Abstract

There are two problems that are rarely considered when drilling a deviated well for the purpose of draining naturally fractured reservoirs:

1. How to intersect enough fractures with the pilot hole to accurately characterize the fracture population.

2. Does reservoir heterogeneity limit vertical permeability, despite fractures, such that a horizontal well provides little benefit in enhanced drainage volume?

The first problem can be addressed with slightly deviated well bores. A 30-degree deviation from vertical will enhance the probability of intersecting a vertical fracture in a 35-ft thick reservoir by up to 6200%. The optimum 6200% occurs if the well-bore azimuth is oriented normal to fracture strike. However, even if fracture strike is unknown, there is a two-thirds chance of intersecting at least half of this percentage with a randomly oriented well-bore azimuth.

The second problem becomes severe in horizontally laminated reservoirs where fractures are bounded by bedding and where lateral well-bore lengths are limited due to lease lines or small reservoir size. In these reservoirs, there is a quantifiable advantage to drilling slant holes with deviations less than about 80 degrees. Increases in reservoir heterogeneity increase the drainage-volume advantage of the short slant hole over that of the horizontal hole. Thus, there is still a need for sedimentary characterization of fractured reservoirs.