Narrow Fractures Research Articles

Displacement of yield-stress fluids in a fracture

We consider a displacement of several yield-stress fluids in a Hele-Shaw cell. The topic is relevant to the development of a model for the flow of multiple phases inside a narrow fracture with application to hydraulically fracturing a hydrocarbon-bearing underground formation. Existing models for fracturing flows include only pure power-law models without yield stress, and the present work is aimed at filling this gap. The fluids are assumed to be immiscible and incompressible. We consider fluid advection in a plane channel in the presence of density gradients. Gravity is taken into account, so that there can be slumping and gravitational convection. We use the lubrication approximation so that governing equations are reduced to a 2D width-averaged system formed by the quasi-linear elliptic equation for pressure and transport equations for volume concentrations of fluids. The numerical solution is obtained using a finite-difference method. The pressure equation is solved using an iterative algorithm and the Multigrid method, while the transport equations are solved using a second-order TVD flux-limiting scheme with the superbee limiter. This numerical model is validated against three different sets of experiments: (i) gravitational slumping of fluids in a closed Hele-Shaw cell, (ii) viscous fingering of fluids with a high viscosity contrast due to the Saffman–Taylor (S–T) instability in a Hele-Shaw cell at microgravity conditions, (iii) displacement of Bingham fluids in a Hele-Shaw cell with the development of fingers due to the S–T instability. Good agreement is observed between simulations and laboratory data. The model is then used to investigate the joint effect of fingering and slumping. Numerical simulations show that the slumping rate of yield-stress fluid is significantly less pronounced than that of a Newtonian fluid with the same density and viscosity. If a low-viscosity Newtonian fluid is injected after a yield-stress one, the S–T instability at the interface leads to the development of fingers. As a result, fingers penetrating into a fluid with a finite yield stress locally decrease the pressure gradient and unyielded zones develop as a consequence.

Feasibility study on application of volume acid fracturing technology to tight gas carbonate reservoir development

Abstract How to effectively develop tight-gas carbonate reservoir and achieve high recovery is always a problem for the oil and gas industry. To solve this problem, domestic petroleum engineers use the combination of the successful experiences of North American shale gas pools development by stimulated reservoir volume (SRV) fracturing with the research achievements of Chinese tight gas development by acid fracturing to propose volume acid fracturing technology for fractured tight-gas carbonate reservoir, which has achieved a good stimulation effect in the pilot tests. To determine what reservoir conditions are suitable to carry out volume acid fracturing, this paper firstly introduces volume acid fracturing technology by giving the stimulation mechanism and technical ideas, and initially analyzes the feasibility by the comparison of reservoir characteristics of shale gas with tight-gas carbonate. Then, this paper analyzes the validity and limitation of the volume acid fracturing technology via the analyses of control conditions for volume acid fracturing in reservoir fracturing performance, natural fracture, horizontal principal stress difference, orientation of in-situ stress and natural fracture, and gives the solution for the limitation. The study results show that the volume acid fracturing process can be used to greatly improve the flow environment of tight-gas carbonate reservoir and increase production; the incremental or stimulation response is closely related with reservoir fracturing performance, the degree of development of natural fracture, the small intersection angle between hydraulic fracture and natural fracture, the large horizontal principal stress difference is easy to form a narrow fracture zone, and it is disadvantageous to create fracture network, but the degradable fiber diversion technology may largely weaken the disadvantage. The practices indicate that the application of volume acid fracturing process to the tight-gas carbonate reservoir development is feasible in the Ordovician Majiagou Formation of lower Paleozoic, which is of great significance and practical value for domestic tight-gas carbonate reservoir development and studies in the future.

