Abstract: Comparison of Borehole Image Logs, Conventional Logs and Cores in Fractured Sandstones

Peter N. Gale, Bruce N. Carpenter, Kevin E. Nick

Open, conductive-mud filled vertical fractures can be detected by conventional Induction-Laterolog type (LL8, LL3, SFL, FL) combinations, since high-conductivity vertical paths affect the laterolog type logs more than the induction logs. This method has been validated by the recent addition of borehole imaging logs which provide excellent documentation of the fractures along the borehole wall. However, these tools apparently make no distinction between naturally occurring and drilling-induced fractures. In most cases the origin of fractures can be determined by examination of core material.

Generation of fractures was considered to be more likely during coring than during rotary drilling since the coring bit transfers more torque to the formation. However, a study of fractured sandstones from two wells in the Cottage Grove Formation, Dewey County, Oklahoma, which were partially cored and partially rotary drilled, finds continuity of fractures that are drilling induced. Analysis of borehole image logs show similarities in fracture characteristics observed in the cores and in the logs and allowed comparison of fractures in the cored and rotary-drilled hole.

Excessive weight on bit and/or elevated hydraulic pressure on the bottom of the hole appears to be the common cause of drilling-induced fractures. The fractures are created downward ahead of the bit and extend laterally beyond the part of the hole drilled or cored. Induced fractures are present in the borehole wall and can be detected by logging tools.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994