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Abstract: Average Fracture Spacing in Subsurface Rock

Wayne Narr

Knowledge of the spacing of fractures in reservoir rocks (i.e. the perpendicular distance between parallel fractures in a subsurface joint set) can lead to a better understanding of the production characteristics of a reservoir and serve to quantify the relative degree of deformation in subsurface rocks. This paper presents a new method for estimating the spacing of subsurface fractures that is easy to use from the standpoint of both data collection and data analysis. Average fracture spacing (Sav) is estimated from an analytical model based on observed borehole-fracture intersections from core or borehole image data. The method develops a ratio between the core/borehole dimensions (diameter Wc and height Hc) and the total fracture height, Hf. In its simplest form, the computation is Sav = WcHc/Hf. Boreholes of any orientation can be analyzed.

Because the likelihood of intersecting fractures increases when a well is deviated orthogonal to a set of fractures, fracture reservoirs are frequently candidates for deviated boreholes. An informed decision on optimal borehole deviation requires prediction of the fracture-intersection frequency as a function of both deviation angle and direction. A new method, based on probabilities of borehole-fracture intersections, uses spacing and height data from subsurface joint-like fractures, and the borehole diameter, to predict fracture-intersection frequencies for all possible well deviations. Solutions are presented for reservoirs having multiple fracture sets and for dipping beds. Graphical output is from a standard commercial spreadsheet program, thus simplifying implementation of the s lution.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela