Fracture-Flow-Enhanced Solute Diffusion into Fractured Rock

PROCEEDINGS, Thirty-Third Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 28-30, 2008 SGP-TR-185 FRACTURE-FLOW-ENHANCED SOLUTE DIFFUSION INTO FRACTURED ROCK Yu-Shu Wu 1 , Ming Ye 2 and E. A. Sudicky 3 Lawrence Berkeley National Laboratory Florida State University University of Waterloo e-mail: YSWu@lbl.gov In recent years, interest has grown in investigating solute transport through fractured rock, driven by environmental concerns related to radionuclide transport in fractured formations (e.g., Liu et al., 2003; Liu et al., 2004a; Hu et al., 2004; Reimus and Callahan, 2007). In addition, application of tracer tests, including environmental tracers and man-made gas- and liquid-tracer injection, has become a standard technique in characterizing fractured rock formations. All of these investigations and experiments require an in-depth, quantitative understanding of fracture-matrix diffusion and interaction. Since the 1970s, while understanding fracture-matrix interaction has been the focus of investigation into flow and transport processes in fractured rock, matrix diffusion has been gradually recognized as one of the most important mechanisms that control radionuclide transport processes in fractured rock (e.g., Neretnieks, 1980; Neretnieks et al., 1982; Maloszewski and Zuber, 1993; Liu et al., 2004a). Even in the laboratory, fracture-matrix diffusion is also found to be dominant (Neretnieks et al., 1982; Sudicky et al., 1985; Wu and Pruess, 2000). However, field tracer testing analyses (e.g., Liu et al., 2003; Liu et al., 2004a) have shown that fracture- matrix interaction through matrix diffusion may be too large (or “enhanced”) to be explained by traditional advection-diffusion theory. To match tracer experimental results from a large-scale field test, Liu et al. (2003) had to increase fracture-matrix interfacial areas by a factor of 2 to 4, or to significantly increase effective matrix diffusion. Several studies have been carried out to look into why such enhanced matrix diffusion at fracture- matrix interfaces occurs, with mechanisms ranging from scale dependency of the effective matrix diffusion coefficient (Liu et al., 2004b) to possible effects of small-scale fractures or large fracture- matrix interaction areas (Wu et al., 2004). Even with the significant progress made in laboratory and field investigations and in mathematical modeling, matrix diffusion at fracture-matrix surfaces has been exclusively considered as a molecular ABSTRACT We propose a new conceptual model of fracture- flow-enhanced matrix diffusion, which correlates with fracture-flow velocity, i.e., matrix diffusion enhancement induced by rapid fluid flow within fractures. According to the boundary-layer or film theory, fracture flow enhanced matrix diffusion may dominate mass-transfer processes at fracture-matrix interfaces, because rapid flow along fractures results in large velocity and concentration gradients at and near fracture-matrix interfaces, enhancing matrix diffusion at matrix surfaces. In this paper, we present a new formulation of the conceptual model for enhanced fracture-matrix diffusion, and its implementation is discussed using existing analytical solutions and numerical models. In addition, we use the enhanced matrix diffusion concept to analyze laboratory experimental results from nonreactive and reactive tracer breakthrough tests, in an effort to validate the new conceptual model. INTRODUCTION Earlier studies of flow and transport in fractured porous media were motivated primarily by concerns related to petroleum and geothermal energy technologies, as well as interest in groundwater resources from fractured reservoirs (e.g., Warren and Root, 1963). In the 1970s and 1980s, chemical or solute transport through fractured porous media received increasing attention from investigators involved in underground natural-resource recovery as well as in subsurface contamination concerns. Since then, fractured rock has been recognized to play an important role in the transport of natural resources or contaminants through subsurface systems. Over the past few decades, significant progress has been made in understanding and modeling transport phenomena in fractured porous media (e.g., Tang et al., 1981; Sudicky and Frind, 1982; Rasmuson, 1982; Huyakorn et al., 1983; Pruess and Narasimhan,