Natural Fractures in the Barnett Shale: Why They Are Important

Gale, Julia F. W.¹, Jon Holder², Robert M. Reed¹ (1) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX

Microseismic monitoring of hydraulic-fracture treatments in the Barnett Shale shows that fractures propagate in an array wider than that expected from knowledge of the present-day stress field. It has been suggested that this is because hydraulic fractures reactivate natural-fracture systems. To investigate this possibility, we analyzed the orientation, size, intensity, and sealing properties of opening-mode fractures in several Barnett Shale cores from the Fort Worth Basin.

Natural fractures in the Barnett Shale are common. They are narrow (< 0.05 mm), sealed with calcite, and present in en echelon arrays. They are typically steep (> 75°), and the dominant trend is WNW. Other sets trend N-S. The narrow fractures are mostly sealed and cannot contribute much to reservoir storage or permeability. The fracture population, however, may follow a power-law size distribution, where the largest fractures in the population are open. A mechanical rock property, the subcritical crack index, was measured for different lithofacies. This index, along with mechanical layer thickness, governs fracture clustering. The subcritical crack index for all lithofacies tested is high, indicating strong fracture clustering. These results suggest that large open fractures exist in clusters spaced several hundred feet apart.

We also tested the effect of calcite-sealed fractures on tensile strength of shale. Samples containing natural fractures have half the tensile strength of those without. The junction between the fracture-wall rock and cement is weak because the calcite cement grows mostly over noncarbonate grains, thus providing a mechanism for hydraulic reactivation of sealed natural fractures.