Vertical And Lateral Variations In Fracture Spacing In Folded Carbonate Sections And Its Relation to Locating Horizontal Wells
Authors: Nelson R.A. , Serra S. 
Published: March 2010

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 make these drilling determinations in fractured carbonate reservoirs, we are faced with determining the relative effect of lithology and structural 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 stratigraphic variation in fracture intensity than hinge zones or areas of higher curvature. In addition, lithologies exhibiting low fracture intensity off-hinge display larger increases when entering the hinge than those with higher initial off-hinge intensity.

The data further indicate that while average fracture intensity is better in hinge zones, flank positions contain layers of optimal properties that have fracture intensities as good if not greater than average intensities in the forelimb or hinge zone. The Conclusion is made that proper deviated or horizontal completions in optimum lithologic layers in flank positions (backlimb or forelimb) could give flow rates as high or higher than average hinge zone completions. Also indicated is a structural style or mode of structural development control on fracture intensity with leading-edge folds containing nearly an order of magnitude more fractures than foreland folds in the same stratigraphic package. In terms of drilling directions, results indicate that backlimb wells should follow optimum stratigraphic horizons, possibly a strike direction; while hinge wells should cross-cut multiple horizons, possibly in a general dip direction.

Introduction

The porosity and permeability of natural subsurface fracture systems are a function of fracture spacing or intensity (how many fractures) and fracture aperture available for fluid flow (how wide they are). Horizontal wells can be used to optimize the contribution of both parameters in fractured reservoirs (Figure 1). Since we can do little in early exploration to actively high-grade fracture aperture, much of our exploration activity in these reservoirs involves high-grading fracture intensity.

Fracture intensity can be defined and predicted by a combination of material property variations (a function of mineral composition, porosity, grain size, and mechanical bed thickness), in situ conditions (depth, pore pressure, temperature, and rate of deformation). and strain distribution within the section (structural position) UI. Because we are mostly interested in determining fracture intensity distributions in individual structures, the environmental parameters at fracturing are usually assumed to have been constant over the vertical and horizontal limits of the field, thus having little effect on relative fracture intensity variations. This leaves us with lithology and structural position as the prime factors to work with in picking optimum well locations, borehole trajectories and completion zones.

Past experiences with fractured carbonate reservoirs have led us to conclude that in many cases lithologic variations have a somewhere larger effect on fracture intensity than does structural position. This conclusion will be detailed with use of four carbonate rock sections of similar age and composition in the remainder of this manuscript.