ESTIMATING AVERAGE FRACTURE SPACING IN SUBSURFACE ROCK

Knowledge of the spacing of fractures in reservoir rocks (i.e., the di stance between parallel fractures in a subsurface joint set) can lead to a better understanding of the production characteristics of a reser voir and serve to quantify the relative degree of deformation in subsu rface rocks. In this paper, I present a new method for estimating the spacing of subsurface fractures; this new method is easy to use from t he standpoint of both data collection and data analysis. The average f racture spacing method can be applied with boreholes of any orientatio n relative to a fracture set. The method is especially powerful when i t is used for the relatively common case of a borehole nearly parallel to a fracture set (e.g., vertical borehole intersecting vertical frac tures). Average fracture spacing is estimated from an analytical solut ion based on observed borehole-fracture intersections and observed fra cture porosity; the only data required are the dimensions of the core (or imaged borehole) and the total height of all sampled fractures. Be cause the likelihood of intersecting fractures increases when a well i s deviated perpendicular to the fractures of a set, fracture reservoir s commonly are candidates for deviated boreholes. An informed decision on borehole deviation requires predicting the fracture intersection f requency as a function of both deviation magnitude and direction. A ne w method, based on probabilities of borehole-fracture intersections, u ses spacing and height data from subsurface joint-like fractures and t he borehole diameter to predict fracture intersection frequencies for all possible well deviations. Fracture intersection frequency solution s are presented with respect to a conventional geographic reference fr ame, thus simplifying even the most complex three-dimentional situatio ns.