Modeling and characterization of fracture roughness and its impact on mass transport processes [electronic resource]
Keywords: Energy Resources Engineering.

Faults and fractures are the main conduits for flow for reservoirs found in tight rocks, such as shale gas and geothermal resources. Both shale gas and geothermal resources have gained significant interest due to their immense potential made possible through hydraulic fracturing. To optimize production from these resources, accurate fracture flow modeling is necessary. Natural fractures have rough surfaces that create complex local aperture distributions and tortuous flow pathways. Numerous experiments and field-scale observations have demonstrated that fracture heterogeneity produces flow channeling behavior. Thus, determining the relationship between fracture roughness and fracture flow behavior is a key component in effective reservoir management. The overall objective of this study was to provide a better understanding of the impact of fracture roughness on fracture flow behavior. To achieve this overall objective, various aspects of rough fractures were considered in this study. One goal of this study was to develop a systematic procedure for modeling the local fracture evolution with stress. In addition, this study proposed consistent and efficient rough fracture characterization techniques. Moreover, this study aimed to use spatial information to investigate fracture flow uncertainty and predict fracture flow behavior at extended length scales. Heterogeneous local aperture distributions and their corresponding permeability evolution due to applied stresses were generated for rough fracture surfaces using the displacement discontinuity boundary element method with integrated complementarity. In addition, comprehensive characterization methods for fracture plane heterogeneity were evaluated and developed. Fracture characterization methods considered in this study included spatial information obtained through variogram models and other nonspatial characterization methods. Furthermore, a geostatistical approach for rough fracture flow behavior characterization and uncertainty quantification was created based on the sequential Gaussian simulation method. Finally, a novel method for fracture scale-up beyond laboratory scale using integrated spatial information was developed through the extension of the sequential Gaussian simulation method. Results from rough fracture permeability-stress relationships derived from the displacement discontinuity boundary element method demonstrated that generalized correlations are inadequate in describing fracture flow behavior. Fracture aperture distributions generated from the displacement discontinuity boundary element method exhibited preferential flow paths perpendicular to the applied shear stress direction. Additionally, heterogeneous local fracture slip distributions were also generated. Based on the intricacy of local fracture aperture distributions found in different fracture data sets, the variogram model was the most effective method for fracture characterization. Variogram parameters described directions of spatial continuity of local apertures within fractures. Using the variogram model, the sequential Gaussian simulation method was used to generate fracture realizations that reproduced the fracture flow behavior characteristics of the original analog fractures. Moreover, the set of generated fracture realizations represented the uncertainty range of flow behavior parameters for fractures with similar aperture distributions in space. Most importantly, the new rough fracture scale-up method developed based on sequential Gaussian simulation predicted the permeability values of fractures successfully at larger length scales using information from the smallest length scale. The developed fracture scale-up method resulted in significant improvements in predictions compared to assigning local permeability values randomly in space. Furthermore, the extension of the developed fracture scale-up method at length scales beyond the laboratory scale predicted results that were consistent with another fractal scale-up method. In summary, the key contributions of this study are the following. First, a comprehensive and physics-based approach was developed to determine the fundamental effects on fracture roughness on permeability-stress relationships. Second, heterogeneous fracture aperture distributions and tortuous flow patterns were demonstrated and characterized. Lastly, this study demonstrated the successful application of geostatistical methods for rough fracture modeling and flow behavior prediction at extended length scales. Recommended future work includes the acquisition of more fracture data at different length scales and the application of other geostatistical methods for rough fracture modeling.