PNNL

Abstract

Fracture systems in diagenetic environments often evolve over millions of years. Slow fracture growth rates increase the likelihood of competing interactions between fracture development and cement precipitation, dissolution, and other potentially slow chemical processes and vice versa. Models that partially account for these interactions produce patterns that differ from those that do not. Chemical processes play a larger role in opening-mode fracture pattern development than has hitherto been appreciated. For fractures formed in diagenetic settings, we review evidence of chemical reactions in fractures and how a chemical perspective helps solve problems in fracture pattern analysis. We also outline the main impediments to subsurface fracture pattern measurement and interpretation, assess implications of recent discoveries in 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. Additional research is needed to develop a basis for accurate prediction of the patterns and mechanical and fluid-flow properties of fracture systems in the subsurface. A literature review and discussions at a workshop sponsored by the Office of Science, U.S. Department of Energy suggests that progress in fracture interpretation and prediction can be made using new observational, experimental, and modeling approaches that view fracture patterns and properties as the result of coupled mechanical and chemical processes. A critical area for future research is reconstructing fracture patterns through time from diverse geologic settings given that such datasets are essential for developing and testing fracture models. Other research topics that need further work include developing accurate models of crystal growth rates at geologic conditions, cement strengthening effects both in host rocks and fracture zones, and subcritical crack propagation. Additionally, work is needed to couple models of fluid flow, fracture propagation, and rock modification processes while accounting for changes in far field stress conditions as well as stratal geometry due to deposition, erosion, faulting, folding, or diapirism over geologic timescales. Geophysical research with a chemical perspective is also needed to correctly identify and interpret subsurface fractures and fracture evolution from geophysical measurements during site characterization and monitoring of subsurface engineering activities.