Abstract

The recent advances in hydraulic fracturing of oil and gas bearing rocks, aka "fracking", have been nothing less than astonishing. However, several aspects of shale fracking such as the topology, geometry, and evolution of the crack system remain not yet understood. In this contribution, 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 fracturing process is characterized by a very dense crack system, with crack spacing of only about 0.1 m. Then, a multi-physics approach to 3D modeling of fluid-driven propagation of a vast network of cracks and open joints in shale is presented. The complex nonlinear and anisotropic mechanical behavior of shale is captured by means of a microplane model. Because the crack spacing must be only about 10 cm, the fracture of shale is analyzed as a softening damage, in a smeared way, by the crack band model. 3D nonlinear equations governing the flow of compressible cracking fluid through the cracks whose opening and length is controlled by fracture mechanics are formulated.

1. INTRODUCTION

In 2010 shale gas production accounted for 23% of the total US gas production and it is projected to reach 50% by 2035 even at the current extraction efficiency [1-3]. This represents a huge opportunity for technological innovation that, leading to increased extraction efficiency, can generate substantial economic, environmental, and societal benefits.

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. These aspects have to be clarified if the current rate gas recovery from the shale strata, which is only about 5&#37: to 15&#37:, is ever to be exceeded.

In this contribution, a model to estimate the topology and evolution of the crack system emerging during the fracturing process is proposed. 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 fracturing process is characterized by a very dense crack system, with crack spacing of only about 0.1 m.

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