Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

The permeability of magma in shallow volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2-m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ~1-m wide, highly-sheared region with relatively low porosity and permeability, and an outer 20-cm wide cataclastic fault zone. The low porosity, highly-sheared rock further exhibits an anisotropic permeability network with slightly higher permeability along the shear plane (parallel to the conduit margin) and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3-m long by ~9-cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with an increased permeability of ~3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e., ductile) shear during ascent up to the point of rupture, estimated by Umakoshi et al. (2008), at ~500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into, and along, the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the spine core with slight displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the upward shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Tunable, Anisotropic Permeability and Spatial Flow of SLM Manufactured Structures

In this study, we report on a novel approach to produce defined porous selectively laser molten structures with predictable anisotropic permeability. For this purpose, in an initial step, the smallest possible wall proximity distance for selectively laser molten structures is investigated by applying a single line scan strategy. The obtained parameters are adapted to a rectangular and, subsequently, to a more complex honeycomb structure. As variation of the hatch distance directly affects the pore size, and thus the resulting porosity and finally permeability, we, in addition, propose and verify a mathematical correlation between selective laser melting process parameters, porosity, and permeability. Moreover, a triangular based anisotropic single line selectively laser molten structure is introduced, which offers the possibility of controlling the three-dimensional flow ratio of passing fluids. Basically, one spatial direction exhibits unhindered flow, whereas the second nearly completely prohibits any passage of the fluid. The amount to which the remaining orientation accounts for is controlled by spreading the basic triangular structure by variation of the included angle. As acute angles yield low passage ratios of 0.25 relative to continuous flow, more obtuse angles show increased ratios up to equal bidirectional flow. Hence, this novel procedure permits (for the first time) fabrication of selective laser molten structures with adjustable permeable properties independent of the applied process parameters.

A New Evaluation of Skin Factor in Inclined Wells with Anisotropic Permeability

Oil and gas well productivity can be affected by a number of different skin factors, the combined influences of which contribute to a well’s total skin factor. The skin caused by deviated wells is one such well-known factor. The present study aimed to investigate skin effects caused by deviated well slants when considering vertical-to-horizontal permeability anisotropy. The research employed computational fluid dynamics (CFD) software to simulate fluid flows in inclined wells through the injection of water with Darcy flow using 3D geometric formations. The present work investigates the effects of four main characteristics—namely, the permeability anisotropy, wellbore radius, reservoir thickness, and deviation angle—of open-hole inclined wells. Additional investigations sought to verify the effect of the direction of perforations on the skin factor or pressure drop in perforated inclined wells. In the case of an inclined open hole well, the novel correlation produced in the current study simplifies the estimation of the skin factor of inclined wells at different inclination angles. Our comparison indicates good agreement between the proposed correlation and available models. Furthermore, the results demonstrated a deviation in the skin factor estimation results for perforated inclined wells in different perforation orientation scenarios; therefore, existing models must be improved in light of this variance. This work contributes to the understanding and simulation of the effects of well inclination on skin factor in the near-wellbore region.

​Study of Anisotropic permeability of Coarse-grained soils using Experimental Model and X-ray Diffraction Analysis

Background: Permeability is one of the most important physical properties of soil used in water engineering science. In order to carry out this research, we designed and tested a device for measuring the horizontal and vertical permeability in a sample with coarse-grained particles. Methods: In the present study, four uniform soil samples and three non-uniform samples under different densities and different water loads were selected to test. In this study, to identify the effect of shape factor on permeability of coarse-grained particle, mineralogy of samples was carried out using X-ray diffraction analysis by XRD. Result: The results of the research show that the permeability in the horizontal direction is often greater than the permeability in the vertical direction and this difference is more pronounced for non-uniform samples compared to uniform samples. The permeability anisotropy rate for uniform samples is between 0.85 and 1.35 and for non-uniform samples in the range of 1.32 to 3.5.

A customized fragment for seepage beneath a dam with a vertical wall

Based on the Method of Fragments for the analysis of steady confined seepage, a customized ‘type E’ fragment is developed, with results presented in the form of charts for the estimation of seepage quantities and exit gradients under embedded water-retaining structures with a vertical cut-off wall. The ‘type E’ fragment is shown to be an extension of previously derived ‘type A and D’ fragments, allowing for both embedment and a cut-off wall. The charts are generated using finite element analysis and cover both isotropic and anisotropic permeability cases. Validation of the fragment is confirmed by comparison with existing Method of Fragments and full finite element analysis. The charts are shown to predict the flow rate and exit gradient more precisely than existing Methods of Fragments. The design charts presented in this paper cover a wide range of confined flow problems of practical interest.