Micromechanics of proppant agglomeration during settling in hydraulic fractures

The micromechanics of proppant settling in quiescent fluid in a rough and relatively narrow rock fracture is investigated. The study focuses on particle–particle and particle–wall interactions in a dense-phase particle settling. The study used a coupled discrete element method and computational fluid dynamics (DEM–CFD) code because DEM–CFD is the most suitable computational method for modeling the frequent interactions of dense assembly of rigid particles and enables modeling of two-way solid–fluid interactions. Due to frequent particle–particle interactions of grains submerged in fluids, the particle interaction model in DEM is improved by the incorporation of the effects of lubrication due to a layer of fluid surrounding particles. Results of the numerical study are compared to previous experimental and theoretical relationships. The findings of the study highlight conditions that lead to proppant aggregation due to the fluid viscosity and fracture width in relation to particle diameter ratio. In the light of the DEM–CFD results, it was found that published relationships are inadequate in describing the settling rates for proppant in a rough and narrow hydraulic fracture and high fluid viscosity. Micromechanical particle interactions during settling and erratic upward and fluid counter-flow may cause proppant trajectories that are not always in the direction of gravity in a rough fracture resulting in clogging of the fracture or forming faster settling particle agglomerates. The maximum packing density 0.3–0.4 (3.9–5.9 lbs/gal) of proppant in a narrow and rough hydraulic fracture was obtained in this study, which is lower than the usually assumed one of 0.5 (9.8 lbs/gal) for any given fracture roughness.

Sustained groundwater level changes and permeability variation in a fault zone following the 12 May 2008, Mw 7.9 Wenchuan earthquake

AbstractWe analyse the groundwater level changes following the Wenchuan earthquake in three wells – NX, DZ and BBLY – located in the same fault zone, in the near field (distance of ~300 km from the epicentre). Co‐seismic falls in the water level and gradual recovery were recorded in these wells but with different recovery periods (from 200 days to more than 1600 days). The response of the groundwater level to Earth tides is used as a proxy to explore the permeability evolution. We found that the permeability increased in response to the Wenchuan earthquake in the three wells but with different post‐earthquake recovery processes. Only BBLY recovered to its pre‐earthquake value 260 days after the Wenchuan earthquake and remained stable. The permeability in NX returned to its pre‐earthquake value over a similar period but then continued to drop. The permeability in DZ returned to its pre‐earthquake value much quicker than that in the other two wells and remained stable below the pre‐earthquake value 200 days after the earthquake. This suggests that the groundwater level changes in the three wells were mainly caused by permeability changes. In the BBLY well, the unclogging/clogging of the fracture flow path mechanism may explain the permeability evolution, whereas mechanisms such as unclogging/clogging or the opening/closing of the fracture associated with blocking of the narrow fracture apertures appears to be responsible for the permeability evolution in the NX and DZ wells. Copyright © 2014 John Wiley & Sons, Ltd.

Study of cement-grout penetration into fractures under static and oscillatory conditions

Grouting of the rock surrounding high-level waste (HLW) can serve to minimize groundwater flow around it and thereby to retard erosion of waste-embedding clay (buffer) and transport of possibly released radionuclides. Earlier attempts have shown the efficiency of superimposing the injection pressure with oscillations for bringing cement-rich grouts into narrow fractures using organic superplasticizers. However, these are short-lived and can produce radionuclide-bearing organic colloids, and should be replaced by inorganic agents. Portland cement in grouts is not long lived and low-pH cements are preferable as is also reduction of the cement content to an absolute minimum. The present study describes the composition and performance of candidate grouts in laboratory experiments with injection into plane-parallel slots with different aperture. The study included development of a simple and quick method for estimating the viscosity on the construction site for adapting the grout recipe to the injectability of the rock. A simple theoretical model for predicting grout penetration gives fair agreement with laboratory data. The longevity of the grout under various conditions is believed to be sufficient for use in HLW repositories implying waste placement in very deep holes.

Experimental Study of the Fracture and Matrix Effects on Free-Fall Gravity Drainage With Micromodels

Summary Free-fall gravity drainage (FFGD) is the main production mechanism in the gas-invaded zone of fractured reservoirs. The gravity and capillary forces are two major forces that control the production performance of a fractured system under an FFGD mechanism. Gravity force acts as a driving force to remove oil from the matrix block whereas the resistive capillary force tends to keep oil inside the matrix. In this study, a series of experiments was performed to study the effects of the geometrical characteristics of the fracture and matrix on the oil-production rate under an FFGD mechanism by use of a glass micromodel. The oil-recovery factor (RF) was also obtained for a single matrix block by use of different patterns. Results from the experiments show that different flow regimes occur during the production life of a single matrix block under a FFGD mechanism. The fluid flow is controlled by the capillary-dominated regime at the early stage and late time of production life, whereas it shows a stabilized bulk flow under a gravity-dominated regime is exhibited at other times. Experimental results revealed that for a narrow fracture opening, fracture capillary pressure has a form similar to that of the matrix block. Also, it was observed that the oil-production rate and RF of the matrix block decreased as the permeability ratio between two media (matrix block and fracture) increased. Lower production rate is achieved in larger-fracture-spacing micromodels. In addition, wider vertical fractures lead to an early breakthrough of gas in bottom horizontal fracture that makes up the main portion of oil traps in the matrix block, and this reduces the RF. Results from this study show that in a heterogeneous layered matrix block, both the drainage rate and RF decrease in comparison with a homogeneous matrix block. Finally, a multiple linear-regression analysis was performed to understand the dimensionless groups affecting the RF of the FFGD process. It was found that the Bond number cannot truly describe the process and other parameters such as the fracture-/matrix-permeability ratio; fracture spacing and fracture opening should also be considered.

