An algorithm based on data obtained during underbalanced drilling operations is described for identifying the location and properties of productive natural fractures that are intersected by the wellbore. The criteria used to identify natural fracture locations are (i) total gas concentration measurements from mud logs and (ii) mud pit volume. The paper also describes a simple approach for estimating the fracture permeability. The algorithm is applied to data from three sets of horizontal parallel wells with spacing of roughly 700 ft. In two of the three well pairs considered, conductive fracture locations identified in one well aligned with corresponding features in the parallel well. An attraction of the proposed methodology is that it makes use of data that is commonly recorded during drilling, reducing the need for expensive image log tests. The information obtained from this type of analysis can be used to improve hydraulic fracture treatment designs.

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