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Achieving the objectives of drilling horizontal wells is greatly dependent on accurate placement of the drainhole in the reservoir. Careful pre-job planning using offset well data and seismic maps helps to reduce the uncertainty to some extent. However, the unpredictability of structural and stratigraphic variation between the planned and offset wells and uncertainty inherent in seismic data are always associated with horizontal well planning. The formation dips computed from seismic and offset wells are therefore prone to some degree of error. Advances in real-time data transmission technology have made it possible to transmit image data uphole while drilling, through improving the robustness of the real-time signal, signal bandwidth and data compression. Logging-while-drilling (LWD) real-time azimuthal resistivity images can be used for accurate estimation of the dip and strike of formations intersected along the well trajectory. In addition, structural surprises such as faults can be detected, and remedial actions taken immediately. Such information is of great value in making geosteering decisions while drilling into reservoirs within structurally complex geology or in an area with sparse geological information. Using image data, trajectory adjustments can be made during drilling yielding optimal well placement. We descibe the use of real-time image interpretation during the drilling of two horizontal wells within a block bounded by major faults on the north and south, with a series of smaller associated faults, anticipated but not confirmed prior to drilling from existing data, in a carbonate field in United Arab Emirates. The image data provided crucial input to guide the well trajectory through the target reservoirs resulting in the wells objectives being achieved. In addition, the image data clarified the regional geology of the area around the wells, thus helping to update the geological model for better description of the structurally complex reservoir. |
