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In this paper we present a novel pressure‐controlled Hele‐Shaw cell to investigate different physical processes in rough fractures using 3D‐printed rock analogs. Our system can measure high‐resolution fracture aperture and tracer concentration maps under relevant field stress conditions. Using a series of hydraulic and visual measurements, combined with numerical simulations, we investigate the evolving fracture geometry characteristics, pressure‐dependent hydraulic transmissivity, flow channeling, and the nature of mass transport as a function of normal stress. Our experimental results show that as the fracture closes and deforms under increasing normal loading: (a) the contact areas grow in number and size; (b) the flow paths become more focused and tortuous; and (c) the transport dynamics of conservative tracers evolve toward a higher dispersive regime. Moreover, under the applied experimental conditions, we observed excellent agreement between the simulated‐ and the experimentally measured‐hydraulic behavior. Key Points: A novel pressure‐controlled Hele‐Shaw cell was introduced to study fracture deformation, flow, and mass transport in 3D‐printed fracturesIn situ aperture maps and tracer concentration fields were measured under relevant field stress conditionsWe linked the changes in mass transport dynamics to the evolution of the aperture field and the channelization degree of the flow [ABSTRACT FROM AUTHOR]

Understanding how wormholes develop and how they change the permeability of the porous medium is important for many natural and industrial processes in subsurface solid‐fluid systems. We conduct core flood tests to study the wormhole formation in a porous medium under different flow rates and at different moments before breakthrough. We show that the wormholes created in core flood tests have lengths following a power‐law distribution, with more short ones than long ones. This statistical nature of the wormhole lengths allow one to experimentally study the wormhole competition in 3‐D: Many wormholes develop initially near the inlet, but only a few develop further. Our experimental results also show that the existing wormhole‐matrix models underestimate wormhole lengths because of the neglected additional pressure drop caused by the radial flow near the wormhole tip. We improve upon the existing models by approximating the radial flow near the wormhole tip with a Rankine ovoid model. Plain Language Summary: Underground fluid flow and chemical reactions often result in wormholes, which are channels that look like tree roots, and significantly increase the permeability of porous rocks. Wormhole formation is relevant in many natural and industrial processes, including the formation of underground caves, CO2 sequestration and enhanced oil recovery. In this study, wormholes are created in the laboratory by injecting water into porous gypsum cores, and are observed with X‐ray CT scanning. We show that the wormhole lengths, like the lengths of many other naturally created patterns (faults, river segments, and fjords), follow a power‐law distribution. By comparing the wormholes formed under different flow rates and at different moments, we show that the wormholes develop by competing for available flow. Many wormholes develop initially, but only a few win the competition to develop further. Our experimental results also show that existing models, which simulate the flow in a porous medium with two parts (one with and one without wormholes), underestimate wormhole lengths. We improve upon these models by accounting for the 3‐D radial flow near the tip of the wormhole, and accurately predict the wormhole lengths observed in the experiments. Key Points: Lengths of wormholes resulting from flow and dissolution in a 3‐D porous medium follow a power‐law distributionWormholes develop by competing for available flow, with many developing initially but a few developing furtherWe improve the wormhole length prediction by approximating the 3‐D radial flow near the wormhole tip with a Rankine ovoid model [ABSTRACT FROM AUTHOR]

Understanding the controlling mechanism and the resulting rate of reactive transport processes is crucial for an accurate prediction of the evolution of the rock‐fluid system in many geological processes and engineering applications. In this study, transport‐controlled dissolution in a single fracture was investigated analytically with the development of the extended Purday solution and experimentally with fracture flow tests. The extended Purday solution simulates dissolution in an evolving fracture and extends the validity domain of the Purday solution from a fracture with a uniform aperture to a fracture with aperture heterogeneity in the flow direction. The fracture flow tests include continuous effluent concentration measurements with a novel experimental setup and three‐dimensional fracture geometry analysis. The modeling and experimental results agree well and show that the high dissolution rate in the entrance region results in a converging fracture geometry (decreasing aperture in the flow direction). This converging geometry, in turn, reduces the overall dissolution rate in the fracture. The comparison between the modeling and experimental results shows that channel formation and sidewalls affect the morphology of the fracture. The resulting cross‐section geometry of the fracture also tends to reduce the overall dissolution rate. This study shows that the extended Purday solution accurately predicts the dissolution rate in an evolving fracture, and that factors, such as channel formation and sidewalls, affect fracture morphology and reduce the overall dissolution rate. Plain Language Summary: Flow and dissolution in underground fractures drive the evolution of the rock‐fluid systems. Examples of this can be found in karst formations, enhanced oil production, and CO2 storage. The flow and dissolution change the fracture geometry, which in turn, reshapes the flow and changes the dissolution rate. This study looked into flow and dissolution in a fracture where the dissolution rate is controlled by the diffusion from the interface to the flowing fluid. The problem is investigated theoretically by developing the extended Purday solution and experimentally using fracture flow tests. The modeling and experimental results agree well and show that the dissolution enlarges the fracture more near the inlet than in the following region, forming a converging fracture geometry. This converging geometry slows down the dissolution in the fracture. The fracture flow tests also show that channels, which are the major flow paths deeply etched in the fracture surface, and the fracture sidewalls affect how the fracture geometry changes and further reduce the overall dissolution rate. The extended Purday solution can be applied to model dissolution in natural fractures or fracture networks. The experimental methods can be used in fracture flow tests to study various geological processes and engineering applications. Key Points: The extended Purday solution models the transport‐controlled dissolution in an evolving fractureThe modeling and experimental results show that the overall dissolution rate decreases as the fracture enlargesChannel formation and sidewalls affect fracture morphology and further reduce the dissolution rate [ABSTRACT FROM AUTHOR]