Reactive transport modeling of the Dixie Valley geothermal area: Insights on flow and geothermometry

A 2D reactive transport model of the Dixie Valley geothermal area in Nevada, USA was developed to assess fluid flow pathways and fluid rock interaction processes. The model includes two major normal faults and the incorporation of a dual continuum domain to simulate the presence of a small-scale thermal spring being fed by a highly permeable but narrow fracture zone. Simulations were performed incorporating fluid flow, heat conduction and advection, and kinetic mineral-water reactions. Various solute geothermometry methods were applied to simulated spring compositions, to compare estimated reservoir temperatures with “true” modeled reservoir temperatures, for a fluid ascending the simulated fracture and cooling on its way to the surface. Under the modeled conditions (cooling but no mixing or boiling), the classical Na–K(–Ca) geothermometers performed best because these are least affected by mineral precipitation upon cooling. Geothermometry based on computed mineral saturation indices and the quartz geothermometer were more sensitive to re-equilibration upon cooling, but showed good results for fluid velocities above ca. 0.1m/d and a reactive fracture surface area 1–2 orders of magnitude lower than the corresponding geometric surface area. This suggests that such upflow rates and relatively low reactive fracture surface areas are likely present in many geothermal fields. The simulations also suggest that the presence of small-scale fracture systems having an elevated permeability of 10−12 to 10−10m2 can significantly alter the shallow fluid flow regime of geothermal systems. For the Dixie Valley case, the model implies that such elevated permeabilities lead to a shallow (less than 1km) convection cell where superficial water infiltrates along the range front normal fault and connects the small-scale geothermal spring through basin filling sediments. Furthermore, we conclude that a fracture permeability on the order of 10−12m2 may lead to near surface temperature >100°C whereas a permeability of 10−10m2 is not realistic because this permeability led to extreme upflow velocities and to a short-circuit of the regional fault zone.

Repair processes of experimental root fractures in rat molars examined by histopathological techniques and 3D micro-CT imaging

AimsThe purpose of this study was to investigate the repair processes of experimental intra-alveolar root fractures of immature rat molars with histopathological techniques and 3-dimensional micro-computed X-ray tomography (3D micro-CT) imaging. MethodsTwelve left upper first molars of 4-week-old rats were dislocated toward the palate with modified Hoe's pliers at a load of 900 g for 20 s, and repositioned. After 1 or 2 weeks, the maxillae were dissected and scanned with micro CT for 3D micro-CT imaging. After standard histological preparation, buccolingual serial sections of the maxillae were stained with hematoxylin and eosin or Masson–Goldner stain and observed by light microscopy. Subsequently, we examined the identified root fracture lesions by 3D micro-CT imaging and histopathological sections. ResultsWe identified 6 intra-alveolar vertical incomplete fracture lines in the mesial roots. Histopathological findings showed that newly differentiated odontoblast-like cells had formed immature reparative dentin on the pulpal side of the fracture line at 1 postoperative week, and deposited reparative dentin with a tubular structure at 2 weeks. While surface resorption and granulation-tissue formation occurred on the periodontal side of the fracture line, the narrow fracture line remained unchanged, and the surface resorption lacunae were partly covered with cementum at postoperative 2 weeks. ConclusionsRoot-fracture investigation requires a combination of histopathological observation and 3D-CT imaging. Intra-alveolar fractured root repair accompanied by tubular reparative dentin formation at the pulpal side, and the deposition of cementum at the periodontal side, may occur when the fracture line is too narrow for the formation of granulation tissue.

