Abstract

The Russkoe gas and oil field was discovered in 1968. No attempts to begin its commercial operation in the former USSR were and have been successful till the present time. The issue of economically attractive development of the Russkoe field is very urgent for TNK-BP since the field is a strategic oil asset to replenish company reserves. The field is located in the Arctic zone, in the north of the Tyumen Oblast, Russian Federation. Russkoe reserves are hard to recover, and the expected oil recovery factor is 14–15%. The main pay zone, PK 1–7, consists of unconsolidated poorly cemented sandstones with a high pelite fraction content. The reservoir is saturated with highly viscous oil (19 API), and low reservoir temperature (23°C) is predominant. Horizontal well profiles (over 500 m) are currently considered to be the most feasible ones from the economic point of view. The field is located in a hard-to-reach region with difficult logistics. With these conditions in view, correct selection of completion systems for sand control or containment is critical for assuring profitable operation of this asset from the point of view of maintaining well productivity, artificial lift and surface equipment loading. This article describes the TNK-BP process and experience in selecting the lower completion systems intended to suppress reservoir sand production in the Russkoe field. Different lower completion systems were tested during the pilot operations in 2006 – 2010. To reduce uncertainty, a set of laboratory tests of completion systems produced by a number of Russian and foreign manufacturers were conducted using the Russkoe field well fluid and core samples. The experience of leading servicing companies as well as experience of developing similar fields in the world was also taken into account. The article discusses in detail the approach chosen, decisions taken, current results, and lessons learned.

The field is located in the Arctic zone in the northeastern part of the Tazovsky oil and gas bearing region in the Yamal Nenets Autonomous Okrug. Within the period of 1966 – 2009, 70 exploration and development wells were drilled. No attempts to begin its commercial operation in the former USSR were and have been successful till the present time. The effective recovery of reserves is complicated primarily for geological reasons: (1) the reservoir pay zone consists of unconsolidated poorly cemented sandstones; (2) sandstone mainly consists of the finegrained fraction with the particle size below 100 mµ; (3) high oil viscosity (150 – 350 cP under reservoir conditions and over 550 cP under surface conditions); (4) tar content in Russkoe oil exceeding 20%; (5) pay zone shallow bedding (down to -870 m) and, consequently, low reservoir temperature (18–23°C); (6) overlying beds prone to caving and falling; склонные к осыпям и обвалам вышележащие пласты; (7) permafrost rocks in the interval of 0–400 m; (8) gas hydrates within the permafrost zone intervals; (9) extensive gas cap; (10) pay zone significant lateral heterogeneity. The reservoir cannot be confidently forecasted while drilling horizontal wells. The expected Russkoe full-field commercial development has made Company urgently seek development technologies, including solution of the primary problem, namely selecting optimum producing well completion systems for the pay zone. The field is located in a hard-to-reach region with difficult logistics. With these conditions in view, correct selection of completion systems for sand control or containment is critical for assuring profitable operation of this asset from the point of view of maintaining well productivity, artificial lift and surface equipment loading. Pilot development has been in progress in the field since 2007 (see Fig. 1 with pilot project zones). By the present time, 112 development wells have been drilled in the field and horizontally completed with wire mesh slotted liners with the cross-section 150 to 750 mμ. The slot size was selected on the basis of the core grain size analysis. As an experiment, one well was completed with a perforated liner, with holes 8 mm in diameter. Horizontal section lengths 168 mm in diameter vary from 600 to 900 m. In the course of operating wells equipped with liners, sharp productivity decrease of producers was observed regardless of the filtering element size. Liner clogging as the result of hole caving is believed to be one of the reasons for this phenomenon. When the well with the perforated liner was operated, its productivity also fell, but in this case it was caused by borehole filling with sand. To restore performance, wellbore were flushed using various technologies and agents (water, dead oil, aerated liquid). None of the technologies applied yielded high-quality results. Probable causes of unsuccessful bottomhole normalization included extensive circulation losses due to low reservoir pressure and high permeability values. Table 1 presents borehole cleaning results.

The operations done have demonstrated that borehole clogging with sand does not directly depend on liner slot sizes as wells equipped with smaller size liners proved to be more exposed to sand precipitation in the wellbore. It can be caused by a damage of the liner wire wrapping in the course of running. To assess wellbore wall stability both while drilling and in the course of operation, a package of studies was carried out, such as: (1) testing core mechanical properties; (2) geomechanical modeling; and (3) combined flow and geomechanical modeling to simulate wellbore wall stability in the course of operation. The results of the work done have shown that the minimum rock strength allowing safe drawdown development should be ~7 MPa. The actual rock strength according to core analysis results is <2 MPa, which is also confirmed by simulation results. Therefore, reservoir destruction in the bottomhole zone is unavoidable under Russkoe field conditions. It is impossible to prevent sand production with drawdown control, drilling direction, wellbore diameter and orientation for these rocks. For horizontal wells, equipment preventing sand infiltration into the well should be used, since significant solids production results in a great number of engineering and technological problems, such as: (1) bottomhole equipment MTBF reduction; (2) frequent wellbore flushing with no technologies properly developed by today; (3) deterioration of reservoir bottomhole zone properties caused by flushing; (4) disposal of sand produced; (5) lack of infrastructure to gather and treat production with high solids content. In view of the Russkoe field pay rock grain size distribution (see Table 2) and in accordance with classical methods of sand control system selection, using gravel packs seems to be the most appropriate choice, which is connected with presence of a great number of fine-grain fractions and poor sandstone sorting. However, in view of high cost and technical risks associated with this completion technique for 1000 m long horizontal boreholes to be drilled, the decision was taken to test alternative sand control screen systems. Thus, it became necessary to select the liner filtering surface with due regard for all oil and rock specific features in the Russkoe field to solve the following problems: (1) containment of the coarse-grained fraction that does not reach the surface with the well production flow and is deposited in the horizontal borehole; (2) giving way to the fine-grained fraction produced in the course of operation; (3) high mechanical strength of the screen structure to prevent damages while running. To solve this problem, various screen designs were lab tested under conditions as close to Russkoe field well conditions in terms of: (1) rock fraction and lithological composition; (2) oil physical and chemical composition; (3) water content in well production.

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