Realtime formation evaluation is critical not only for various geomechanical applications but also for quick drilling decision making in both exploration and development scenarios. The study focuses on a horizontal development well in a carbonate field which has a thin oil column overlain by a large gas cap. Large exposure multi-lateral development wells are used to improve drainage of the oil column by reducing the tendency for gas coning.

The real time formation evaluation proved formation fluid in the landing section as gas, and as a result a quick decision was made to change the well trajectory. The well was drilled 20m down in TVD as a pilot well to perform down hole fluid profiling using the Modular Formation Dynamics Tester (MDT), however, MDT could not be carried out due to heavy mud losses and the well was plugged. Finally, it was decided to sidetrack the well.

MDT with Live Fluid Analyzer (LFA) and Compositional Fluid Analyzer (CFA) was deployed to perform downhole fluid profiling in the side tracked well to identify the hydrocarbon type and composition at 3 stations. With the help of inclinometry information well trajectory and probe orientation were monitored so that sampling points were selected with probe on the high side of hole and outside of sumps if present. Fluid scanning stations using CFA/LFA clearly indicated the fluid type as gas. Fluorescence was monitored in real time to ensure that gas samples were collected above dew point. Samples collected at 3 depths were examined at surface and to confirm the CFA result. The CFA provided the C1, C2-C5, C6+ weight fractions, GOR and apparent hydrocarbon density in real-time. The fluid analysis recorded by the CFA matched well with the known gas composition and fluid density after corrections for mud filtrate were applied.

This paper shows the importance of LWD in terms of quick real-time decision making over wire line logs which are recorded after the well is drilled up to target depth. The downhole fluid profiling in sidetracked well helped in taking quick decision as not to drill further.

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