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.
The subject field is located onshore Abu Dhabi in the United Arab Emirates. Production is from an alternating sequence of high and low porosity calcite layers. To minimize the environmental impact of the field development, the wellheads are clustered, thereby keeping roadworks, piping and other obstructions at surface to a minimum. As a result of this surface clustering the wells are drilled away from the location of the cluster and then turned parallel to the maximum formation stress orientation.
Early development of the field involved drilling clusters of wells on either side of a fault corridor that runs through the field. Figure 1 shows the existing wells (red lines) drilled from surface clusters and the faults (black lines) indicating the presence of a fault corridor through the field.