Department of Mechanics: Seminar: Abstract Bažant 2015

Why fracking works and why not well enough

The astonishing success of the U.S. industry with horizontal drilling and hydraulic fracturing, aka fracking or frac, drastically improves the energy prospects of the U.S. Many aspects of fracking, including the propagation of a single crack with the flow of pressurized incompressible fluid through the crack, are well understood by now. However, the topology, geometry and evolution of the crack system still remains an enigma. This makes mechanicians wonder: Why fracking works? The answer must be sought in the stability of interacting hydraulic cracks. Based on: 1) the known gas permeability of shale, 2) the known percentage of gas extraction from shale stratum, 3) the observed time to peak flux of gas at the wellhead and 4) the observed halftime of flux decay, it is shown that the crack spacing must be only about 10 cm. This roughly coincides with rock joint spacing and implies about 106 interacting vertical cracks per frac stage. Attainment of such a small spacing requires preventing localization in parallel crack systems. This is a stability problem analogous to a system of parallel cooling or shrinkage cracks studied at Northwestern in 1970s. Formulated is a hydro-thermal analogy which makes it possible to transfer solutions from cooling to hydraulic cracks. From this analogy, and from new numerical solutions of stability of a system of pressurized circular equidistant vertical cracks, it is concluded that, at slow flow, the localization instability can be avoided if the hydraulic pressure profile along the cracks can be made almost uniform. How it depends on the rate and history of pumping of viscous frac water, and on the proppants, gellants and acids in the water, is being studied. Preventing localization in a vast system of growing cracks interacting with the flow of frac water through the cracks (or open rock joints) is, from the fracture mechanics viewpoint, what makes fracking work. But not well enough, since currently only 5-15% of gas gets extracted from the shale strata. More extensive suppression of fracture localization will be one way to increase the gas extraction percentage. It will also reduce the amount of frac water per unit amount of extracted gas and diminish the potential for seismicity since narrower and denser cracks make smaller dynamic jumps at inhomogeneities. This will mitigate the environmental footprint. Finally, similarities with deep sequestration of waste water and CO2 are pointed out.

References:

• Bažant, Z.P., Salviato, M., Chau, Viet T., Viswanathan, H., and Zubelewicz, A. (2014): Why fracking works, ASME J. of Appl. Mech. 81, 101010-1---101010-10
• Bažant, Z.P., and Ohtsubo, H. (1977): Stability conditions for propagation of a system of cracks in a brittle solid, Mech. Res. Communications 4, 353--366

Biography: Born and educated in Prague (Ph.D. 1963 at CTU), Bažant joined Northwestern in 1969, where he has been WP Murphy Professor since 1990 and simultaneously McCormick Institute Professor since 2002; and Director of Center for Geomaterials (1981-87). He was inducted to NAS, NAE, Am. Acad. of Arts & Sci. and Royal Soc. London; to the national academies of Italy, Austria, Spain, Czech Rep. and Lombardy; to Academia Europaea and Eur. Acad. of Sci. & Arts. Honorary Member: ASCE, ASME, ACI, RILEM. Received 7 honorary doctorates (Prague, Karlsruhe, UC Boulder, Milan, Lyon, Vienna, Ohio State); Honors: ASME Timoshenko, Nadai and Warner Medals; ASCE von Karman, Newmark, Biot, Mindlin and Croes Medals and Lifetime Achievement Award; SES Prager Medal; RILEM L'Hermite Medal; Exner Medal (Austria); Torroja Medal (Madrid); Šolín Medal (Prague), etc.; was President of SES, founding pres. of IA-FRAMCOS & IA-CONCREEP; was editor (in Chief) of JEM, ASCE. He is Illinois Registered Structural Engineer. Authored six books: Scaling of Structural Strength, Inelastic Analysis, Fracture and Size Effect, Stability of Structures, Concrete at High Temperatures, and Concrete Creep. Citations: 45,000, H-index: 103 (Google June 2015, incl. self-cit.), i10 index: 492. He is one of the original top 100 ISI Highly Cited Scientists in Engineering (of all fields); www.ISIhighlycited.com.