Controls on Fracture Visibility in Vertical and Horizontal Boreholes

Fractures play an essential role in many unconventional reservoirs, yet our ability to see and characterize them is often limited. It is common to observe few vertical fractures in vertical image logs and many in horizontal or inclined well images. Core gives the highest resolution and the best characterization but has limited application because of the time and cost involved. Image logs are acquired more frequently and can be obtained from wells of all orientations and over long intervals. After de-biasing and comparing fracture intensities between vertical cores, vertical image logs and inclined/horizontal images there is a noticeable difference in the ability to detect/resolve fractures from the three data sources. Many fractures that are visible to the eye in core are not resolved in a wellbore image. There is clearly better visibility of fractures in the horizontal images than in the vertical images. In addition to the effect of well orientation on sampled fracture density, the effects of image coverage, obscuring features and altered stress at the wellbore wall influence the visibility of fractures. This paper examines these effects and compares the observed fracture abundance to a minimum size-intensity relationship derived from core observations.