ABSTRACT

An integrated approach for evaluating reservoir fractures using borehole acoustic-log data is presented. This approach combines borehole ultrasonic imaging, Stoneley-wave reflection, cross-dipole anisotropy, and the recently developed shear-wave reflection imaging techniques to provide an overall assessment of fracture permeability, orientation, and its connectivity around the wellbore. Evaluating Stoneley reflection over different frequency ranges allows us to detect permeable structures in formation. These structures, when aligned in a preferential direction, give rise to significant acoustic anisotropy from the cross-dipole measurement that characterizes them as permeable fractures. However, the fractures measured by these conventional acoustic techniques could also be the drilling-induced fractures that do not extend into the formation. Integrating the conventional measurements with shear-wave reflection imaging allows us to assess the connectivity of the fractures in the near-well region. In addition to the borehole guided flexural mode a dipole acoustic tool generates shear body waves that radiate away from the borehole and strike a fracture surface, and records the shear reflection from the fracture. The reflection of the shear waves, particularly the shear waves that polarize parallel to the fracture surface, are sensitive to open fractures in the vicinity of the well, allowing the fractures to be imaged using the dipole-shear reflection data. We use case examples to demonstrate the effectiveness of this integrated acoustic fracture evaluation approach.

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