The Yamal region of Western Siberia holds enormous reserves of gas and condensate across many geologic layers including the Achimov deposits of the Late Jurassic and Early Cretaceous. The Achimov however, is among the most challenging layers in the Yamal area with deep bedding, very low permeability, thin laminations and abnormally high reservoir pressures that all greatly complicate the appraisal and production of hydrocarbons. In this regard, accurate formation evaluation is essential to ensure efficient and economically reasonable methods of production. Modern methods of openhole logging, including NMR, acoustic and wireline formation testers (WFT) provide advanced information about the formation and can aid in the most efficient development. In this article we present the results of advanced methods of openhole logging that provides greater understanding of the characteristics of the Achimov reservoir. Special NMR measurements were used to estimate the residual fluid saturation which was confirmed with WFT tools designed for downhole fluid analysis and sampling. We also show how to overcome the negative impact of supercharging on measurements of formation pressure in the Achimov formations and the necessity of carrying out such measurements to validate the hydrodynamic reservoir model. To understand the validity of the samples acquired downhole a simulation was carried out further showing the range of possible variations of the basic PVT properties of hydrocarbons during the sampling. The results of advanced acoustic logging allows to estimate the anisotropy of the mechanical properties of the Achimov layers. The use of the data allowed us to model the fractures resulting from hydraulic stimulation and showed significant differences in the geometric characteristics of the fracture between wells and explains why the lower section of the Achimov are often depleted with respect to the upper sections.

For economic production, the heterogeneous low-permeability Achimov formation of the Urengoiskoe field requires stimulation with high-volume hydraulic fracturing. Shales acting as stress barriers are present within the reservoir, dictating the necessity of fracture geometry monitoring and design accounting for local changes in mechanical properties. A new slim dipole sonic tool enables a reliable identification of fracture geometry in the near-wellbore region. The tool dimensions allow it to be run in environments unsuitable for conventional-sized sonic tools such as completions with a small inner diameter. Analysis of sonic data in three wells reliably identified unintended fracture growth from the lower target Ach6 reservoir through the shale barrier into the upper Ach5 reservoir. Calibration of the fracturing model using the measured fracture height reveals that the main uncertainty in fracture geometry prediction originates from the input formation mechanical properties and stress profile. Therefore, fracture design optimization should include updates to the geomechanical model with account for the presence of structural VTI-anisotropy as well as justified dynamic-to-static conversions, refined pore pressure profiles, etc. This paper demonstrates the use of dipole sonic pre- and post-frac logs to determine the real fracture height as well as the corresponding impact on the calibrated fracture model. A way forward is proposed to improve geomechanical models by including sonic-based anisotropy parameters to improve fracture geometry predictability in target zones.