Making the Most of the Geometric Relationships Between Deviated Well Bores and Regional Natural Fractures

LORENZ, JOHN C., Sandia National Laboratories, Albuquerque, NM

A randomly oriented, horizontal well-bore azimuth has a 67% chance of intersecting more than 50% of those regional, vertical, natural fractures that it would intersect if it were normal to fracture strike. Similarly, if subsurface fracture strike can be determined to only within +30 degrees, a well bore can still be oriented to intersect 287% of the fractures. This percentage, or efficiency, is the sine of the angle between the well-bore azimuth and fracture strike. If horizontal well bores are not an option, the sine-function relationship works to the operator's advantage in well bores with only moderate deviations. Thus, if subsurface fracture strike is known, a well bore can be deviated through the reservoir at 30 degrees from vertical in the direction normal to fracture strike and will intersect 50% of the fractures that would be hit by a horizontal well bore of similar length with the same azimuth. Core from a 30 degrees well bore where fracture strike is unknown would still be extremely useful for fracture characterization because it significantly improves chances for recovering fractures in core; such a well bore has a 67% chance of a greater than or equal to 25% fracture-intersection efficiency. Moreover, fractures in deviated core can be oriented without an orientation survey by using bedding and the well-bore survey. An asymmetric scribe shoe (0 degrees -130 degrees -200 degrees ) is valuable for determining uphole direction of the core. In both thin-bedded and heterogeneous fractured reservoirs, where fractures are confined to bedding, there is a quantifiab e trade-off between number of beds intersected and number of fractures intersected as the well bore deviates from vertical. An intermediate well-bore deviation may offer more drainage efficiency than horizontal or vertical wells and maximize opportunities for fracture characterization.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)