Three-dimensional flow measurements in rock fractures, 1999, Dijk P. , Berkowitz B.

Nuclear magnetic resonance imaging is applied to measure flow patterns in natural, water-saturated, rough-walled rock fractures. From three-dimensional water density and velocity vector images the fracture morphology and flow patterns are determined. The parabolic nature and asymmetry of the velocity profiles, and thus the accuracy of local cubic law flow rate predictions, vary greatly. This depends on the degree of wall roughness. Particularly complex flow patterns are found in one sample which contains a sharp fracture wall discontinuity. A power law for the flow rate versus aperture for the low-flow region was found without considering the hydraulic gradients.

Investigation of flow in water-saturated rock fractures using nuclear magnetic resonance imaging (NMRI), 1999, Dijk P. , Berkowitz B. , Bendel P.

The application of nuclear magnetic resonance imaging (NMRI) to the direct three-dimensional measurement of flow in rough-walled water-saturated rock fractures is presented for the first time. The study demonstrates the abilities of NMRI to noninvasively measure rock-water interfaces and water flow velocities in these fractures and investigates the effects of wall morphology on flow patterns inside a typical rock fracture. Two- and three-dimensional flow-encoded spin-echo pulse sequences were applied. The stability and reproducibility of the water flow patterns were confirmed by analyzing two-dimensional velocity images. A variety of geometrical and hydraulic features were determined from three-dimensional velocity images, including the rock-water interfaces, the fracture aperture distribution, and the critical aperture path; velocity profiles and volumetric flow rates; flow and stagnant regions; and the critical velocity path. In particular, the effects of a sharp step discontinuity of the fracture walls and the applicability of the cubic law were examined. As a result of the complex three-dimensional geometry, velocity profiles are generally parabolic but often highly asymmetric, with respect to the fracture walls. These asymmetric velocity profiles are clustered together, with significant correlations; they are not just local random phenomena. However, theoretical considerations indicate that the effects of the measured asymmetry on volumetric flow rates and hydraulic conductivities are insignificant, in that the overall flow inside rough fractures still obeys the cubic law. The features discussed in this study emphasize the strong heterogeneity and the highly three-dimensional nature of the flow patterns in natural rock fractures and consequently the need for three-dimensional flow analysis.

Buoyancy-driven dissolution enhancement in rock fractures, 2000, Dijk Pe, Berkowitz B,

The structures of geological formations, as well as flow and chemical transport patterns within them, are profoundly affected by chemical dissolution and precipitation processes (i.e., the interactions among flow, chemical transport, buoyancy, and dissolution and precipitation reactions). These processes are intrinsically hard to measure, and therefore are not well understood. Nuclear magnetic resonance imaging is applied to study the dynamic behavior of coupled flow and dissolution in natural rock fractures. Our findings reveal that flow and transport in evolving fractures are far more unpredictable than commonly assumed, due to complex interactions among fracture morphology, flow, dissolution, and buoyancy. This can explain physical processes causing catastrophic collapse and subsurface structural instabilities, such as sinkholes and land subsidence

Buoyancy-driven dissolution enhancement in rock fractures, 2000, Dijk, P. E. , Berkowitz, B.

The structures of geological formations, as well as flow and chemical transport patterns within them, are profoundly affected by chemical dissolution and precipitation processes (i.e., the interactions among flow, chemical transport, buoyancy, and dissolution and precipitation reactions). These processes are intrinsically hard to measure, and therefore are not well understood. Nuclear magnetic resonance imaging is applied to study the dynamic behavior of coupled flow and dissolution in natural rock fractures. Our findings reveal that flow and transport in evolving fractures are far more unpredictable than commonly assumed, due to complex interactions among fracture morphology, flow, dissolution, and buoyancy. This can explain physical processes causing catastrophic collapse and subsurface structural instabilities, such as sinkholes and land subsidence.

Measurement and analysis of dissolution patterns in rock fractures, 2002, Dijk P. E. , Berkowitz B. , Yechieli Y.

Nuclear magnetic resonance imaging (NMRI) is applied to noninvasively measure flow and dissolution patterns in natural, rough-walled, water-saturated halite fractures. Three-dimensional images of water density and flow velocity acquired with NMRI allow quantification of the developing fracture morphology and flow patterns. The flow patterns are correlated strongly to the local apertures and the large-scale wall roughness. The correlations of the dissolution patterns to the fracture morphology, flow patterns, and mineralogical composition of the rock matrix are a function of the overall dimensionless Damköhler number. At high Damköhler numbers the dissolution patterns are dominated by the flow structure. In addition, at high Damköhler numbers buoyancy (stratified flow) becomes important. In such cases the dissolution patterns also depend on the orientation and elevation of the fracture walls, resulting in preferential upward dissolution. At low Damköhler numbers the dissolution patterns depend mainly on the mineralogical composition of the rock matrix. These findings suggest that coupled flow and dissolution processes are much more complex and unpredictable than commonly assumed, even under simplified conditions.

NMR Imaging of Fluid Exchange between Macropores and Matrix in Eogenetic Karst, 2009, Florea L. J. , Cunningham K. J. , Altobelli S.

Sequential time-step images acquired using nuclear magnetic resonance (NMR) show the displacement of deuterated water (D2O) by fresh water within two limestone samples characterized by a porous and permeable limestone matrix of peloids and ooids. These samples were selected because they have a macropore system representative of some parts of the eogenetic karst limestone of the Biscayne Aquifer in southeastern Florida. The macroporosity, created by the trace fossil Ophiomorpha, is principally well connected and of centimeter scale. These macropores occur in broadly continuous stratiform zones that create preferential flow layers within the Hydrogeologic units of the Biscayne. This arrangement of porosity is important because in coastal areas, it could produce a preferential pathway for salt water intrusion. Two experiments were conducted in which samples saturated with D2O were placed in acrylic chambers filled with fresh water and examined with NMR. Results reveal a substantial flux of fresh water into the matrix porosity with a simultaneous loss of D2O. Specifically, we measured rates upward of 0.001 mL/h/g of sample in static conditions, and perhaps as great as 0.07 mL/h/g of sample when fresh water continuously flows past a sample at velocities less than those found within stressed areas of the Biscayne. These experiments illustrate how fresh water and D2O, with different chemical properties, migrate within one type of matrix porosity found in the Biscayne. Furthermore, these experiments are a comparative exercise in the displacement of sea water by fresh water in the matrix of a coastal, karst aquifer since D2O has a greater density than fresh water.