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.