Optimum drilling direction and attitude in fractured reservoirs are afunction of the in situ stress field, the orientation of the natural fracturespresent, and the 3-d variation in their fracture intensity or spacing. To makethese drilling determinations in fractured carbonate reservoirs, we are facedwith determining the relative effect of lithology and structural position onsubsurface fracture intensity. Work on several North American folded carbonatesections indicates that weakly deformed or lower curvature portions of thefolds display an overall greater stratigraphic variation in fracture intensitythan hinge zones or areas of higher curvature. In addition, lithologiesexhibiting low-fracture intensity off-hinge display larger increases whenentering the hinge than those with higher initial off-hinge intensity.
The data further indicate that while average fracture intensity is better inhinge zones, flank positions contain layers of optimal properties that havefracture intensities as good if not greater than average intensities in theforelimb or hinge zone. The conclusion is made that proper deviated orhorizontal completions in optimum lithologic layers in flank positions(backlimb or forelimb) could give flow rates as high or higher than averagehinge zone completions. Also indicated is a structural style or mode ofstructural development control on fracture intensity with leading edge foldscontaining nearly an order of magnitude more fractures than foreland folds inthe same stratigraphic package. In terms of drilling directions, resultsindicate that backlimb wells should follow optimum stratigraphic horizons, possibly a strike direction; while hinge wells should cross-cut multiplehorizons, possibly in a general dip direction.
The porosity and permeability of natural subsurface fracture systems are afunction of fracture spacing or intensity (how many fractures) and hydraulicfracture aperture (how wide they are). Horizontal wells can be used to optimizeboth parameters in fractured reservoirs (Figure 1). Since we can do little inexploration to actively high-grade fracture aperture, much of our explorationactivity in these reservoirs involves high-grading fracture intensity.