Multiscale Hessian fracture filtering for the enhancement and segmentation of narrow fractures in 3D image data

Narrow fractures—or more generally narrow planar features—can be difficult to extract from 3D image datasets, and available methods are often unsuitable or inapplicable. A proper extraction is however in many cases required for visualisation or future processing steps. We use the example of 3D X-ray micro-Computed Tomography (µCT) data of narrow fractures through core samples from a dolomitic hydrocarbon reservoir (Hauptdolomit below the Vienna Basin, Austria). The extraction and eventual binary segmentation of the fractures in these datasets is required for porosity determination and permeability modelling.In this paper, we present the multiscale Hessian fracture filtering technique for extracting narrow fractures from a 3D image dataset. The second-order information in the Hessian matrix is used to distinguish planar features from the dataset. Different results are obtained for different scales of analysis in the calculation of the Hessian matrix. By combining these various scales of analysis, the final output is multiscale; i.e. narrow fractures of different apertures are detected. The presented technique is implemented and made available as macro code for the multiplatform public domain image processing software ImageJ. Serial processing of blocks of data ensures that full 3D processing of relatively large datasets (example dataset: 1670×1670×1546 voxels) is possible on a desktop computer. Here, several hours of processing time are required, but interaction is only required in the beginning. Various post-processing steps (calibration, connectivity filtering, and binarisation) can be applied, depending on the goals of research.The multiscale Hessian fracture filtering technique provides very good results for extracting the narrow fractures in our example dataset, despite several drawbacks inherent to the use of the Hessian matrix. Although we apply the technique on a specific example, the general implementation makes the filter suitable for different types of 3D datasets and different research goals.

Crustal fracturing and chert dike formation triggered by large meteorite impacts, ca. 3.260 Ga, Barberton greenstone belt, South Africa

The ca. 3260 Ma contact between the largely volcanic Onverwacht Group and overlying largely sedimentary Fig Tree Group in the Barberton greenstone belt, South Africa, is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the Mendon Formation (Onverwacht Group). In the Barite Valley area, these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the Mendon Formation. Spherules that formed during a large meteorite or asteroid impact event occur in a wave- and/or current-deposited unit, spherule bed S2, which widely marks the Onverwacht–Fig Tree contact, and as loose grains and masses within some chert dikes up to 50 m below the contact. Four main types of chert dikes and veins are recognized: (Type 1) irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy; (Type 2) small vertical dikes, most <1 m wide, which are restricted to the lower half of the Mendon chert section; (Type 3) small crosscutting veins, most <50 cm across, filled with precipitative silica; and (Type 4) small irregular to bedding-parallel to irregular veins, mostly <10 cm wide, filled with translucent precipitative silica. Type 2 dikes formed first and reflect a short-lived seismic event that locally decoupled the sedimentary section at the top of the Mendon Formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower, lithified part of the sedimentary section. Later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section, widespread liquefaction of soft, uppermost Mendon sediments, and flowage of the liquefied sediments and loose impact-generated spherules into the open fractures. Late-stage tsunamis everywhere eroded and reworked the spherule layer. The coincidence of crustal disruption, dike formation, spherule deposition, and tsunami activity suggests that all were related to the S2 impact or impact cluster. Crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact Fig Tree Group, including a small conglomeratic fan delta in the Barite Valley area. Remobilization and further movement of debris in the subsurface continued for some time. Locally, the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike, causing foundering of S2 and ∼1–2 m of overlying baritic sediments into the dike. Spherule beds occur at the base of the Fig Tree Group over wide areas of the Barberton belt, marking the abrupt change from ∼300 m.y. of predominantly anorogenic, mafic, and komatiitic volcanism of the Onverwacht Group to orogenic clastic sedimentation and associated felsic volcanism of the Fig Tree Group. This area never again returned to Onverwacht-style mafic and ultramafic volcanism but evolved ∼100 m.y. later into the Kaapvaal craton. These results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca. 3260–3240 Ma.

