East Siberia is a new and promising centre of oil and gas industry in Russia. Reserves explored in the region amount to ~11 percent of oil, ~14 percent of natural gas, and ~16 percent of gas condensate reserves of Russia. However, East Siberia is currently producing no more than 0.3 percent of Russian oil and about 1 percent of gas [1]. Major causes of such a low production level include remoteness of the region, poorly developed infrastructure, complicated geology, numerous faults, high mineralization of formation water in pay zones, etc.

The largest oil- and gas-bearing object of East Siberia is the Vendian terrigenous complex, which transgressively and discordantly overlies the eroded surface of Riphean rock. The complex is represented by the Parfenovskiy Horizon (the Kovykta Field), the Vanavarskaya Suite (the Sobinskoye Field), the Verhnechonskiy Horizon (the Verhnechonskoye Field), the Botuobinskiy Horizon (Srednebotuobinskoye and Chayandinskoye fields), etc. The present publication focuses on our experiences of applying geosteering to drilling in the Verkhnechonskiy Horizon interval of the Verkhnechonskoye oil and gas condensate field.

The key specific feature of the Verkhnechonskiy Horizon reservoir is its almost all-round yet very uneven Salinization [2].

Moreover, some of the rock series contain significant volumes of accessory minerals of anomalous physical properties [3].

Risks of drilling horizontal wells in poorly productive intervals are significant due to highly abrupt variability of reservoir rocks of the terrigenous complex in interwell space. With application of LWD, it is possible to drill horizontal sections of wells through the most favourable subsurface zones and to obtain reliable determinations of resistance-contrasting boundaries of formations along the entire wellbore [4]. It is generally an adeqaute condition for timely adjustment of well trajectories and for keeping wellbores within target interval boundaries.

Geosteering makes it possible to achieve a 10 percent increase in penetrating the best reservoir by horizontal wellbores in comparison with the originally designed wellbores. The average total effect of geosteering in more than 20 wells came to 67 tpd per well.

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