In the parallel paper by Li et al. (2019; https://doi.org/10.1029/2018JB017238), an effluent chemistry monitoring system was designed and used in core flood experiments to continuously measure the effluent concentration and study the evolution of the rock‐fluid system. In this study, the results from the parallel paper were used for interpretation and modeling of the dissolution and wormhole formation. Based on the behavior of the effluent concentration, two transient states and two quasi‐steady states were defined to describe the dissolution in the rock‐fluid system. Dimensional analysis was used to identify the controlling mechanisms of the dissolution and transport in the matrix and the wormholes. The dimensional analysis showed that the dissolution in the matrix was reaction controlled, while the dissolution in the wormholes was diffusion controlled. It also showed that the rock‐fluid system evolved from reaction‐controlled dissolution to diffusion‐controlled dissolution during the core flood tests. A continuum model and the extended Graetz solution were used to model the dissolution in the matrix and in the wormholes, respectively. In the continuum model, this study estimates the effective surface area as a function of the flow rate (injection flux), to account for the effect of flow conditions on dissolution. Finally, a semiempirical model combining the continuum model and the extended Graetz solution was developed to simulate the formation of wormholes and the evolution of the dissolution kinetics during core flood tests. Key Points: The continuous effluent concentration data were used to identify the four states of the dissolution processes during core flood testsThe dimensional analysis showed that the dissolution was reaction controlled in the matrix and diffusion controlled in the wormholesA semiempirical model was developed to simulate the formation of wormholes and the evolution of the dissolution kinetics [ABSTRACT FROM AUTHOR]

Core flood tests were conducted to study the effect of flow rate on the dissolution of the gypsum rock matrix and the formation of wormholes. An effluent chemistry monitoring system was designed and integrated into a triaxial system to provide continuous effluent concentration measurements, in addition to the pressure and flow measurements during the core flood tests. X‐ray computed tomography (CT) was used to study the geometry of the wormholes after the tests. The core flood tests showed agreement with experiments reported in the literature regarding permeability evolution and wormhole breakthrough. By continuously monitoring the effluent concentration, the effluent chemistry monitoring system advanced the experimental study by showing how the dissolution kinetics evolved with the formation of wormholes. Three‐dimensional topological and morphological algorithms were developed to analyze the CT data and provide quantitative descriptions for the wormhole geometry. The CT analysis showed that higher flow rates resulted in more complex wormhole geometries regarding the number of wormholes and branches. Key Points: A new device was introduced in core flood tests to study the evolution of the overall dissolution rate in the rock‐fluid systemThree‐dimensional topological and morphological algorithms were developed to analyze and compare the complex wormhole geometries quantitativelyWormhole growth rate and breakthrough pore volume as functions of the injection flow rate were compared with the relations in the literature [ABSTRACT FROM AUTHOR]

The evolution of the pore space of mudrocks induced by drying shrinkage and effective stresses is of importance to several areas of research. Drying is a prerequisite for most mudrock characterization methods, while effective stresses have a direct impact on mudrocks properties such as permeability, compressibility, and strength. Mercury porosimetry intrusion has been widely utilized to show that drying shrinkage and effective stresses lead to the collapse of large pores only (~ > 50 nm), while small pores (~ < 50 nm) remain unaffected. However, the validity of mercury porosimetry intrusion‐derived pore size distributions is greatly doubted in the literature due to the fact that most pores in mudrocks are not directly accessible to the surrounding mercury. This study follows a different approach by utilizing a suite of methods including imaging by transmission electron microscopy and scanning electron microscopy at cryogenic temperature after high‐pressure freezing (cryoSEM), and gravimetric porosity measurements to investigate the influence of volumetric changes on the pore space of mudrocks. Contrary to previously published results, we show that volumetric changes induced by drying and effective stresses lead to the collapse of pores of all sizes. Furthermore, we show that porosity measured from SEM images is dependent on SEM resolution and reveals only a fraction of the actual porosity. These results provide valuable insights used to interpret the results of characterization methods requiring drying and modeling of effective stress influence on the properties of mudrocks. Key Points: Volumetric changes in mudrocks induced by drying and effective stresses lead to the collapse of pores of all sizesThis contradicts results obtained using mercury porosimetry intrusion showing that large pores collapse while small pores remain unchangedPorosity measured from scanning electron microscopy images is dependent on SEM resolution and reveals only a fraction of total porosity [ABSTRACT FROM AUTHOR]

