Geological Storage of <scp>CO</scp>₂: Modeling Applications for Large‐Scale Simulations<i>Geofluids</i> (2012) NordbottenJ.M., CeliaM.A., <i>Wiley &amp; Sons</i>, Hoboken, New Jersey, USA241<i> pp. ISBN 978‐0‐470‐88946‐6</i>

Book Review Geological Storage of CO 2 : Modeling Applications for Large-Scale Simulations Geofluids (2012) Nordbotten J.M., Celia M.A., Wiley & Sons, Hoboken, New Jersey, USA 241 pp. ISBN 978-0-470- One of the big film box-office suc- cesses of 1985 was the comedy ‘Back to the Future’, in which a 17-year- old boy is sent back in time from 1985 to 1955, with the help of a mad scientist named Dr. Emmett ‘Doc’ Brown. By helping his future father stand up against a bully and meet his future mother, he ensures (in addition to his own existence) that his family’s miserable present-day life changes into a brighter future. As this article is not a movie review, one may ask what ‘Back to the Future’ has to do with Nordbot- ten and Celia’s book about modeling approaches for CO 2 storage in the subsurface. The answer is simple: Just as young Marty McFly builds a time machine using an amped-up DeLore- an, the authors rely on simplifying analytical approaches dating decades or centuries back, dusting them off with some ingenious mathematics, and developing them into powerful simulation tools. These tools track the multiphase processes occurring when CO 2 captured from industrial sources is injected into appropriate formations deep underground. The comparison stops here, though: Nordbotten and Celia are no mad scientists like ‘Doc’ Brown; this is a well-written book, one that is useful for a wide audience interested in por- ous media flow processes and large- scale simulation applications relevant to carbon storage. The basic idea conveyed by Nord- botten and Celia is intriguing and in fact wider than the CO 2 storage application at the center of this book. The buzz word is heterogeneous multiscale method (HMM), a new modeling philosophy borrowed from the mathematics community, which allows fine-scale models to inform coarse-scale simulation, even if the coarse-scale equations cannot be explicitly derived. The idea is to reduce a fine-scale model to a less complex ‘coarse’ representation, via a compression operator, so that the modeler is able to simulate very large lateral domains while retaining suffi- cient resolution. The coarse-scale solution is then converted back into a fine-scale solution using a reconstruc- tion operator. The authors apply the concept of HMM to develop a con- sistent set of simplifying assumptions for CO 2 -brine systems, and they explain why these are suitable based on the analysis of the spatial and tem- poral scale of the problem at hand. The compression and reconstruc- tion operations proposed in this book are all based on the well-established assumption of vertical equilibrium behavior in aquifers, which goes back to Jules Dupuit’s 1863 paper on sin- gle-phase groundwater flow. For the multiphase problem of less dense CO 2 migrating in a brine aquifer, vertical equilibrium can be recon- structed, for example, as two density- segregated fluids forming a sharp interface, or, with a bit more com- plexity, using a capillary-fringe model that honors the balance between grav- itational and capillary forces. Nord- botten and Celia make a convincing case that the analytical and numerical solutions built upon compression and reconstruction allow for efficient modeling of carbon-storage problems at scales spanning many kilometers horizontally, while retaining essential information about complex phenom- ena at much smaller scales. The book is divided into six main chapters, each with a list of recom- mendations for further reading. Chapter 1, ‘The Carbon Problem’, discusses the possible role of carbon sequestration in the portfolio of emis- sion–reduction strategies and describes the basic geologic features that make a suitable CO 2 storage site. Chapters 2 and 3, on single- and two-phase flow, respectively, introduce the clas- sical theory of fluid flow in the sub- surface and review basic solution strategies for the governing equa- tions, together with common simplifi- cations. While both chapters briefly consider the concept of compression and reconstruction via vertical averag- ing, Chapter 4 provides a much more involved mathematical framework for HMM, based on considerations of large length and timescales. In Chapters 5 and 6, with large- scale equations at hand, the authors develop analytical and numerical solu- tion methods that can solve for large- scale injection, migration, and leakage of both CO 2 and brine, while also accommodating interphase mass exchange and subsequent transport of dissolved CO 2 . Each chapter ends with practical applications to real- world CO 2 storage projects. Most interesting is the Alberta Basin appli- cation in Chapter 6 that involves modeling probabilistic risk assessment of hypothetical yet realistic storage operations in a multilayer subsurface domain of 50 km 9 50 km lateral extent. Because of past and ongoing oil and gas operations, the region is perforated by about 1 250 wells, which could potentially act as vertical pathways for fluids. Nordbotten and Celia simulate 50 years of CO 2 injec- tion into this domain to assess the potential risks arising from the migra- tion and possible leakage of CO 2 . These simulations—utilizing a semi-

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