Evidence for silicate dissolution on Mars from the Nakhla meteorite

AbstractVeins containing carbonates, hydrous silicates, and sulfates that occur within and between grains of augite and olivine in the Nakhla meteorite are good evidence for the former presence of liquid water in the Martian crust. Aqueous solutions gained access to grain interiors via narrow fractures, and those fractures within olivine whose walls were oriented close to (001) were preferentially widened by etching along [001]. This orientation selective dissolution may have been due to the presence within olivine of shock‐formed [001](100) and [001]{110} screw dislocations. The duration of etching is likely to have been brief, possibly less than a year, and the solutions responsible were sufficiently cool and reducing that laihunite did not form and Fe liberated from the olivine was not immediately oxidized. The pores within olivine were mineralized in sequence by siderite, nanocrystalline smectite, a Fe‐Mg phyllosilicate, and then gypsum, whereas only the smectite occurs within augite. The nanocrystalline smectite was deposited as submicrometer thick layers on etched vein walls, and solution compositions varied substantially between and sometimes during precipitation of each layer. Together with microcrystalline gypsum the Fe‐Mg phyllosilicate crystallized as water briefly returned to some of the veins following desiccation fracturing of the smectite. These results show that etching of olivine enhanced the porosity and permeability of the nakhlite parent rock and that dissolution and secondary mineralization took place within the same near‐static aqueous system.

Fracture-scale model of immiscible fluid flow

We present a method for modeling immiscible fluid flow in narrow fractures. The method combines a parallel-plate flow model with a recoloration approach adapted from multiple-component lattice-Boltzmann methods. The resulting numerical method is straightforward to implement and accurately reproduces the relevant fluid behavior. To demonstrate the method, single-droplet simulations are compared to analytical solutions that isolate the contributions from the in-plane and normal curvature. Simulations reproducing capillary forces inside a widening aperture are also presented. Excellent agreement is found in all cases considered. Finally, the method's ability to model fracture flow is demonstrated by deriving relative permeability curves for flow through a heterogeneous aperture created between two fractal surfaces.

Conductivity of narrow fractures filled with a proppant monolayer

A simplified approach for calculating the conductivity of narrow fractures filled with a sparse monolayer of proppant particles is developed. The proppant particles are modelled as rigid spheres and the deformation of the fracture faces is assumed to be purely elastic. Hertz contact theory and the principle of superposition are utilised to obtain the fracture opening profile as a function of the proppant concentration and the value of confining stress. The conductivity of the deformed fracture channel is determined by using computational fluid dynamics.It has been demonstrated that some optimal proppant concentration exists, which maximises the fracture conductivity. Based on the simplified approach developed in this paper, the optimal concentration has been obtained as a function of the confining stress and the material properties of the fractured medium. The results are compared with previously published experimental data, which also indicate the existence of an optimal proppant concentration. A reasonable agreement between theoretical and experimental results is observed.

Patterned ground in martian high northern latitudes: Morphology and age constraints

We performed a systematic survey of typical surface patterns and small (5–50m) impact craters in a zone between 50° and 70° latitude in the Northern Lowlands of Mars. We found that small-scale (<10m) polygonal pattern formed by narrow fractures is typical in the northern half of the studied zone. We interpret this pattern to be formed by active thermal contraction polygons associated with the present-day shallow ground ice. These fresh small polygons are superposed over a larger-scale (∼15–30m) polygonal network of smooth gentle troughs and mounds. The latter pattern extends southward from the shallow ground ice boundary to the southern edge of the study zone. We interpret this pattern to be a relic from different climate conditions in the geologically recent past (possibly, to the most recent periods of high obliquity of Mars spin axis). The population of small craters is sparse and indicates intensive resurfacing in the recent geological past. The retention age of 5–10m diameter craters in the northern part of our study zone is surprisingly young, on the order of hundreds to thousands of years, which means that quick obliteration of such craters occurs under the present-day spin/orbit configuration of Mars. The crater obliteration time scale increases for larger craters and toward the southern edge of the study area.

