LAUBACH, STEPHEN E., and ELOISE DOHERTY
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Abstract: Natural Fracture Analysis Using Drilled Sidewall Cores

A procedure to obtain oriented drilled sidewall cores and to evaluate large natural fractures using microfractures was successfully tested in three gas wells in Silurian Medina Group Appalachian Basin. Microfractures from these cores document the strike of fractures that control production. They also indicate fracture size distribution and degree of occlusion of fracture porosity by authigenic cement. Sidewall core holes (and thus core azimuth) are readily visible on borehole image logs. The main challenge to orienting these cores is determining "up" direction for the 1-inch-diameter cores, because sedimentary facing indicators are rarely present and thus cannot be relied on, and currently deployed sidewall coring devices do not mark (scribe) cores. To find sidewall core tops we used fracture-surface marks induced on core ends during rotation of the core during recovery and a thorough examination of subtle evidence visible on borehole image logs and sidewall drilling records. Success rate for this core-orientation method is remarkably high, in these examples close to 85 percent. This shows that cost-effective fracture evaluation using drilled sidewall cores is feasible.

Despite the importance of fractures as fluid conduits in many reservoirs, the cost of full-diameter core or image logs may not be justified by the completeness and usefulness of the data that can be reliably obtained. Sidewall cores and microstructures are a practical additional source of information on fracture attributes that is not subject to the same sampling limitations as conventional methods.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas