Fracture Statistics Derived From Digital Ultrasonic Televiewer Logging
Authors: Schmitt Douglas R. 
Published: March 2010

A 300 m section of core from a highly fractured igneous formation is directly compared to digital ultrasonic televiewer images of the borehole wall. The two different images consist of a mapping of the amplitudes travel-times of ultrasonic pulses reflected from the wall rock with respect to depth and magnetic north. These data were acquired during logging using a specially designed high-speed digitization system. The number of planar features seen in the amplitude and travel-lime images were 867 and 322, respectively, whereas 567 fractures and veins were observed in the core. The depth distributions of planar features seen in the core and in the travel time image appear to correlate. This correlation suggests that the travel-time log features in this paper are indicative of those in situ fractures which are open and permeable as determined by other means. Further, the travel-time planar features dip in the same direction as that expected for the greatest compressive stress. Conversely, this observation implies that the travel-time feature orientations possibly reveal in situ stress directions. Core examination indicates that the observed fractures are predominantly shear and, for a given state of stress, the orientations of these type of fractures differ from the orientations of extensional joints which open perpendicular to the direction of the least compressive stress. Extensional joints have been reported in televiewer logs by other workers. Hence, to delineate in situ stress directions from fractures observed in televiewer images, it must be distinguished whether a fracture results from either shear or extensional failure.

Introduction

Both natural and artificial fractures enhance fluid flow in the crust. In dense limestone, tight gas sand, coal and most igneous and metamorphic rock formations where the intrinsic permeability is low, economic extraction of hydrocarbon and geothermal resources can only originate from natural or artificial joints and fractures. Further, natural joints also impede and complicate underground construction. Hence, some knowledge of the density and orientation of in situ fractures is desirable for the efficient completion of many different engineering endeavours and both fracture detection and evaluation is important.

A short listing of fracture detection methods includes borehole and surface seismics(1, 2, 3), radar(4), temperature gradient perturbations(5), sonic logging(6), resistivity scanning(7), and ultrasonic borehole televiewer analysis(5, 8, 9, 10). Each of these methods provides a piece of relevant information on fracture density, aperture, orientation, permeability, or location. Differentiating which fractures are open and permeable from those which arc closed and impermeable is especially crucial. Because the resolution of the fracture sensing techniques varies from a few metres for seismic measurements to centimetres for some wireline logging procedures, accurate characterization of in situ joints and fractures requires application or a number of methods.

Of the above techniques mentioned, high resolution ultrasonic borehole televiewer logging(8) has been commercially exploited for nearly two decades. In conventional analog usage the televiewer provides a two-dimensional linear grey scale image of the borehole wall by mapping me strength, i.e. amplitude, of ultrasonic pulses reflected from rock from the interface between the wellbore fluid and the wall rock.