Most fractures below 2,500 ft (762 m) are nearly vertical or of high inclination. Even if there are horizontal fractures, they probably tend to close owing to the effect of overburden.

What are the possibilities of intercepting a vertical fracture with a vertical hole? Most likely those probabilities are very slim. Therefore, when looking for naturally fractured reservoirs, the chances for success would be better if, instead of drilling vertical holes, directional holes, perpendicular to the orientation of the fractures, were drilled.

If the fractures are not intercepted, it is nearly impossible to detect potential hydrocarbon intervals from logs utilizing conventional techniques, as these techniques are based on invasion of the fractures by mud filtrate. Furthermore, a drill-stem test will probably recover only some mud and the pressure will not increase significantly because of the very tight characteristics of the primary porosity system. Under these circumstances, the prospect probably would be abandoned. The problem is that a potential hydrocarbon reservoir might be abandoned.

Directional holes have advantages over vertical holes in naturally fractured reservoirs. A method is available which allows recognition of naturally fractured intervals even though the fractures are not intercepted. Finally, the effects of small and large fracture treatment differ when the fractures are or are not intercepted by the borehole.

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