Using advanced logging methods combining NMR and spectroscopy an Oil Saturation Index (OSI) is computed that correlates well with the productive layers of the Bazhenov formation. This article describes a method of estimating the OSI from log data, proposes a model for log interpretation in the Bazhenov Formation sediments and compares the results to core analysis. We present results from three wells.

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.

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The maintenance of Western Siberia oil production levels has required the industry to evaluate and develop ever more complex reservoirs. In this case study we look at the evaluation of the heavy oil bearing field located in the north part of Western Siberia and specifically the unconsolidated Pokurskaya formation of the Cenomanian age. The specific evaluation requirements included an understanding of the oil properties, especially viscosity and its variations, and an understanding of recovery factor by an analysis of residual oil saturation. In order to accomplish these objectives an extensive suite of log data was acquired including standard triple combo data, cross-dipole acoustic, NMR, micro-images, spectroscopy and formation tester samples and pressures.In this paper we show how an innovative use of NMR data allowed us to improve the fluid model and irreducible water saturation calculations. Combining NMR with standard data allowed a prediction of the residual oil saturation. Additionally, from the NMR data we were also able to extract oil viscosity information, however we needed a calibration point and for that a formation oil sample was required.Unconsolidated formations saturated with viscous oils are notoriously difficult to sample with formation testers. Sand production can lead to contaminated samples, tool plugging and even borehole collapse. To avoid these issues we included dipole sonic data to estimate formation strength in order to select the optimal sampling locations. Additionally a new 'dual intake' straddle packer configuration was used to optimize the fluid sampling. As a result of these precautions samples were successfully acquired and the resulting PVT data was used in field development planning as well as calibration points for a continuous NMR-derived viscosity curve. Advanced logging suite for Pokurskaya formation studying PetrophysicsAs industry attention turns to ever more difficult and complex reservoirs to exploit the complexity of the attendant petrophysical interpretations increase. Case in point is the Pokurskaya formation of the Messoyakha field in the Yamalo-Nenets Autonomous Okrug. To date no integrated log-core data analysis has been performed on this formation due in part to poor core data/recovery and incomplete log data sets. In this paper we describe how we performed such a study accounting for complexities such as multiple clay and matrix components, sorting variations, variable fluid saturations and viscous oil. We divided out study into two phases, an elementary analysis of the mineral composition and a fluid model. The primary outputs of the elementary analysis (ELAN) is a total pore volume and a clay volume. By using geochemical log data from a neutron capture spectroscopy log (ECS*) it is possible to evaluate the mineralogical components, resolve the clay components and then refine the total porosity estimation. We note that the neutron capture spectroscopy measurements is independent of external information, a significant advantage in our case as we had no core data...

Commercial production from the thinly-laminated Turonian deposits of North West Siberia has been proven in many wells. But despite the fact that we see these layers in many fields and they are in fact the primary development target, the reservoir properties are not well studied and thus their ultimate potential is unclear. To date, the obstacle has been the sand shale laminations that we encounter are on the order of a few millimeters to even fractions of a millimeter thick. Standard log interpretation method have proven to be inadequate, including the application of the latest deconvolution techniques of using a high resolution measurement such as a microimager to inform the layering of standard resolution devices. Even core analysis is ambiguous due to the heterogeneous and anisotropic nature of the reservoirs. In this paper we discuss a complete method of analyzing these thinly-laminated layers with a view to resolving a fuller petrophysical understanding.