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№ 1 (January 2008)
Until recently, the inconvenience of complex geological sections has hampered development of production drilling in Eastern Siberia
By A. Baluta, A. Sharipov, A. Lozukov
Wider application of PDC drilling tools is one way to increase drilling efficiency in Eastern Siberian fields. However, the question arises of whether extensive introduction of PDC drill bits in Eastern Siberia will have the same success as in Western Siberia. Results of applying drill bits and core drill bits from the NPP (Research and Production Enterprise) Burintekh show this to be quite possible. Table 1 presents Surgutneftegaz’ results at the fields of the Republic of Sakha-Yakutia in 2007.
It bears mentioning that in certain drilling intervals, the rate of penetration of PDC core drill bits is inferior to that of rolling-cutter core drill bits. However, PDC core drill bits have two significant advantages: higher core recovery percentage and no accidents related to the destruction of rolling cutters at the bottomhole.
The efficiency of PDC core drill bits is increasingly notable with well depth. Lenaneftegaz’s comparison of various core drill bits for the drilling of exploratory wells at the Oldonskaya group of fields is shown in Table 2.
The upper drilling interval is the most complicated for the use of rock-cutting tools with PDC technologies. Currently, it is impractical to use PDC drill bits for surface hole drilling shallower than 400 m.
Burintekh’s Center for Drilling Tool Design is therefore focusing on the selection of drill bits for production hole drilling.
Since the beginning of 2007, the company has collaborated with the Drilling Office of Surgutneftegaz on a program to optimize PDC drill bit design for maximal economic and operating efficiency at the fields of the Republic of Sakha-Yakutia. This program systematically analyzes the parameters for successful drill bit operation with the goal of improving the design and practicality of the bits. In general, optimizing PDC bit design requires selecting the appropriate size and density of cutters, a bit profile and a hydraulic system best suited to the specific geological and technical conditions.
The data on drilling and bit performance are recorded and analyzed, and the results are used to develop the bit designs for the next test. This research cycle is repeated until it produces the optimal design.
To date, two drill bits have passed field tests (see Table 3).
Results show that PDC bits can drill rocks of up to 6 or 7 category hardness, with rates of penetration exceeding those of rolling-cutter bits by 50 to 100 percent and with higher meterage per bit. The offered bit designs operate well in the hole angle buildup intervals when positive displacement downhole motors are used. Problems arise in intervals with highly fractured rocks and alternating rocks of different hardness categories. In these intervals, PDC drill bits are subject to significant dynamic loads, causing intensive cutter destruction.
In lower intervals of terrigenous deposits, PDC drill bits can be also more efficient than rolling-cutter ones.
An illustrative example is bit BIT 215.9 V 913 N, used in the interval 2,897-3,121 m (nonuniformly anhydritized hard and medium-hard rocks, consisting of sandstone alternated with rock salt, marl, dolomite, argillite and siltstone) during the drilling of an exploratory well in the Krasnoyarsk Territory. With a total meterage of 172 m and an average penetration rate of 0.64 m/h (287 h drilling time), 15.5 days were saved compared to three-rolling cutter drill bits.
Another example is BIT 142.9 V 613 UN, run by Lenanafetgaz to drill an exploratory well at the Oldonskaya group of fields through the Parshinskaya formation with alternating argillite and sandstone of drillability index ranging from 4 to 7. Comparative results are shown in Table 4.
Wide application of PDC technologies for drilling in Eastern Siberia might seem confined to the future. However, the promising results of utilizing PDC drill bits and core drill bits at various fields allow optimism about the prospects of implementing these rock cutting tools and of further optimizing their designs