Real-Time Geological Positioning of a Well in the North Sea

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Abstract This paper describes the pre-job and real-time geological modeling of a horizontal well in a field in the North Sea. Using the PayZone Steering™ (PZS™) system, LWD gamma ray and resistivity data were used to continuously locate the wellbore in the geological section, modifying the geologic model in real-time as needed. Drilling was assisted by the real-time transmission and display of the LWD information, including downhole pressure and computed ECD, at multiple locations both onshore and offshore. The well was planned to develop the Brent Sand by traversing two fault blocks, the second being downthrown and the primary target. The original geologic interpretation suggested major lithologic and stratigraphic differences between the two blocks. Therefore pre-job modeling was performed using three offset wells within the second fault block. In the drilled well, the formation in the second block proved approximately 20 meters higher than expected as confirmed by comparison with the offset wells. Significant facies changes in the channel sands made detailed correlations with the offset wells difficult. Fortunately for concurrent detailed analysis, the geology in both blocks proved similar and the drilled well penetrated the target sand in both the upthrown and downthrown blocks. As a result, the LWD logs acquired in the first (upthrown) block provided the best correlations with the horizontal section, allowing for accurate determination of the well's geologic position. The project team was thus able to successfully detect formation tops that were different from the modeled geology, continuously locate the position of the wellpath in the geologic section, and update the geologic model in a timely fashion. This information was then used to dynamically modify the drilling program.