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Theme 4: Mechanics

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Fracture gradient curves

Variations of the fracture gradient in a normal faulting environment to illustrate the influence of B (1/E) and n on the degree of stress; relaxation (modified after Xu et al., 2017).
Bn: Barnett, Hv: Haynesville, Ef: Eagle Ford, FSJ: Fort St. John, Lp: Lodgepole, MB: Middle Bakken, LB: Lower Bakken, ThF: Three Forks, RV:Reedsville, UU: Upper Utica, BU: Basal Utica, PP: Point Pleasant, LX - Lexington, TR - Trenton. The circles are samples deformed normal to bedding, the inverted triangles are samples deformed parallel to the bedding. From Xu et al. (2017).

experimental apparatus

Schematic of CT-scannable core holder capable of applying triaxial stress (Glatz et al 2018). The core holder is built, tested, and available for this EFRC. The system is capable of 14 MPa confining pressure, 69 MPa vertical load, and 230 °C. Typical spatial resolution is 200 by 200 by 625 mm.

 

 

Time dependent deformation curves

Time dependent deformation as a function of clay content in the Barnett shale and Haynesville shale and as function of loading direction on samples of the Eagle Ford of similar composition. Unconventional formations are anisotropic both elastically (being more compliant normal to bedding) and viscoplastically (creeping more normal to bedding).

 

 

mohr circles

Schematic diagram illustrating how in normal and strike-slip faulting environments, viscoplastic strain relaxation in shales result in an increase in the magnitude of the least principal stress.
The cartoon on the left shows a moderate increase in the magnitude of the least principal stress above the upper sand resulting from a minor amount of viscoplastic strain relaxation whereas a greater amount of stress relaxation in the shale below the sand creates a larger stress stress difference and thus a more effective barrier to vertical fracture growth.

Transport through fractures is strongly correlated with the stress state of unconventional rocks, rock properties, and the type of fluid saturating the pore space. While water is the hydraulic fracturing fluid of interest, replacement of fresh water is considered through creation of a knowledge base of geomechanical properties of unconventional shales in the presence of nonaqueous fluids such as carbon dioxide and nitrogen. New laboratory measurements under triaxial conditions will be conducted and results generalized. Many of these tests will be imaged using X-ray CT.
Micromechanical computations will provide further insight and advance the numerical description of geomechanics from the grain to the continuum scale. A major component of the envisioned effort is to improve fundamental understanding of the role of physical mechanisms of shale mineral, kerogen, and nonaqueous and aqueous pore fluid interactions with respect to mechanical properties. Ultimately, such knowledge contributes to better design of hydraulic fractures.

Anthony Kovscek
Tony Kovscek
Hamdi Tchelepi
Hamdi Tchelepi