Why Fracking Works

Abstract

Although spectacular advances in hydraulic fracturing, also known as fracking, have taken place and many aspects are well understood by now, the topology, geometry, and evolution of the crack system remain an enigma and mechanicians wonder: Why fracking works? Fracture mechanics of individual fluid-pressurized cracks has been clarified but the vital problem of stability of interacting hydraulic cracks escaped attention. First, based on the known shale permeability, 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 0.1 m. Attainment of such a small crack spacing requires preventing localization in parallel crack systems. Therefore, attention is subsequently focused on the classical solutions of the critical states of localization instability in a system of cooling or shrinkage cracks. Formulated is a hydrothermal analogy which makes it possible to transfer these solutions to a system of hydraulic cracks. It is concluded that if the hydraulic pressure profile along the cracks can be made almost uniform, with a steep enough pressure drop at the front, the localization instability can be avoided. To achieve this kind of profile, which is essential for obtaining crack systems dense enough to allow gas escape from a significant portion of kerogen-filled nanopores, the pumping rate (corrected for the leak rate) must not be too high and must not be increased too fast. Furthermore, numerical solutions are presented to show that an idealized system of circular equidistant vertical cracks propagating from a horizontal borehole behaves similarly. It is pointed out that one useful role of the proppants, as well as the acids that promote creation of debris in the new cracks, is to partially help to limit crack closings and thus localization. To attain the crack spacing of only 0.1 m, one must imagine formation of hierarchical progressively refined crack systems. Compared to new cracks, the system of pre-existing uncemented natural cracks or joints is shown to be slightly more prone to localization and thus of little help in producing the fine crack spacing required. So, from fracture mechanics viewpoint, what makes fracking work?–the mitigation of fracture localization instabilities. This can also improve efficiency by fracturing more shale. Besides, it is environmentally beneficial, by reducing flowback per m3 of gas. So is the reduction of seismicity caused by dynamic fracture instabilities (which are more severe in underground CO2 sequestration).

Growth model for large branched three-dimensional hydraulic crack system in gas or oil shale

“…(i) The hydraulic crack system must reasonably be expected to consist of roughly orthogonal vertical cracks (whose spacing should, according to previous analysis [11], be of the order of 0.1 m). This crack system geometry simplifies the analysis.…”

“…Because of the previous conclusion [11] that the spacing of water-filled parallel cracks or joints must be of the order of 0.1 m, and that the cracks must be nearly vertical and mutually orthogonal, the shale mass will be subdivided into six-node cubic (hex) finite elements (with linear shape functions; figure 4). Each element may contain two vertical, mutually orthogonal (or orthorhombic) cracks normal to the x 1 -and x 2 -axes, coinciding with the principal tectonic stresses T 1 and T 2 .…”

“…Based on the equations relating the measured gas flux histories at the wellhead (at Fayetteville shale basin in Oklahoma) to the typical permeability of shale [8][9][10], recent detailed analysis [11] showed that the crack spacing must be of the order of 0.1 m. If the spacing were 1 m or 10 m (as inferred by some from the typical 10 m spacing of perforation clusters), then the gas extraction would take about 100 times and 10 000 times longer, respectively, than it does. The inescapable conclusion is that currently the fracked portion of a fracking stage (only about 5-15% of the total volume) must involve about 10 5 vertical branched intersecting cracks, and that about 10 6 cracks would be required to frac the whole fracking stage (typically 70 m long, 150 m deep and 500 m wide).…”

Crack deflection in brittle media with heterogeneous interfaces and its application in shale fracking

“…While the main stream of research on fracture is targeted at avoiding fracture, promoting fracture in a controllable way has gained attention in recent years owing to the interest in exploiting underground energy resources such as shale gas ( Bažant et al, 2014 ). To achieve a high yield of shale gas collection, breakage of the low-permeability shale reservoirs where gas is encapsulated is desired.…”

The influence of crack-orientation distribution on the mechanical properties of pre-cracked brittle media

“…One key technology that exploits these resources is hydraulic fracturing, more commonly known as fracking ( Bažant et al, 2014 ). To achieve effective fracking, engineers must thoroughly understand the macroscopic mechanical properties of kerogen-rich shales, including their elastic properties and fracture strength.…”