AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Role of Strain in Controlling Orientation of Natural Hydraulic Fractures in Gas Shales
(1) Penn State, University Park, PA.
Horizontal, bedding-parallel veins constitute the most common fracture type displayed in core and in borehole images from the Jurassic Haynesville gas shale of Louisiana. Vertical joints and veins are more common in both outcrops and core from the Middle Devonian Marcellus gas shale of the northern Appalachian Basin. Differences between the orientation of mesoscopic cracks populations in the Haynesville and Marcellus begs a hypothesis based on the premise that crack orientation is controlled by stress state with cracks propagate normal to the least effective stress at the time of propagation. This means that crack propagation in the Haynesville took place while the least effective stress was vertical whereas crack propagation in the Marcellus took place when the least effective stress was horizontal.
One might presume that differences in tectonic history between the Haynesville and Marcellus were responsible for the difference in stress orientation. The Haynesville was deposited in a restricted basin on a passive margin. If there was any tectonic overprint, it was a subtle down-dip extension toward the Gulf of Mexico. The Marcellus deposited in an interior seaway that was later subject to a significant layer parallel shortening. In the Haynesville, down-dip extension might serve to relax a horizontal stress, thereby maintaining a stress state represented by normal faulting (Sv > Shmin = σ3). After deposition in the Devonian, the Marcellus was subject to foreland shortening during the Alleghanian orogeny, thereby being subject to a stress state represented by thrust faulting (Shmin > Sv = σ3). If crack propagation by hydraulic fracturing were active during these tectonic states, the Haynesville should have been populated with vertical cracks and the Marcellus should have been populated with vertical cracks. Because the opposite is true there must be another explanation for difference in predominate crack orientation in the Haynesville and Marcellus. The difference in maximum pore pressure gradient (0.95 psi/ft3 vs. 0.7 psi/ft3) might provide the clue. The interaction between pore pressure and stress is captured in a coupled behavior defined as poroelasticity. The hypothesis constructed in this paper is that poroelastic deformation coupled with strain (uniaxial vs. triaxial) is the root cause of a stress state for the Haynesville (Sv > Shmin = σ3) and Marcellus (Shmin > Sv = σ3).