Experience with dual completions in TNK-BP
Authors: Muslimov E. , Medvedev P. 
Published: October 2012

The majority of fields in Russian Federation contain multiple stratified oil reservoirs. In many cases the fluid properties, geological parameters and reservoir pressure conditions are such that each reservoir has to be developed separately, as dictated by legislation. This means additional drilling grids or producing zones consecutively over time using the same wellbore. The first option implies heavy CAPEX commitments, while the other means significant production delay with a drastic negative impact on NPV. The alternative is dual completion technology. Its advantages include drilling cost reduction, production acceleration, independent control and optimal drawdown on each layer, elimination of undesirable fluids' mixing, ability to continue production if one of the pumps fails, elimination of well shutdown to test separate zones, reduced environmental footprint and full compliance with legal requirements. There is a range of dual completion and dual monitoring technologies which can be applied to meet the legal requirement with a certain degree of accuracy. Selection of the right technology is critical to obtain the optimal value from the investments. The article discusses the challenges in the domain of completion and production technology for multilayered reservoirs in TNK-BP; covers main objectives of dual completions /dual monitoring and key functional requirements; highlights the classification of dual completion types (covering single lift and dual lift categories and subgroups) including high level overview of specific technologies available on the market. The current situation with multilayered reservoirs in TNK-BP regions will be presented with highlights on the results of the performed dual completion pilots as well as outlook on future activities.

Drilling separate wells in every reservoir is economically unviable in some cases. Aside from that, construction of many wells entails technical or logistical restrictions and limitations. Successive development of reservoirs slows down the production of reserves. Developing different reservoirs with the same well via dual completion technologies can decrease the scope of drilling, increase production, accelerate development of reserves, and help mitigate adverse environmental impact. It should be noted that the use of dual completion technologies must meet the requirements of FDP documents and provisions for production allocation, provide the required level of reliability and cost efficiency, and fulfill the requirements to optimal development of reservoirs. According to Rostekhnadzor's resolution, dual completion of several reservoirs with the same well shall be permitted when replaceable downhole equipment is available for separate metering of produced hydrocarbons, separate testing and logging of every reservoir, and safe servicing of wells with due consideration for pressure differential and the properties of reservoir fluids. The actual operating conditions of TNK-BP's fields are such that 98 percent of wells are worked by artificial lift, 90 percent of them using electrical submersible pump (ESP) units. The average depth of wells exceeds 2,000 m, the diameter of most production casings is 168 mm or less, and hydraulic fracturing technologies are used extensively (approximately 1,500 hydraulic fracturing jobs a year). Many fields have three or more reservoirs and a number of complicating factors, such as asphalt-paraffin deposits, salts, abrasive suspended solids, a high gas-oil ratio, and frequent crossflows behind casings. In such conditions, the requirements for dual completions equipment are more stringent. Despite the fact that dual completion systems do not constitute an essentially new technology, their use requires application of advanced methods of artificial lift, geophysical studies, metrology, and well completion.

The main risk inherent in dual completion technologies is that the flow rate of wells or permeability of new reservoirs under development is not known exactly and the distribution of production from reservoirs with an active well stock is usually uncertain. Other risks include unsatisfactory or unknown mechanical conditions of candidate wells (casings, cement sheath, etc.) and the poor susceptibility of technologies to commercial integration. Furthermore, the fact that the reliability of a complex system is lower than that of its constituent elements should always be taken into consideration in technology application planning. According to the Tyumen Petroleum Research Center (TNNC), TNK-BP's potential well stock usable for dual completions as of the beginning of 2012 stood at approximately 3,000 wells, and the oil reserves that may be developed and recovered using dual completion technologies are estimated at several tens mln tons. In future, this potential is expected to rise with the number of new fields brought into development. A classification of the key systems required for application of dual completion technologies in the assets of TNK-BP is shown in Fig. 1. There are systems for dual production (DP), dual injection (DI), and dual production and injection (DP&I).