Abstract

Since 1998 Rosneft SMNG (Sakhalinmorneftegas) has been developing the northern part of the Odoptu structure, near-shore to eastern Sakhalin Island, using ERD (extended reach drilling). Discovered in 1977, North Odoptu is alone among Sakhalin ERD projects in that it is entirely Russian operated and utilizes a unique hybrid of both Russian and international capabilities to address the considerable challenges of ERD well construction. To date 21 wells, with horizontal displacement/TVD aspect ratios up to 4.1, have been drilled and completed from two land-based drilling locations.

The use of rotary steerable (RSS) technology has more than doubled net drilling performance compared with early wells drilled using positive displacement motors (PDM's). A hole-cleaning challenge remains however, due to a practical necessity for back-reaming while tripping out of the hole. This has led to serious delays as localized unstable sections of the well-bore collapse due to mechanical disturbance, forming cavities that complicate hole-cleaning operations with pack-offs and stuck-pipe.

Low-resolution 2-D seismic has been a limitation on understanding the detailed nature of the over-burden stratigraphy and thus the ability to characterize the cavities. Geo-mechanical models, while clarifying the impact of the regional stress regime, have indicated that the origin of this instability phenomenon may not be due to compressive shear-failure.

The primary focus has therefore been on the development of cost-effective and innovative drilling optimization techniques to predict and identify cavity zones early in the drilling operation, and application of a fit-for-purpose, rig-based, risk-management approach to well-bore instability events. The progressive application of these techniques and the adopted risk-mitigation strategies is documented. This approach, in conjunction with the use of a regularly updated geo-mechanical model, has resulted in a successful outcome for well construction in a previously severely problematic region of the field.

The ability to recover well from instability-related setbacks has helped to foster a co-operative, continuous-improvement culture among the extended drilling team - Russian operator and international contractors alike. Recommendations are made for follow-on technologies and strategies that will have the greatest impact on the next phase of field development.

Historical Perspective

The first ERD well was drilled in the Odoptu area in 19731. It was an exploration well with a total drilled depth of 3406m and a horizontal displacement of nearly 2.5km. Though it failed to reach hydrocarbons it remained a drilling record in Russia for the next 10 years and is the earliest inspiration for today's successful ERD developments of Sakhalin Island more than 30 years later.

After a seismological survey of over 30,000 km2, a program of exploration/appraisal drilling was carried out in the latter part of the 1970's. This outlined the near-shore Miocene sandstone accumulations of Odoptu in 1977 and its sister field, Chayvo in 1979 2. Yuzhno-based engineering institute SakhalinNIPImorneft subsequently recommended long-radius ERD wells, in contrast to an ice-resistant offshore platform, for development. At the time, land-based drilling was determined to be the favored development concept due to an estimated cost 4 to 5 times less than an offshore installation.

Field dimensions are approximately 4 km E/W × 12km N/S with two crestal areas known as the North and South domes at 1500–1700m m TVD. The productive portion of the North Dome lies 4–6 km offshore, while the South Dome, slightly further from the nearest shoreline, is readily reachable with 8–10km displacements. Overburden consists of Pliocene and Upper Miocene sand-shale sequences of the Nutovo formation suite (fig. 1).

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