In Resedimented Boston Blue Clay (RBBC), a low-plasticity glacio-marine illitic mudrock, the ratio of the horizontal to vertical permeability (the permeability anisotropy, rk) increases from 1.2 to 1.9 as the porosity decreases from 0.5 to 0.37 and the permeability decreases by more than 1 order of magnitude. Backscattered Scanning Electron Microscope (BSEM) images taken at formation stress levels reveal that particles rotate perpendicular to the axial loading direction by ∼22°, with larger particles rotating more significantly and achieving more uniform alignment than smaller particles. We show experimentally that preferred platy particle orientation can explain our permeability anisotropy measurements. The permeability anisotropy of mechanically compressed mudrocks is minimal, <2.5. We use a novel approach (cubic specimens) to measure the evolution of permeability anisotropy in different directions on the same specimen, unlike most other methods. Modified analytic techniques allow calculation of the permeability anisotropy for a specimen using directional constant head permeability methods. A better understanding of the evolution of permeability anisotropy during sediment burial is important for modeling subsurface transport processes, including hydrocarbon migration and contaminant transport, as well as estimating in situ conditions such as pore pressure, overpressure, and effective stress. [ABSTRACT FROM AUTHOR]

A technique for visualizing particle transport in the interior of a porous medium is presented. The technique, which includes the construction of a translucent medium and the use of laser-induced fluorescence for particle tracking, was used to examine the behavior of a dilute suspension of negatively charged, micron-sized particles in the interior of uniform glass bead packs during one-dimensional, downward flow. Particle behavior as a function of pore fluid velocity and bead surface roughness was observed at both the macroscopic and microscopic levels. Experimental results show that particle filtration occurred only at solid-solid contact points (contact filtration) in smooth bead packs, while particle filtration occurred at the top of bead surfaces (surface filtration) as well as at solid-solid contact points in rough bead packs. Particle contact filtration was the result of physical straining at solid-solid contact points, while surface filtration was the result of particles interlocking on surface asperities. In both smooth and rough bead packs the filtration capacity of the medium decreased with the pore fluid velocity. In smooth bead packs the filtration capacity was approximately invariant with transport distance, while in the rough bead packs the filtration capacity showed a decrease with transport distance. This decrease was attributed to the early surface filtration of larger particles as a result of gravitational sedimentation. The accumulation of reversibly attached particles was observed even when particle pore fluid concentrations were stable. [ABSTRACT FROM AUTHOR]

We describe a study undertaken to investigate the potential of the geotechnical centrifuge as an experimental tool to study dense nonaqueous phase liquid (DNAPL) behavior in fracture systems. Scaling laws are developed that link observations made in a centrifuge model of a fracture system to the full-scale problem (prototype). Centrifuge experiments carried out in synthetic fractures, described by smooth-walled vertical capillary tubes, demonstrate for the first time that the geotechnical centrifuge can be used to model DNAPL transport in a simplified system provided that inertial forces are negligible in both the model and the prototype. Experiments also confirm, as predicted by Kueper and McWhorter [1991], that DNAPL invades a smooth-walled, vertical fracture when the DNAPL pool height at its entrance reaches its entry pressure. A simple mathematical model describing the displacement of the DNAPL-water interface is also presented. The predictions of this model and the experimental data are in good agreement with each other in the lower half of an invaded fracture. However, at the early stages of DNAPL invasion into the fracture, our observed DNAPL-water interface velocities are slower than predicted by the model. We attribute this to an apparent dependence of the local capillary pressure on the interface displacement velocity. [ABSTRACT FROM AUTHOR]

The link from suction or water content to effective stress and shear strength is unclear. Particle contact forces and the effect of moisture on them are formulated in parallel with triaxial shear tests on specimens of uniform glass spheres. The effect of suction is found to be different from applying an external isotropic stress to the soil. In continuum soil mechanics, the mechanical behavior of an element of soil is related to the effective stress, which is a measure of the average stress transmitted through the solid matrix in the form of contact stresses. In unsaturated soils, the coexistence of water and air within the soil pore space complicates this concept because the microscopic distribution of each fluid phase in soil pores cannot be known. Because it is thus not possible to physically measure effective stress in unsaturated soils, it is often estimated through the experimentally measurable shear strength. A conceptual model based on contact shear forces was built on the contact normal force based models from the literature to link effective stress and shear strength of the continuum to total stress and microscopic pore fluid forces for the simplified case of uniform spherical particles at low water contents. The validity of both models was examined by comparing model predictions of shear strength with the results of drained triaxial compression tests performed on specimens of uniform glass spheres. Although the shear strength calculated by the new model was consistently larger than that calculated via the contact normal force models, values measured in the experiments were even larger. Nevertheless, the test results fit what the new model analytically showed: the friction angle of a soil does not change with desaturation, and introducing moisture in the form of liquid bridges at contacts is fundamentally different from applying an equivalent isotropic stress to the soil externally. [ABSTRACT FROM AUTHOR]