Phosphonic/Hydrofluoric Acid: Tortuosity Remediation for Fracturing Treatments

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 152624, ’Phosphonic/Hydrofluoric Acid - A Promising New Weapon in the Tortuosity-Remediation Arsenal for Fracturing Treatments,’ by Ricardo Melo, James Curtis, SPE, Julio Gomez, SPE, Alexandre Melo, Fernando Garcia, and Helio Pedrosa, SPE, Baker Hughes, prepared for the 2012 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, 6-8 February. The paper has not been peer reviewed. Tortuosity is one of the biggest challenges for shale and tight gas hydraulic-fracturing treatments, leading to high near-wellbore frictional pressure loss, premature screenouts, reduced treating rates, and poor production results. A new solution demonstrated substantial success in overcoming particularly stubborn tortuosity problems. The method builds on the use of a nondamaging, single-stage phosphonic-/hydrofluoric-acid system (SAS) for matrix-stimulation treatments. Introduction Hydraulic fractures are initiated and kept open by pressure. The pressure is transmitted by the pumped fluid, so as the pumping rate increases, so does the pressure. Following this process, stress is induced in the rock around the well-bore. If the pumping rate continues to increase, so does the stress. Eventually, a stress limit will be reached at which point the rock can no longer sustain the applied stress and it fractures. Over a long perforated interval or in deviated wells, fractures can initiate anywhere, provided that the fluid pressure gradient exceeds the fracture gradient, either to initiate fractures or to reopen natural fractures. Generally, the rock will fail at its weakest point and the fracture will initiate there. However, if the pressure continues to rise, additional fractures may be formed because of changes in near-wellbore stress, as shown in Fig. 1. Later, the fracture will reach its natural direction, according to the formation’s stress orientation. Every perforation is a potential source of fracture initiation. Many of these fractures will be very small, but some may be large enough to take a significant portion of the treatment fluid. Despite the artificial-stress environment generated around the well-bore, hydraulic-fracture treatments tend to produce a small number of larger fractures. Because of increased stresses around the fracture faces, individual fractures tend not to join together. The stress regime around the fracture faces can cause fractures to repel each other. However, with the complex stresses around the wellbore and perforations, fractures can join together at some point, connecting through narrow paths, sometimes with bends toward a main large fracture. Therefore, the treating fluid must flow from a region containing many small narrow fractures to a region containing a few large fractures. In following this path, the fluid must move through a series of convoluted, bending, and narrow fractures (i.e., a tortuous path). This tortuosity can produce a significant pressure loss, resulting in a smaller-than-expected fracture or in early screenout. Such screenouts might also be caused directly by tortuosity because these channels through the rock often are not wide enough for the proppant to pass through, causing the proppant to bridge and prevent further flow of proppant.

Low temperature alteration of serpentinized ultramafic rock and implications for microbial life

Alteration (serpentinization) of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water. Hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere. In this study we focus on ongoing low-temperature water–rock processes in partially serpentinized dunite from the Leka Ophiolite Complex, mid-Norway. We aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities. The study is based on the chemical and physical alteration features of a 51m deep rock core as well as the chemical composition of groundwater from the core's borehole. The borehole intersects three main fracture zones. Observations during drilling indicated that water infiltration largely takes place along the deepest zone. The rock in the fracture zones is nearly completely altered to serpentine, brucite, magnetite and minor calcite. An increase in pH and decrease in Mg, Si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water–rock reactions where dissolution of brucite, serpentine and calcite and precipitation of chrysotile, hydromagnesite and iron-hydroxide are most likely to occur. Detection of both H2 and O2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water/rock ratios and to dissolution of Fe(II)-containing brucite and serpentine resulting in the formation and successive diffusion of H2 into the oxic borehole. Oxidation of H2, reduced Fe and Mn, NH4+ and CH4 are all potential energy-yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources. Methane could result from microbial oxidation of H2 and reduction of CO2, or from the degradation of organic matter. The results demonstrate that low-temperature water–rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities.

Geochemical and morphological characteristics of coarse-grained tabular beryl from the Xuebaoding W–Sn–Be deposit, Sichuan Province, western China

