Understanding hydraulic fracturing: a multi-scale problem

Abstract

Despite the impact that hydraulic fracturing has had on the energy sector, the physical mechanisms that control its efficiency and environmental impacts remain poorly understood in part because the length scales involved range from nanometres to kilometres. We characterize flow and transport in shale formations across and between these scales using integrated computational, theoretical and experimental efforts/methods. At the field scale, we use discrete fracture network modelling to simulate production of a hydraulically fractured well from a fracture network that is based on the site characterization of a shale gas reservoir. At the core scale, we use triaxial fracture experiments and a finite-discrete element model to study dynamic fracture/crack propagation in low permeability shale. We use lattice Boltzmann pore-scale simulations and microfluidic experiments in both synthetic and shale rock micromodels to study pore-scale flow and transport phenomena, including multi-phase flow and fluids mixing. A mechanistic description and integration of these multiple scales is required for accurate predictions of production and the eventual optimization of hydrocarbon extraction from unconventional reservoirs. Finally, we discuss the potential of CO2 as an alternative working fluid, both in fracturing and re-stimulating activities, beyond its environmental advantages.This article is part of the themed issue 'Energy and the subsurface'.

A Multi-Parameter Optimization Model for the Evaluation of Shale Gas Recovery Enhancement

“…It has been observed that hydraulic fracturing treatments still leave behind 70-80% of the estimated gas in place [7]. At the early stage of production, the methane in large fractures (hydraulic fracture) and their adjacent natural fracture networks is released quickly, which leads to the rapid initial decline in production [7]. Next, matrix block with high gas exchange rate that depends on the distribution of natural fracture sustains the production at intermediate stage of production.…”

“…However, current extraction efficiency is low. It has been observed that hydraulic fracturing treatments still leave behind 70-80% of the estimated gas in place [7]. At the early stage of production, the methane in large fractures (hydraulic fracture) and their adjacent natural fracture networks is released quickly, which leads to the rapid initial decline in production [7].…”

“…The original free and desorbed gases flow from matrix to natural fractures, then to hydraulic fractures and finally to the horizontal well. The gas flow shows significant multi-scale characteristics [7] and is significantly affected by the uncertainties of many reservoir parameters for matrix, natural fractures, and hydraulic fractures. Therefore, an optimal set of these parameters is expected to maximize the gas recovery rate from a shale gas reservoir.…”

“…The commercial development of shale gas has been driven by three key advances in technology and science: (1) horizontal well plus multi-stage hydraulic fracturing [1][2][3][4][5][6], (2) the understanding of gas storage mechanisms [1], and (3) the understanding of multi-scale mechanisms of gas flows from shale matrix to hydraulic fracture [7][8][9][10]. A shale gas reservoir has free gas stored in matrix pores and natural fractures, and adsorbed gas on the organic matter [11,12].…”

Introduction: energy and the subsurface

“…The simulation captures the primary fracture network (outlined in the experiment), as well as bedding plane fractures and densely fractured regions, and illustrates the calculated stress intensity within the specimen. Reproduced from Hyman et al [2].…”