Abstract

We present a systematic visual technique describing and quantifying fractures in core. It’s primary advantage over the traditional method used in industry is that it provides better and more pertinent data for use in both the exploration and development of fractured reservoirs. Use of this method can aid exploration by improving, first, the ability to identify mechanical facies and, second, to assess the relative importance of lithologic and structural controls of fracturing. The technique can also aid in development decisions by providing reservoir engineers with better data with which to predict fracture permeability and porosity away from the borehole.

The systematic procedure involves the measurement of fracture spacings, apertures (openings), and orientations. It also requires the accumulation of statistics on fracture terminations in core, from which one can estimate average lengths of fractures that extend beyond the borehole. The traditional method of logging fractures, in which the number of fractures per unit length of core is counted, can provide a crude estimate of fracture-prone and fracture-free intervals. That technique is appropriate for small fields whose development will be limited to one or two wells. However, it is inadequate for large fields, in which drilling strategies depend, first, on assessing the relative importance of lithology and structural position to fracture distribution; and, second, on predicting fracture permeability and porosity, quantitatively, throughout the reservoir.

This method is, to be sure, more time-consuming than the traditional technique; with the former, experienced personnel log at rates of 20 to 100 ft/day as opposed to 100 to 500 ft/day with the latter. However, numerous applications have shown that our systematic method rewards the added effort involved by providing useful information – data that would otherwise be unavailable. The traditional method, on the other hand, yields few if any meaningful results.

Our systematic technique of logging fractures also improves the reproducibility of fracture density data both from one core to the next and from one logger to the next. Better reproducibility allows quantification of the lithologic and structural controls of fracture density to an extent never before possible.