The dominant crystal faces on beryl are usually hexagonal prisms, a direct reflection of the lattice structure. However, coarse-grained beryl from the Xuebaoding W–Sn–Be deposit, China, has a tabular habit. To evaluate the genesis of this well-crystallized tabular beryl, we analysed the crystals using inductively coupled plasma mass spectrometry (ICP–MS), X-ray fluorescence (XRF) spectrometry, X-ray powder diffraction (XRD), and differential interference contrast microscopy (DICM). The beryl unit cell dimensions are 9.223–9.232 Å for a 0 and 9.183–9.244 Å for c 0. The ratio c 0 /a 0 = 0.995–1.002. The beryl has a high alkali content, including Li2O (2.60–3.10 wt.%), Rb (65–120 ppm), Cs2O (0.70–0.92 wt.%), and Na2O (1.29–1.66 wt.%); other components include TiO2 (0.00–0.01 wt.%), TFe2O3 (1.03–1.29 wt.%), MnO (0.01 wt.%), MgO (0.09–0.12 wt.%), and CaO (0.03–0.04 wt.%). These analyses indicate that the Xuebaoding material is a Na–Li–Cs beryl with a dominance of tetrahedral substitution in the crystal structure. Based on a systematic statistical study of the relationship between chemical composition and habit, a high content of alkalis leads to a lower growth rate of c {0001} compared with m , producing the short prismatic/columnar habit that typifies the Xuebaoding beryl. The difference in the number of pyramidal growth hillocks on s faces that face in opposing directions strongly suggests the influence of unidirectional hydrothermal fluid flow on growth rates. The oriented flow also led to variable growth rates for other crystal faces, including m and c {0001}, and caused the growth of many thick, distorted crystals of scheelite, cassiterite, tabular apatite, etc. Apparently, the m faces were outgrown by other, more rapidly growing faces and consequently tended to disappear. Accordingly, we discuss two mechanisms that underlie the formation of tabular beryl crystals: (1) the geochemical compositions and especially the high amount of alkalis at the Be site and in the channel structures of the crystals; and (2) the directional flow and supply of hydrothermal fluids in narrow fractures during crystal growth.

Interface approximation of Darcy flow in a narrow channel

A mixed formulation is introduced for the singular problem of Darcy flow in a porous medium in a region containing a narrow fracture or channel with width and high permeability . The solution converges as ε → 0 to that of Darcy flow coupled to tangential flow on the lower‐dimensional interface or boundary. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Experimental study of water flow behaviour in narrow fractures of cementitious materials

This paper reports an experimental investigation on water flow behaviour in narrow fractures of cementitious materials. A permeation device was developed especially for water flow in fractures. Five cementitious materials were retained, specimens were prepared in disk form and fractures were obtained by splitting specimens in three-point bending mode. The topography of fractured surfaces of specimens was scanned by laser profilometer and the surface roughness was quantified by 3D fractal dimension analysis. The fractured specimens were then closed and mounted into the permeation device for flow test by deionized water during 48 h. Furthermore, repetitive flow tests were performed on one fractured specimen to study the sealing effect of fractures. It is observed that the geometrical opening aperture in fractures is very different from the hydraulic equivalent value and the sealing effect of narrow fractures can be mainly attributed to the non-Darcian characteristic of liquid flow between rough surfaces under low pressure gradient.

Revisiting dirt cracking as a physical weathering process in warm deserts

A half century ago C.D. Ollier proposed that insolation-driven temperature changes expand and contract fill in fissures enough to widen cracks, a process that would permit progressively deeper penetration of fissure fills, that would in turn generate a positive feedback of greater and greater strain until desert boulders and bedrock shatters. Although desert physical weathering by “dirt cracking” has occasionally been cited, this hypothesized process remains without support from subsequent research. Here, field observations, electron microscopy, X-ray powder diffraction, particle-size analysis, and laboratory experiments shed new light on dirt cracking. Little clear evidence supports the original notion of expansive pressures from thermal fluctuations. However, mineralogical, high resolution transmission electron microscopy, back-scattered electron microscopy, and experimental evidence support two alternative processes of widening fractures: wetting and drying of fills inside fissures; and the precipitation and remobilization of calcium carbonate. A re-envisioned dirt-cracking wedging process starts with calcium carbonate precipitating in fissures less than 5 μm wide. First precipitation, and then ongoing dissolution of this laminar calcrete, opens enough space for dust to penetrate into these narrow fractures. Wetting of expansive clays in the fissure fill exerts enough pressure to widen and deepen the fissure, allowing the carbonate precipitation process to penetrate even deeper and allowing even more dust to move into a fracture. As the dust infiltrates, its texture changes from a chaotic mix of particles to an alignment of clays parallel to fissure sides. This parallel alignment could increase the efficiency of fill wedging. Ollier's concept of a positive feedback remains supported; each increment of fracture deepening and widening permits more, even deeper infiltration of laminar calcrete and dust. Field and electron microscope observations of rock spalling in the winter of 2010 are consistent with Ollier's hypothesis that dirt cracking is a common physical weathering process in deserts that splits rocks of all different sizes.