The West Salym is a Salym Petroleum Development oil field in the Khanty-Mansi region, 120 km southwest of Surgut, Russia. The West Salym oil field was discovered in 1987 and was brought on stream in 2004. The reservoirs vary from fluvial/deltaic to shallow marine deposits. Primary development of the central area of West Salym Field is complete, but the field edges remain undeveloped and potentially attractive.

The edges of the field are presented by a mouth bar and characterized by significant structural formation changes, including unknown formation dips and local carbonate concretions and stripes. The sand thickness of the target layer is 15 m, with a minimum oil height of 1 m, which is caused by structural dips and nearby oil/water contact (OWC). These conditions make it difficult to drain the area with geometrically placed wells within the hydrocarbon-saturated layer using well correlation and 3D seismic-interpretation results. Another challenge is the low resistivity contrast between the shale, oil-bearing, and water-bearing layers. This poor contrast complicates the evaluation of reservoir properties and the ability to distinguish different fluid saturations. Two horizontal wells (500 m each) were drilled for the first time in West Salym Field to evaluate capabilities of modern reservoir-navigation technology using deep-azimuthal resistivity technology and advanced data-interpretation software. Drilling of both horizontal wells was improved with a standard wireline logging suite (gamma ray, spontaneous potential, resistivity, density, and neutron) and pressure-testing results available from pilot holes.

Logging-while-drilling (LWD) deep-azimuthal resistivity technology has been used in field development, contributing to proactive reservoir navigation. This technology provides input for interpreting an extensive set of multicomponent, multispacing, and multifrequency measurements. These data are usually sufficient to resolve the formation properties in the vicinity of several meters from the wellbore and to adjust the direction of the well trajectory. However, because of time restrictions, very simple resistivity models and only a subset of the data are often used for real-time interpretation. Moreover, the structure of the data subset is often predefined to provide the maximum depth of investigation, neglecting the resolution quality of the formation parameters. In some fields, it can lead to increased uncertainties during reservoir navigation.

The data-interpretation software mentioned in this paper has excellent performance and enables real-time processing of the full set of downhole measurements derived from multilayered formation models. This case highlights the first use of this software application in the Russian Federation.

The software is dependent on the method of the most-probable parameter combination, and it maintains an optimal balance between the information recovered from the measured data and all available a priori knowledge about the formation structure. The ability to accurately involve a priori information enhances the capability to resolve layers with low resistivity contrasts. Moreover, the inversion software is user-guided, enabling precise monitoring of lateral and vertical changes in the geology.

The data-inversion software ensured successful reservoir navigation in the challenging conditions of the West Salym Field. All steering decisions were made according to a consistent and reliable multilayered formation resistivity model that was constructed in real time during drilling. A good net/gross ratio was achieved of approximately 75% for one well and 50% for the other. Post-drilling analysis showed that geometric drilling without reservoir-navigation technology would lower the net/gross ratio to less than 40%.

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