Containment of Natural Fractures in Brittle Beds of the Austin Chalk

ABSTRACT: The vertical continuity of natural fractures in a formation such as the Austin Chalk is dependent upon whether or not the fractures tend to be throughgoing or are contained within brittle chalk layers separated by shale/marl layers. Holifield (1992) emphasizes the economic importance of recognizing a horizontally stratified reservoir when planning the trajectories of vertical, slant and horizontal boreholes. Field observations of the Atco Member of the chalk at the Leigh Quarry in Waco indicate that fractures tend to propagate upward from brittle layers; fan and die out in the shale/marl members, and exhibit a spacing about equal to brittle-bed thickness. Laboratory experiments involving the propagation of a mode 1 fracture in a brittle layer toward a clay layer at 10 and 50 MPa confining pressures indicate that fracture containment in the brittle layer is enhanced by increasing thickness of the clay layer, decreasing effective confining pressure, using wet versus room dry clay, reducing the displacement rate and changing the brittle member from Berea Sandstone to Austin Chalk.