Earth Sciences Division
Fundamental and Exploratory Research Program
Annual Report 1998-1999
Identifying Limits of Film Flow In Unsaturated Fractures Research
Objectives
be associated with the matric potential at which the local rock matrix is Tetsu K. Tokunaga In an earlier study (Tokunaga and e ffe c t i ve ly saturated (satiated), and and Jiamin Wan Wan,1997), we identified film flow as the upper limit associated with local a mechanism that can contribute to saturation of the fracture aperture. Contact: Tetsu Tokunaga fast flow along unsaturated fractures. The critical matric potential at (510) 486-7176, tktokunaga@lbl.gov The combination of low-permeability which the rock matrix is effectively rock, large fracture aperture and nearsaturated and ab ove which wa t e r zero matric potential was qualitatively identified as necessary for films can begin to emerge on fracture surfaces is approximately development of fast film flow. However, more quantitative deterequal to the air-entry matric potential. Because of hysteresis in minations of the ranges of matrix permeabilities, fracture aperthe potential-saturation relation, and also because of generally tures and matric potentials that permit development of thick, unknown wetting history, the actual critical matric potential will transmissive films along fracture surfaces remained unspecified. typically have a magnitude between about 50% and 100% that of This study is directed at identifying the approximate region of the air-entry value.The lower range of matric potentials,at which stability for thick films, within the parameter space defined by thick films begin to emerge from fracture surfaces,was estimated rock matrix permeability, local fracture aperture and matric through correlations between matrix permeabilities and airpotential. entry matric potentials of a wide range of porous media, including soils, glass bead packs, rocks and ceramics (Figure 1).We seek Approach only rough correlations since the range of permeability spans nearly 10 orders of magnitude. This correlation was shown to Our initial studies on flow in unsaturated fractures identified predict the air-entry matric potential within one order of magnifilm flow as a mechanism capable of permitting fast flow and tude, for 92% of the data (N = 76).The correlation was also fairly transport. That study revealed limitations of earlier conceptual consistent with predictions based on Miller-Miller geometric models for unsaturated flow in fractured rock, introduced the similitude, even though the highly varied sample set does not rigfilm flow hypothesis and provided experimental results suporously conform to prerequisites for geometric similitude. The portive of the hypothesis. The water “filmsâ€? investigated in the exponent in the regression fit is -0.425, whereas Miller-Miller similitude predicts a value of -0.5. previous study as well as the present one develop on rough surUpon effectively saturating the rock matrix, local topographic faces typical of rock fractures, range in average thickness from minima on the fracture surface become progressively waterabout 1 to 50 Âľm and flow in the laminar regime. The initial filled as the matric potential is brought closer to zero. Filling of study showed that film flow becomes important when matric local topographic minima on fracture surfaces progresses from potentials along rock surfaces are high enough (less negative) to finer roughness features to coarser ones, as the matric potential effectively saturate the surficial rock matrix. Here, more specific is brought closer to zero, in accordance with continuously constraints are identified for parameter ranges within which film increasing the radius of curflow can occur. vature characteristic of the The first step in this study air-water interface. Thus, the consisted of identifying a sinave rage film thickness on gle parameter to represent rough fra c t u re surfa c e s each of the primary system i n c reases as the matric components, the ro ck potential appro a ches zero, matrix, fracture and water. In primarily because of surface this simplification, the capillary re l a t i o n s . In this selected para m e t e rs we re progression to thicker averthe rock matrix permeability, f ra c t u re apert u re and age films, the transmissivity matric potential, re s p e cand hydraulic diffusivity also tively. Since the matrix perincrease, as shown in earlier meability and local fracture work. aperture are essentially fixed In typical rock fractures, at any given location along a film thickening has a finite fra c t u re surface, one can limit imposed by the fracture then consider the range of aperture. The upper range of matric potentials over which matric potentials, at which stable thick films exist. The Figure 1. Correlation between permeabilities and air-entry matric potentials for a films give way to locally satlower energy limit will then wide variety of porous media (glass beads, sands, clays, rocks and ceramics). urated fracture apertures, is 19