VERTICAL AND LATERAL VARIATIONS IN FRACTURE SPACING IN FOLDED CARBONATE SECTIONS AND ITS RELATION TO LOCATING HORIZONTAL WELLS

Optimum drilling direction and attitude in fractured reservoirs are a function of the width and orientation of the natural fractures present , and the 3-D variation in their fracture intensity or spacing. To mak e these drilling determinations in fractured carbonate reservoirs, we are faced with determining the relative ;effect of lithology and struc tural position on subsurface fracture intensity. Work on several North American folded carbonate sections indicates that weakly deformed or lower curvature portions of the folds display an overall greater strat igraphic variation in fracture intensity than hinge zones or areas of higher curvature. In addition, lithologies exhibiting low fracture int ensity off-hinge display larger increases when entering the hinge than those with higher initial off-hinge intensity. The data further indic ate that while average fracture intensity is better in hinge zones, fl ank positions contain layers of optimal properties that have fracture intensities as good if not greater than average intensities in the for elimb or hinge zone. The conclusion is made that proper deviated or ho rizontal completions in optimum lithologic layers in flank positions ( backlimb or forelimb) could give flow rates as high or higher than ave rage hinge zone completions. Also indicated is a structural style or m ode of structural development control on fracture intensity with leadi ng-edge folds containing nearly an order of magnitude more fractures t han foreland folds in the same stratigraphic package. In terms of dril ling directions, results indicate that backlimb wells should follow op timum stratigraphic horizons, possibly a strike direction; while hinge wells should cross-cut multiple horizons, possibly in a general dip d irection.