--> Abstract: Fracture Spacing in Slant/Horizontal Core, Mesa Verde Formation, CO: Comparison with Outcrop and Vertical-Core Data, by J. C. Lorenz; #91004 (1991)

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Fracture Spacing in Slant/Horizontal Core, Mesa Verde Formation, CO: Comparison with Outcrop and Vertical-Core Data

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

Core from the U.S. Department of Energy's Slant Hole Completion Test well (SHCT-1) shows an irregular but remarkably close fracture spacing in flat-lying reservoirs of the Mesaverde Formation in northwestern Colorado. Core was taken from 30 ft thick lenticular sandstones where the wellbore is inclined 60 degrees to vertical (266 ft of core), and from a 60 ft thick marine blanket sandstone where the wellbore is near-horizontal (115 ft of core), at the vertical depths of about 7100 and 7850 ft, respectively. In both zones, fractures cut across the core at a near-orthogonal angle to the core axis, with true lateral spacing averaging about 3 ft/fracture. Fracture spacing is not proportional to bed thickness. Fractures occur in swarms of up to five fractures each, and swarms are somewhat m re regularly spaced at 8-10 ft, although individual fracture spacing ranges from less than an inch to 17 ft. Only one fracture was present in the same zones in 175 ft of 4 in. diameter core taken in nearby vertical wells. Outcrops of the same facies, however, show irregular spacings that average on the order of 1.5 ft (lenticular sandstones) and 3 ft (blanket sandstones). Extrapolation of outcrop fracture data, or of fracture data from vertical wellbores, to engineering models of subsurface fracture spacing should be undertaken cautiously. However, subsurface fractures in flat-lying reservoirs can be more closely spaced than is commonly acknowledged.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)