normal rate even in the vicinity of the subglacial lake surface.

During the seasonal work of the 64th Russian Antarctic Expedition in 2018-2019 at the “Vostok” drilling facility named after B.B.Kudryashov (“Vostok” station, Antarctic) specialists of Saint Petersburg Mining University conducted experimental investigations on the process of drilling by melting with simultaneous expansion of wells in the ice mass. A test bench and a full-scale model of a thermohydraulic reamer-drilling tool were developed, manufactured and tested for the research. The first bench tests of the full-scale model proved its efficiency and suitability for experimental drilling with simultaneous expansion of wells in ice mass; its operational capabilities were determined and the drawbacks that will be taken into account in future were found out. The article substantiates the choice of constructive elements for thermohydraulic reamer-drilling tool. It is determined that the technology of full diameter drilling with simultaneous expansion of the well in ice mass can be implemented by combining contact drilling by melting and convective expansion with creation of forced near-bottomhole annular circulation of the heated heat carrier. Dependencies of expansion rate on main technological parameters were determined: active heat power of heating elements in penetrator and circulation system, mechanical drilling rate, pump flow rate. According to the results of investigations, the experimental model of thermohydraulic reamer-drilling tool will be designed and manufactured for testing in conditions of well 5G.

No abstract