REVIEW ARTICLE The Role of Chemistry in Fracture Pattern Development 10.1029/2019RG000671 and Opportunities to Advance Interpretations Key Points: • A chemical perspective helps solve of Geological Materials challenges to understanding S. E. Laubach1 , R. H. Lander2, L. J. Criscenti3 , L. M. Anovitz4 , J. L. Urai5 , subsurface fractures: inadequate 6 7 8 9 10 11 samples, ambiguous analogs, and R. M. Pollyea , J. N. Hooker , W. Narr , M. A. Evans , S. N. Kerisit , J. E. Olson , difficulties determining which T. Dewers3 , D. Fisher7 , R. Bodnar6, B. Evans12 , P. Dove6, L. M. Bonnell2, models are correct from M. P. Marder13 , and L. Pyrak‐Nolte14 observations • Many tools of chemical analysis, 1Bureau of Economic Geology, The University of Texas at Austin, Austin, TX, USA, 2Geocosm LLC, Durango, CO, USA, experiment, modeling, and theory 3 4 have yet to be brought to bear on Sandia National Laboratories, Albuquerque, NM, USA, Chemical Sciences Division, Oak Ridge National Laboratory, understanding how fracture patterns Oak Ridge, TN, USA, 5Institute of Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, develop at geological timescales Germany, 6Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, • Chemical and mechanical 7Department of Geosciences, Pennsylvania State University, University Park, PA, USA, 8Chevron Energy Technology investigations together have great 9 potential to solve challenging Company, Houston, TX, USA, Department of Geological Sciences, Central Connecticut State University, New Britain, 10 11 practical problems in subsurface CT, USA, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA, USA, Hildebrand science Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX, USA, 12Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA, 13Department of Physics, The University of Texas at Austin, Austin, TX, USA, 14Department of Physics, Purdue University, West Lafayette, IN, USA Correspondence to: S. E. Laubach, [email protected] Abstract Fracture pattern development has been a challenging area of research in the Earth sciences for more than 100 years. Much has been learned about the spatial and temporal complexity inherent to Citation: these systems, but severe challenges remain. Future advances will require new approaches. Chemical Laubach, S. E., Lander, R. H., Criscenti, L. J., Anovitz, L. M., Urai, J. processes play a larger role in opening‐mode fracture pattern development than has hitherto been L., Pollyea, R. M., et al. (2019). The role appreciated. This review examines relationships between mechanical and geochemical processes that of chemistry in fracture pattern influence the fracture patterns recorded in natural settings. For fractures formed in diagenetic settings (~50 development and opportunities to advance interpretations of geological to 200 °C), we review evidence of chemical reactions in fractures and show how a chemical perspective helps materials. Reviews of Geophysics, 57, solve problems in fracture analysis. We also outline impediments to subsurface pattern measurement 1065–1111. https://doi.org/10.1029/ ‐ 2019RG000671 and interpretation, assess implications of discoveries in fracture history reconstruction for process based models, review models of fracture cementation and chemically assisted fracture growth, and discuss promising paths for future work. To accurately predict the mechanical and fluid flow properties of fracture Received 30 JUL 2019 ‐ Accepted 6 AUG 2019 systems, a processes based approach is needed. Progress is possible using observational, experimental, and Accepted article online 22 AUG 2019 modeling approaches that view fracture patterns and properties as the result of coupled mechanical and Published online 13 SEP 2019 chemical processes. A critical area is reconstructing patterns through time. Such data sets are essential for developing and testing predictive models. Other topics that need work include models of crystal growth and dissolution rates under geological conditions, cement mechanical effects, and subcritical crack propagation. Advances in machine learning and 3‐D imaging present opportunities for a mechanistic understanding of fracture formation and development, enabling prediction of spatial and temporal complexity over geologic timescales. Geophysical research with a chemical perspective is needed to correctly identify and interpret fractures from geophysical measurements during site characterization and monitoring of subsurface engineering activities. Plain Language Summary Fracture patterns in rock strongly affect directions, magnitudes, and heterogeneities of both fluid flow and rock strength. Accurate and testable predictions of patterns are