The fate and transport of groundwater contaminants depends partially on groundwater velocity, which can vary appreciably in highly stratified aquifers. A high‐resolution passive profiler (HRPP) was developed to evaluate groundwater velocity, contaminant concentrations, and microbial community structure at ∼20 cm vertical depth resolution in shallow heterogeneous aquifers. The objective of this study was to use mass transfer of bromide (Br−), a conservative tracer released from cells in the HRPP, to estimate interstitial velocity. Laboratory experiments were conducted to empirically relate velocity and the mass transfer coefficient of Br− based on the relative loss of Br− from HRPP cells. Laboratory‐scale HRPPs were deployed in flow boxes containing saturated soils with differing porosities, and the mass transfer coefficient of Br− was measured at multiple interstitial velocities (0 to 100 cm/day). A two‐dimensional (2D) quasi‐steady‐state model was used to relate velocity to mass transfer of Br− for a range of soil porosities (0.2–0.5). The laboratory data indicate that the mass transfer coefficient of Br−, which was directly—but non‐linearly—related to velocity, can be determined with a single 3‐week deployment of the HRPP. The mass transfer coefficient was relatively unaffected by sampler orientation, length of deployment time, or porosity. The model closely simulated the experimental results. The data suggest that the HRPP will be applicable for estimating groundwater velocity ranging from 1 to 100 cm/day in the field at a minimum depth resolution of 10 cm, depending on sampler design.

Gas sorption can lead to the volumetric swelling of the shale matrix and reduction of the effective pore volume, which further impacts the gas transportation in micro-and nanopores in shale. At present, it is very challenging to directly measure the pore volume shrinkage (i.e., pore volumetric strain) in the laboratory. In this study, an innovative method is proposed to quantify the pore volumetric strain resulting from the sorption-induced matrix swelling in shale. More specifically, Gibbs methane sorption capacities of the Barnett and Eagle Ford shale core samples were determined via volumetric and gravimetric methods, respectively. Meanwhile, the bulk volume swelling of the shale core sample was also measured during the gas sorption process. Correlations between the sorption-induced bulk and pore volumetric strains were developed to calculate the pore volume shrinkage ascribed to the gas sorption, which was further validated with the measured gas sorption capacities. It is found that the pore volume shrinkage is 3.7−4.8 times greater than the bulk volume swelling during the gas sorption process for the Barnett shale, while such a ratio reaches as high as 59.8−67.1 times for the Eagle Ford shale. In addition, the sorptioninduced bulk and pore volumetric strains follow a power law relationship for both Barnett and Eagle Ford shales, which yields a nearly identical absolute gas sorption isotherm with the ones determined using volumetric and gravimetric methods. The results of this study give insight into the feasibility of characterizing the sorption-induced pore volume shrinkage in shales and illustrate the benefits of applying both gravimetric and volumetric methods to evaluate the gas sorption behaviors.

Understanding fracture initiation and propagation from perforated wellbores is essential to designing a perforation scheme to achieve an efficient hydraulic fracture stimulation treatment. The effect of perforation design on hydraulic fracture propagation has been extensively studied using experimental and analytical methods. Because the experimental investigation of hydraulic fracture is complicated, expensive, and often returns limited results, numerical methods can be applied as an efficient way to simulate fracture propagation from perforations. An Extended Finite Element Method (XFEM) was used to develop a model to investigate the effects of various parameters on fracture propagation from a set of perforations. These parameters included perforation orientation, perforation length, stress anisotropy, and elastic properties of the formation. Fracture propagation patterns from the XFEM model were first matched against published experimental studies and exhibited good agreement. The model was then used to broaden the study of perforation effects. Results of the modeling proved the effects of perforation orientation and length on hydraulic fracture propagation pattern. Horizontal stress anisotropy and rock mechanical properties were observed to strongly influence fracture propagation. It was also observed that, when two or more perforations are positioned at different orientation angles at the same depth, a fracture tends to propagate from the less deviated perforation. In these cases, the more deviated perforation can develop a short fracture, following a propagating pattern that could be caused by stress shadowing/interference. Stress interference between two perforations positioned closely together results in either perforation breakdown or fracture propagating away from one another. The simulation results from this study offer methods to enhance perforation design for hydraulic fracture treatment, particularly in the case of high stress anisotropy and high uncertainty in a preferred fracture plane. Analyzing competing perforations suggests that a technique based on this concept can be applied when high uncertainty exists regarding the direction of the principal horizontal stresses through increasing perforation density.

Summary In this paper, a fully coupled 2‐dimensional poroelastic displacement discontinuity method is used to investigate the refracturing process in horizontal wells. One of the objectives of refracturing is to access new reserves by adding new hydraulic fractures in zones that were bypassed in the initial fracturing attempt. Pore pressure depletion in the vicinity of old fractures directly affects the state of stress and eventually the propagation of newly created hydraulic fractures. Thus, a poroelastic analysis is required to identify guidelines for the refracturing process, in particular to understand the extension of the pore pressure depletion, and eventually, the orientation of new as well as old fractures. We propose a fully coupled approach to model the whole process of child fracture propagation in a depleted area between 2 parent fractures in the same wellbore. This approach omits the need of using multistep workflow that is regularly used to model the process. The maximum tensile stress criterion (σ criterion) is used for hydraulic fracture propagation. The proposed method is verified using available analytical solutions for total stress and pore pressure loading modes on a line fracture in drained and undrained conditions. Then, test cases of multifractured horizontal wells are studied to calculate the time evolution of the stress and pore pressure fields around old fractures and to understand the effect of these fields on the propagation path of newly created fractures. Finally, the effect of the pore pressure depletion on the propagation path of the newly created fractures in the bypassed area of the same wellbore is studied. The results show that the depleted areas around old fractures are highly affected by the extent and severity of the stress redistribution and pore pressure depletion. It is observed that a successful creation of new fractures may only happen in certain time frames. The results of this study provide new insights on the behavior of newly created fractures in depleted zones. They also clarify the relationship between stress change and pore pressure depletion in horizontal wells.

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