Influence of the Weak Bedding Plane in Michigan Antrim Shale on Laboratory Hydraulic Fracture Orientation

摘要

The test results from both phases of the study showed that horizontal fractures were consistently created in samples with a confining (horizontal) stress in excess of 100 psi. Samples subjected to confining stresses less than 25 psi displayed only vertical fractures. Tests in the region between 25 and 100 psi showed either vertical or horizontal failures. Vertical fractures from both phases of testing showed a preferred fracture direction, suggesting a degree of anisotropy in the plane of bedding. Existing hydraulic fracturing theory was expanded for transversely isotropic rock to include the effect of two distinct tensile strengths. Experimental breakdown pressure trends were notably similar to those theoretically predicted for both vertical and horizontal failures. Miniature straddle packer operation was studied in unconfined plexiglass samples to observe the packer's sealing mechanics. Using a soft rubber (60 to 70 durometer) for the inflatable packer elements, it was found that the packer formed an 0-ring type seal with the borehole wall. A low (300 psi) packer set-pressure was all that was necessary to insure that the packer formed a physical plug in the packer-borehole annulus. Borehole pressures of 5000 psi were held by the miniature straddle packer. Hydraulic fracturing tensile strengths in the plane of bedding were measured for cores from Dow/ERDA well 100 (Sanilac County, Michigan). The tensile strength was found to increase with depth for the upper 165 feet of the formation. (ERA citation 05:018406)