To Successfully Address Sub-Surface Faulting Uncertainties – Integration of Seismic and Geosteering Operation Analysis

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Abstract Fault zones pose significant operational challenges and risks, including wellbore stability, formation damage, changes in lithology, porosity, permeability, and stress regime. These complexities lead to uncertainties in drilling, well placement, formation evaluation and completion operations, potentially affecting the overall success of field development. This paper aims to introduce the best practice to overcome these challenges based on years of operational experience in one major offshore field. Faults can be identified utilizing different methods including seismic interpretation, geosteering interpretation and high-resolution resistivity image log interpretation. Seismic interpretation is the only way to predict fault in the pre-drilling phase and to guide the well planning, however, it has the highest uncertainty of +/- 500 ft. Geo-steering interpretation has relatively less uncertainty of +/- 50 ft and enables the adjustment of well trajectory during drilling operation but can only confirm fault with throw or with drilling response (losses, torque & drag abnormity, etc.). Borehole image log has the least uncertainty of +/- 1 ft, it is preferred to identify any possible fault while well placement, and other primary and secondary structural features, which can be incorporated to optimize the modelling. Image log should be included in data gathering program for wells expected to encounter faults along lateral section, however, it requires processing and interpretation of the memory data, which is only available after drilling the well. Therefore, seismic interpretation, geosteering interpretation and resistivity image log should be integrated throughout well planning, operation and interpretation process of wells expected to encounter faults. Multiple wells were drilled in the fault zone of the discussed field, which has significant uncertainties of local formation dip, reservoir thickness, complex lithology, and heterogeneity. Faults were countered in different wellbores with different scales. A fault was encountered near to the landing point in one of the wells, however well landed successfully by implementing the structural information in geo-steering model. In another well, a major scale fault (~70 ft throw) was observed in lateral section, however well position was confirmed shortly. The Geo-steering model was updated in real-time to bring the well back into the target formation. Borehole image data confirmed the actual fault while others were local features. All the wells were also completed successfully after integrating the porosity, calibrated NMR permeability, saturation, fracture/fault analysis, azimuthal caliper, and pressure testing data into the completion design. The objective for field development was achieved. The case history demonstrated the importance of understanding the geological challenges addressed by fault zones, and necessity of integrating different disciplines to optimize drilling, well placement, formation evaluation and completion operations.

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