Photoelastic Observation of Toughness-Dominant Hydraulic Fracture Propagation Across an Orthogonal Discontinuity in Soft, Viscoelastic Layered Formations

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Photoelastic Observation of Toughness-Dominant Hydraulic Fracture Propagation Across an Orthogonal Discontinuity in Soft, Viscoelastic Layered Formations International Journal of Rock Mechanics & Mining Sciences 134 (2020) 104438 Contents lists available at ScienceDirect International Journal of Rock Mechanics and Mining Sciences journal homepage: http://www.elsevier.com/locate/ijrmms Photoelastic observation of toughness-dominant hydraulic fracture propagation across an orthogonal discontinuity in soft, viscoelastic layered formations Soo-Min Ham, Tae-Hyuk Kwon * Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, South Korea ARTICLE INFO ABSTRACT Keywords: Hydraulic fracture (HF) unavoidably encounters natural pre-existing discontinuities in geologic formations, Hydraulic fracture which complicates the propagation and containment behavior across a discontinuity. This study explored the HF Fracture visualization propagation across an orthogonal discontinuity in layered formations by exploiting the photoelastic, transparent, Photoelasticity soft (or deformable), and viscoelastic characteristics of gelatin. First, photoelastic HF experiments in homoge­ Stress intensity factor neous gelatin media were carried out while monitoring the fluidpressure and stress intensity factor (SIF). The SIF Fracture containment Discontinuity was observed to stay constant during steady-state HF propagation. Second, photoelastic HF experiments in layered gelatin media were conducted, in which biwing-type fractures encountered the bounding layers with different levels of stiffness. Two contrasting containment behaviors — crossing or dilation/arrested — were observed. The fracture crossed the discontinuity when it encountered the bounding layer with a lower toughness. By contrast, the fracture was arrested by the bounding layer and/or dilated the discontinuity, propagating along the discontinuity when the bounding layer had a greater toughness. In addition, competition between debonding of the layer interface and creating a fresh fracture in the bounding layer was found to play a significant role in fracture containment behavior. This study provides unique experimental data with photoelastic images that are comparable to analytical or numerical models. Furthermore, the results contribute to a better understanding of HF propagation and containment behaviors across a discontinuity in viscoelastic media. 1. Introduction discontinuity (or interface) — whether it will cross the interface (crossing or passing), be arrested by the interface (arrested or confined), Generation of fractures by fluid injection, also referred to as “hy­ or propagate along while dilating the interface (dilation).8,9 Various draulic fracturing”, enhances fluidtransport in geologic formations, and efforts using physical experimentation have attempted to answer this thus it is widely implemented in geo-engineering practices, including question and to identify the factors affecting such containment behavior enhanced hydrocarbon recovery,1,2 enhanced geothermal heat recov­ at a discontinuity. These include material properties, in situ stress con­ ery,3,4 and isolation of hazardous waste.5 Natural geologic formations ditions, discontinuity condition, and injecting-fluid properties. The contain innate discontinuity, such as stratification (layering), natural salient findings from previous experimental studies are summarized in faults and fractures, and tiny fissures and cracks. Previous mine-back Table 1. However, physical experimental results on HF propagation and experiments and microseismic fracture mapping studies at a field scale containment behavior in layered formations with discontinuity are still have reported the complex patterns of hydraulic fracture (HF) propa­ scarce, and experimental data that are interpretable and comparable to gation in fractured rocks or layered formations.6,7 Prediction of the analytical and numerical models are particularly limited. Moreover, generated HF geometry in natural geologic formations with layering and because the rock materials are opaque and the fracture propagation is a discontinuity is one of the important issues in safe and economic design fairly fast and dynamic process, observation of HF propagation patterns of hydraulic fracturing practices. at a discontinuity in physical experimentation still poses challenges. One of the intriguing questions concerns the containment behavior This hampers further advancement in the fundamental understanding of of a propagating fracture when it encounters a pre-existing natural HF containment behavior at a discontinuity. * Corresponding author. E-mail addresses: [email protected] (S.