essential for understanding many societally important processes in the Earth and for effectively managing subsurface engineering operations. Chemical processes play a larger role in opening‐mode fracture pattern – ©2019. The Authors. development than has hitherto been appreciated. For fractures formed at depths of ~1 10 km and This is an open access article under the temperatures of 50–200 °C, new evidence shows chemical reactions are common and more diverse than terms of the Creative Commons previously recognized. We describe how viewing fracture formation and evolution from a chemical Attribution License, which permits use, distribution and reproduction in any perspective helps to solve problems in fracture pattern analysis. We outline the main impediments to medium, provided the original work is subsurface fracture pattern measurement and interpretation, assess implications of recent discoveries in properly cited. LAUBACH ET AL. 1065 Reviews of Geophysics 10.1029/2019RG000671 fracture history reconstruction for process‐based models of fracture and cement accumulation, review models of fracture cementation and chemically assisted fracture growth, and discuss promising paths for future work. Potential exists for basic scientific investigations to lead to progress on what has been one of the most refractory practical problems in subsurface science. Results suggest that progress in fracture interpretation and prediction can be made using observational, experimental, modeling, and theoretical approaches that view fracture patterns as the result of coupled mechanical and chemical processes. 1. The Challenge of Natural Fractures in the Earth Accurate and testable predictions of fracture patterns in rock are essential to understand many societally important processes in the Earth and for effective management of subsurface engineering operations. Fractures strongly affect directions, magnitudes, and heterogeneities of both fluid flow and rock strength (Bear et al., 1993; Berkowitz, 2002; Bonnet et al., 2001; de Dreuzy et al., 2012; Olson et al., 2009; Tsang & Neretnieks, 1998; Wang, 1991). Consequently, they have a first‐order control on the production of water, hydrocarbons, and geothermal energy; flow of pollutants in the subsurface; safe storage of CO2 and waste- water; and seismicity, whether natural or induced (National Research Council, 1996; Pyrak‐Nolte & DePaolo, 2015). Understanding the role of chemically assisted subcritical fracturing in rock deformation is recognized to be important for the exploitation of hydrocarbons, geothermal resources, and the long‐term stability of geological reservoirs for CO2 and nuclear waste sequestration (e.g., Bergsaker et al., 2016; Eppes & Keanini, 2017; Olson et al., 2009). Both natural and induced fractures influence resource extraction in the unconventional resources that are the focus of much recent hydrocarbon exploration and develop- ment (Gale et al., 2014; Narr et al., 2006; Pedersen & Eaton, 2018; Solano et al., 2011). Understanding preex- isting fractures is key to anticipating and controlling engineered fractures (e.g., Wang et al., 2018). Because fracture data are difficult to acquire and interpret (Figures 1 and 2 and section 1.2.1), fracture patterns remain stubbornly indistinct and unpredictable despite more than a century of progress in a number of important areas including detection, sophisticated mechanics‐based and statistical modeling approaches, and expensive drilling and well testing campaigns in the private and public sector. A review of the challenges reveals that severe limitations remain despite a new round of research drilling (e.g., Gale et al., 2018) and a revolution in outcrop fracture pattern description based on remote and drone‐based imaging (e.g., Pollyea & Fairley, 2011; Menegoni et al., 2018; Wüstefeld et al., 2018). These limitations persist because subsurface patterns, particularly on scales of a meter or more, cannot be reliably inferred from borehole samples as they are currently analyzed, and models are difficult to validate. Thus, among the most important and challenging problems in geoscience is identifying and understanding key influences on fracture pattern development and how to recognize these influences with the limited samples that are typically available. Here, we investigate the proposition that in diagenetic settings chemistry is a key missing ingredient for accu- rate fracture pattern predictions and meaningful interpretations. Mechanics is an indispensable tool for deci- phering how fracture patterns form in rock (Pollard & Aydin, 1988). But is mechanics sufficient? Fracturing is inherently a physiochemical process that involves