Host: Hanchen Huang
Date: Friday, November 14, 2014
Location: 305 Shillman
Time: 10:30am – 11:30am
Refreshments at 10:00am
Sponsored by the MIE department
Abstract: The astonishing success of the U.S. industry with horizontal drilling and hydraulic fracturing, aka fracking, drastically improves the energy prospects. Many aspects of fracking, including the propagation of a single fluid-pressurized crack, are well understood by now. However, the geometry and evolution of the crack system still remains an enigma and mechanicians wonder: Why fracking works? The answer must be sought in the stability of interacting hydraulic cracks. Based on the known gas permeability of shale, on the known percentage of gas extraction from shale stratum and on two key features of the measured gas outflow which are 1) the time to peak flux and 2) the halftime of flux decay, it is shown that the crack spacing must be only about 10 cm. 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 long ago. 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 the localization instability can be avoided if the hydraulic pressure profile along the cracks can be made almost uniform. Whether it can depends on the rate and history of pumping of the fracking water. Of course, the proppants, gellants and acids in the fracking water play a role, too. Preventing localization in a vast system of fluid-filled pressurized cracks and open joints is, from the fracture mechanics viewpoint, what makes fracking work. Since currently only <15% of gas gets recovered from the shale strata, the localization is being prevented only partially. More extensive suppression of localization will be one way to increase the gas extraction percentage. This will also reduce the amount of fracking water per unit amount of extracted gas and thus mitigate the environmental footprint of fracking.