Numerical Simulation of Fracking in Shale Rocks: Current State and Future Approaches

Abstract

Extracting gas from shale rocks is one of the current engineering challenges but offers the prospect of cheap gas. Part of the development of an effective engineering solution for shale gas extraction in the future will be the availability of reliable and efficient methods of modelling the development of a fracture system, and the use of these models to guide operators in locating, drilling and pressurising wells. Numerous research papers have been dedicated to this problem, but the information is still incomplete, since a number of simplifications have been adopted such as the assumption of shale as an isotropic material. Recent works on shale characterisation have proved this assumption to be wrong. The anisotropy of shale depends significantly on the scale at which the problem is tackled (nano, micro or macroscale), suggesting that a multiscale model would be appropriate. Moreover, propagation of hydraulic fractures in such a complex medium can be difficult to model with current numerical discretisation methods. The crack propagation may not be unique, and crack branching can occur during the fracture extension. A number of natural fractures could exist in a shale deposit, so we are dealing with several cracks propagating at once over a considerable range of length scales. For all these reasons, the modelling of the fracking problem deserves considerable attention. The objective of this work is to present an overview of the hydraulic fracture of shale, introducing the most recent investigations concerning the anisotropy of shale rocks, then presenting some of the possible numerical methods that could be used to model the real fracking problem.

On the undrained and drained hydraulic fracture splits

“…Examples are studies that considered nonlinear fluid rheology and proppant transport, HF propagation in porous media, acoustic emissions, solid nonlinearity due to bulk plasticity and cohesive fractures, and nonlocal plasticity and damage models . For a detailed review of recent advances in the simulation of HFs, the reader is referred to related works . The focus of this manuscript is on the first area of improving the numerical techniques used in HF models.…”

“…52,53 For a detailed review of recent advances in the simulation of HFs, the reader is referred to related works. 1,2,[54][55][56] The focus of this manuscript is on the first area of improving the numerical techniques used in HF models.…”

Peridynamics review

“…Various numerical methods have been developed to study these phenomena, among which of particular interest here is PD. For instance, Hattori et al [238] examined various numerical methods, among which PD, to analyze hydraulic fracking in shale rocks. For further details on hydraulic fracturing in the context of PD, see [41,[239][240][241][242] among others.…”

Numerical simulation of MZF design with non-planar hydraulic fracturing from multi-lateral horizontal wells

“…The other region is the broken cohesive zone, where the crack 120 surfaces are traction free and the crack is filled with fracturing fluid. Based on the Cohesive Crack Model (CCM) (Needleman, 1987;Hillerborg, 1985), two different types of crack tip are identified, these being the "real crack tip" and the "fictitious crack tip" (Carpinteri, 2012;Hattori et al, 2017). The former implies the point delineating the stress-free area from the FPZ, whereas the latter is the point delineating the FPZ from the uncracked material.…”