This paper presents a seismic-attributes-guided method developed to assist drilling engineering both at design and execution stages.

In recent years, shale gas has been a focus of development in the Sichuan basin (southwest China). The formations are over 400 million years old; they have been affected by multiple major tectonic events during geological times. As a result, of successive stress and strain, the reservoir rocks now display a large number of small-scale faults, folds and multi-directional fractures. To maximize the value from wells, while avoiding drilling hazards, a detailed structural delineation is required for: reservoir-scale geosteering, well trajectory control and drilling risk prevention and mitigation.

Seismic is the only data available for 3D structural characterization. For an accurate result, high fidelity seismic (amplitude preserved high resolution data) is required. First a pre-stack data is selected (from gathers), conditioned and prepared as a micro cube for interpretation. Then a reservoir scale structural interpretation is carried out and a detailed structural map constructed, to analyze micro structural features from surface based attributes (amplitude, dip, curvature, etc.). In parallel multiple volume attributes are derived from the micro cube, including: variance, Ant-Tracking, apparent polarity and instantaneous bandwidth. In a final step surface attributes and volume attributes are integrated along with the planned well path to help anticipate possible drilling risk. ‘Ant-Tracking’ and ‘Seismic Discontinuity Planes’ algorithms were used to extract fault planes. The apparent polarity attribute reveals the sign of the reflection coefficient; it was found useful for delineating small events. Instantaneous bandwidth, represents the standard deviation of the instantaneous power spectrum about its mean, delineating vertical variation along the structural surface.

We were able to predict drilling risk and formation dip with the integration of seismic attributes.

The predictions were in good agreement with drilling events observed in the horizontal wells, such as: tight pull; mud loss; stuck pipe and drilling breaks. The predicted formation dip trend was confirmed. In one well, the original structural interpretation showed only one fault while the attributes analysis showed 11 faults. While drilling the well, for two of these faults, "bit drops" of 0.5m and 0.1m were observed. Only two predicted faults had no corresponding drilling event, but larger rock fragments were observed in cuttings, suggesting faulted zones may have been encountered.

This paper is based on several real-time-tracked horizontal wells, drilled with the assistance of integrated seismic attribute analysis (including particularly apparent polarity and instantaneous bandwidth).

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