-M. Ham), [email protected] (T.-H. Kwon). https://doi.org/10.1016/j.ijrmms.2020.104438 Received 22 January 2020; Received in revised form 27 May 2020; Accepted 7 July 2020 Available online 17 August 2020 1365-1609/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). S.-M. Ham and T.-H. Kwon International Journal of Rock Mechanics and Mining Sciences 134 (2020) 104438 “Photoelasticity” refers to the optomechanical characteristics of 2. Materials and methods materials that become doubly refractive (or birefringent) when sub­ jected to stress. The resulting fringe pattern in a photoelastic material 2.1. Materials under stress makes possible the analysis of the stress distribution near a fracture tip and, hence, computation of the stress intensity factor (SIF) Gelatin was chosen as an analog to a rocklike medium because of the – (linear elastic fracture mechanics, or LEFM).10 12 Hence, the photo­ following advantages. (a) The gelatin is characterized with its visco- elasticity often has been used to calculate the SIF during fracture elasticity and it shows a remarkably wide elastic range, up to a verti­ – propagation in homogeneous media,13 16 although the majority of cal strain of ~30–50%.25,26 Therefore, the gelain can be a good analogy previous studies investigated the fractures driven by external mechani­ to soft rocks, such as shale.27,28 (b) The gelatin is optically transparent, cal loads. However, no photoelastic investigation on fluid-driven frac­ such that the fracture can be visualized during propagation. (c) Because ture propagation in homogeneous media or with the discontinuity has gelatin is known to become doubly refractive (or birefringent) when been reported to date. subjected to stress, this photoelasticity of gelatin makes possible the Furthermore, HF propagation behavior significantlydiffers with the analysis of the SIF and the stress distribution near a fracture tip.12 (d) material plasticity or viscosity with respect to, but not limited to, the fluid Finally, the specimen can be remolded into various shapes, and the injection pressure response, fracture geometry, and propagation veloc­ stiffness can be readily controlled with the gelatin concentration, which – ity.17 19 For instance, the pressure significantlydrops in hard and brittle provides a good control for physical experimentation. media with no or extremely low viscosity. However, in soft (or deform­ In this study, the gelatin stiffness was controlled to four different able) and viscous media, it gradually decreases as the fracture is initiated levels by varying the gelatin concentration in solution prior to gelation, and propagates. The fracture tip is sharp in brittle media, whereas it is following previous work.19 The gelatin concentration varied as follows: comet-shaped in deformable and viscous media. In brittle media, the HF 7.41 wt% for low stiffness (L), 12.28 wt% for medium stiffness (M), propagation velocity decreases with time, while it is kept at a steady state 13.79 wt% for high stiffness (H), and 16.7 wt% for very high stiffness in deformable, viscous media.19,20 There have been a few experimental (VH). Hereafter, the gelatin samples are named after their stiffness level: – studies on HF propagation in viscoelastic media19,21 23; however, no L-gelatin, M-gelatin, H-gelatin, and VH-gelatin. The Young’s modules attempt has been made to investigate the HF propagation across the and fracture toughness of these gelatin samples were determined using discontinuity in viscoelastic media. the indentation tests29 and the wire cutting method,30 and summarized Therefore, in this study, the HF propagation across discontinuities in in Table 2. Given the unique soft characteristics of gelatin, the values viscoelastic and layered plate media were investigated by employing a obtained herein provide good estimates on the elastic moduli and photoelastic experiment method. Previously, the steady-state HF prop­ toughness of gelatin, though the values may differ slightly from the ones agation in two-dimensional plate media made of transparent, visco­ from the ISRM-suggested standard methods, such as uniaxial compres­ elastic gelatin samples has been successfully visualized and sion test and Brazilian tensile test. The Poisson’s ratio was assumed to be – characterized.19 First, as a follow-up to that previous study, photoelastic 0.5 as the gelatin is an incompressible material.31 33 investigation in HF propagation in homogeneous, viscoelastic gelatin The photoelastic constant was determined by applying the loads onto media was carried out, in which the fracture geometry and photoelastic the disk-shaped gelatin samples with a thickness of 10 mm and a radius fringe patterns were visualized while monitoring the fluid injection of 9.3 mm. Fig. 1 shows the photoelastic images of L-, M , H-, and VH- pressure. Second, the effect
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