This article is a description of an integrated engineering approach to solutions selection for efficient and safe drilling of unstable intervals represented by interbedded coal and argillite layers. Due to specific features of these formations, any significant mechanical stress, as well as penetration of drilling fluid filtrate, may lead to wellbore instability regardless of the drilling mud type used. The paper presents a description of the features of drilling in unstable intervals with various types of drilling muds (WBM/OBM) in Yamalo-Nenets Autonomous Okrug (YNAO).

Experience has shown that drilling through coal intervals prone to instability may cause significant non-productive time (NPT). Such wells should be designed with an integrated engineering approach, which covers the entire cycle: starting with the well path planning, casing setting depths selection, BHA design and drilling regimes of the risk interval. No less important, detailed development of solutions for drilling muds.

During the study the main causes of issues in wells drilled through the interbedded formations of coal and argillites in two fields were identified. As a result, a set of measures was developed to minimize risks for each type of mud (OBM and WBM):

  • Revision of the initial well design solutions. Selection of optimal mud weight based on the experience gained and the geomechanical model

  • Revision of chemicals concentrations together with the use of additional additives

  • Placement of stabilizing pills across unstable intervals

  • Well path optimization

  • Development of safe drilling procedures.

The measures developed for various types of drilling muds allowed minimizing the NPT and successfully completing the wells on time. The experience gained formed the basis for recommendations to prevent issues associated with the coal layers instability in the region.

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