AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Importance of Subcritical Index for Determining Fracture Spacing in the New Albany Shale, Illinois Basin
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
(2) Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX.
Production of gas from the New Albany Shale in southern Indiana and western Kentucky, where gas is mostly thermogenic, is partly dependent on understanding how the natural fracture system interacts with hydraulic fracture treatments. A key component of this interaction is natural fracture spacing, which includes the distance between individual fractures as well as the spacing between clusters or swarms of fractures. Static analysis of the stress field around opening-mode fractures suggests that fractures should be regularly spaced at an interval approximately equal to bed thickness. However, observations from outcrops show that fractures in New Albany Shale can be regularly spaced at widths less than bed thickness or clustered in swarms of fractures. Thus, we find that mechanical layer thickness alone is insufficient to explain observed fracture spacing patterns.
Natural opening-mode fractures propagate at subcritical velocities that are lower than the critical rupture velocity of the material. The subcritical index of the material relates the propagation velocity of the fracture to the stress intensity at the fracture tip. Subcritical indices are influenced by lithologic properties and the fluid composition at the crack tip. Low values (n=5) promote the development of clusters of fractures with irregular spacings much less than bed thickness, intermediate values (n=20) result in regularly-spaced fractures with intervals proportional to layer thickness, and high values (n=80) cause fractures to form widely-spaced clusters. Dual-torsion beam tests have been performed on 27 samples from 2 New Albany Shale cores, with subcritical index values ranging from n=24 to n=208, and having a mean of n=61. Subcritical index values are lower than those of Barnett Shale samples we have tested; these commonly have values of n>100 indicating widely spaced clusters. We explore the variability in the New Albany subcritical index data by altering the saturating fluid of the test, as well as examining our samples petrographically to determine the influence of macro- and micro-scale lithologic variation on the test results. Overall, our results suggest that parts of the New Albany have evenly spaced fractures and other parts have widely spaced clusters, and that the underlying controls on these differences are lithologic controls on subcritical index.