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 Volga Urals basin of Western Russia has a very long production history and consequently many of the original target reservoirs have become depleted. The Uzenskoe field is no exception. Originally placed on production in 2008 the original target layers in the lower Cretaceous formations have since depleted. It was assumed that potentially productive reservoirs exist in the overlying layers; however, these had never been evaluated. Using a variety of through-casing measurements these layers were investigated for productive potential. Integrated interpretation of the acquired data was used evaluate lithology, rock composition, saturation and porosity to identify potentially productive layers. These layers were then perforated and tested with a wireline conveyed formation tester inside casing. Fluid type was verified, samples were acquired and pressure transient analysis was performed to determine layers permeability. The entire evaluation took less than three days of rig time comparing very favorably with alternative methods.
In this paper we discuss the planning, execution and interpretation of this evaluation exercise.
The Vikulovskaya formation of Western Siberia is characterized by thinly-bedded, sand-shale layers. The vertical thickness of these layers ranges from a few millimeters to a few centimeters. This layered feature presents well known challenges for petrophysical analysis from standard logging suite data. These layers are typically beyond the vertical resolution of the standard tools so net-to-gross cannot be derived directly. The shale layers suppress the resistivity readings in the oil strata and the resulting low resistivity contrast makes it difficult to determine the oil-water contact. Finally, the ability to resolve the individual sand layers makes it impossible to accurately determine their water saturation.In this paper we discuss how these challenges were surmounted when performing a petrophysical evaluation of a dataset acquired in a recently drilled well in the Krasnoleninskoe field. This dataset consisted of full bore core and traditional 'triple combo' data. Additionally, we had NMR data, high resolution micro-imager data and formation tester pressure and fluid analysis data. By combining the measurements from the traditional tools with the resolution of the micro-imager data we were able estimate the desired petrophysical properties of the thinly-bedded layers individually. By using tools with different physics we were able to realize an independent quality control of the interpretation: stationary NMR measurements were used as porosity and irreducible water saturation reference, and formation tester data of direct inflow composition were used as a reference for fluid saturations. As a final check on our method we performed a digital integration of core and micro-imager data to validate our findings. The resultant workflow is concisely explained such that it can be easily applied to similar evaluation environments.
The determination of saturation and therefore fluid contacts can be challenging in certain low permeability reservoirs of the Yamal peninsula. The low water salinity and high shale content make contact determination with traditional petrophysical methods difficult. The low permeability also complicates contact determination using pressure gradient analysis. Downhole fluid analysis seems a logical choice, but it too is fraught with difficulties in low perm, near saturated reservoirs.In this paper we show how to apply DFA for contact determination and how to mitigate, by interpretation and hardware configuration, the various challenges. Additionally we investigate the quality of the obtained samples with respect to their suitability for PVT studies.
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