Cross Fractures Research Articles

Flow in fracture Intersections: Deflection flow assumption reexamined

Deflection flow, which represents the process of flow redistribution at fracture intersections, is essential for accurately predicting fluid infiltration and contaminant transport in fracture networks. However, previous studies focused on the outcome of flow redistribution but neglected the process, leading to poor knowledge of deflection flow at intersections. In this study, numerical investigations of the deflection flow in cross fractures were carried out based on Navier-Stokes equations and the results were compared with previous experiments. Geometrical characteristics such as angle(φ), aperture ratio(k), and hydraulic characteristics like inlet flow ratio(n) were considered to explore the mechanism of deflection flow. The deflection flow intensity(Xφ) was introduced to characterize the magnitude and direction of the deflection flow. The results showed that the deflection flow is caused by unequal local head loss. The effect of φ on Xφ is limited by n and k. Increasing n or k enhances the deflection flow but the directions of the induced deflection flow are reversed. The obtained relationship was further supported by theoretical results derived from the cubic law. Subsequently, sensitivity analysis of impact factors to the deflection flow was carried out by the Morris method, the sensitivity in descending order was k, n, φ. Finally, based on the analysis of Xφ, the conditions to limit the occurrence of deflection flow was obtained as a linear positive correlation between the critical inlet flow ratio(nc) and aperture ratio(k). The results from this study may provide insights and physical constraints on construction of flow models based on discrete fracture networks.

Effect of ultrasonic amplitude and riveting speed on mechanical properties of Ti-45Nb riveted lap joints

Ultrasonic vibration-assisted machining has been introduced as a new process in riveting aircraft components. In this paper, the effects of different ultrasonic amplitudes and riveting speeds on the quality, shear properties, and cross-tension properties of riveted joints are investigated. The failure sample fracture was characterized using a scanning electron microscope and the cause of the fracture was discussed. The results show that the ultrasonic amplitude is negatively correlated with the height of the riveted joint and positively correlated with the diameter of the riveted joint. The maximum shear failure load was obtained at a riveting speed of 5 mm/s with an ultrasonic amplitude of 35.3 μm, and the maximum cross tensile failure load was obtained at an ultrasonic amplitude of 23.3 μm. All shear fractures occurred in the rivet shanks and all cross fractures occurred in the rivet manufactured head, and the failure modes of both shear and cross-tension fracture were mixed brittle and plastic fracture modes.

Fracture propagation and evolution law of indirect fracturing in the roof of broken soft coal seams

Indirect fracturing in the roof of broken soft coal seams has been demonstrated to be a feasible technology. In this work, the No. 5 coal seam in the Hancheng block was taken as the research object. Based on the findings of true triaxial hydraulic fracturing experiments and field pilot under this technology and the cohesive element method, a 3D numerical model of indirect fracturing in the roof of broken soft coal seams was established, the fracture morphology propagation and evolution law under different conditions was investigated, and analysis of main controlling factors of fracture parameters was conducted with the combination weight method, which was based on grey incidence, analytic hierarchy process and entropy weight method. The results show that “士”-shaped fractures, T-shaped fractures, cross fractures, H-shaped fractures, and “干”-shaped fractures dominated by horizontal fractures were formed. Different parameter combinations can form different fracture morphologies. When the coal seam permeability is lower and the minimum horizontal principal stress difference between layers and fracturing fluid injection rate are both larger, it tends to form “士”-shaped fractures. When the coal seam permeability and minimum horizontal principal stress between layers and perforation position are moderate, cross fractures are easily generated. Different fracture parameters have different main controlling factors. Engineering factors of perforation location, fracturing fluid injection rate and viscosity are the dominant factors of hydraulic fracture shape parameters. This study can provide a reference for the design of indirect fracturing in the roof of broken soft coal seams.

Investigation of additive-assisted microbial-induced calcium carbonate precipitation in 3D printed cross fractures

Microbially induced carbonate precipitation (MICP) with additives is a promising approach for seepage control in the field of geotechnical engineering. This study designed and fabricated four cross-fracture models using three-dimensional printing. One of the models did not include additives and was treated with a mixed injection strategy. The other three models were prepared using different types of additives such as calcium carbonate, jute fiber, and gravel particles, which were treated with a staged injection strategy to improve our understanding of the optimized MICP process. The increased mass of calcium carbonate in the model was determined by the difference in the model’s weight before and after four-cycle intervals to evaluate the effect of the three additives and two injection strategies on seepage control and precipitation patterns. The results indicated that the masses of the precipitates increased the most for the model without additive owing to gravity during the MICP process; however, the clogging effect was weakest during the seepage experiment. This may be because the smooth inner surfaces of the model and a higher flow rate prevented the floc and cementation solution from settling. The models that included plant fibers and gravel particles provided more surfaces that facilitated calcium carbonate precipitation and contributed to seepage control. The model with calcium carbonate produced a considerable amount of calcium carbonate and interacted positively with the fracture surfaces, leading to the largest decrease in seepage. In the case of calcium-carbonate additives, a layer of precipitates was formed, which increased the inner surface roughness and enhanced the boundary-layer effect. This promoted the precipitation and settling of floc and the formation of more reliable bridges within the cross-fracture model compared with those of other additives.

Study on seepage characteristics of cross fractures in adjacent and convective flow modes

The distribution of fractures in the underground rock mass is complex, so it is of great significance to study the parameter characteristics of fracture seepage for geothermal exploitation, land subsidence, and other fields. Based on the Weierstrass-Mandelbrot function, several X-shaped fracture models with different roughness are established, and the fracture seepage law under different roughness and inlet pressure is studied. At the same time, the center (cross-region) of the X-shaped fracture is selected as the research object, and the seepage difference characteristics of the cross-region under the two inlet modes of adjacent flow and convective flow are compared and analyzed. The results show that the outlet velocity of fracture fluid is inversely proportional to the fractal dimension and is directly proportional to the inlet pressure.

A Test Method for Finding Early Dynamic Fracture of Rock: Using DIC and YOLOv5.

Intelligent monitoring and early warning of rock mass failure is vital. To realize the early intelligent identification of dynamic fractures in the failure process of complex fractured rocks, 3D printing of the fracture network model was used to produce rock-like specimens containing 20 random joints. An algorithm for the early intelligent identification of dynamic fractures was proposed based on the YOLOv5 deep learning network model and DIC cloud. The results demonstrate an important relationship between the overall strength of the specimen with complex fractures and dynamic fracture propagation, and the overall specimen strength can be judged semi-quantitatively by counting dynamic fracture propagation. Before the initiation of each primary fracture, a strain concentration area appears, which indicates new fracture initiation. The dynamic evolution of primary fractures can be divided into four types: primary fractures, stress concentration areas, new fractures, and cross fractures. The cross fractures have the greatest impact on the overall strength of the specimen. The overall identification accuracy of the four types of fractures identified by the algorithm reached 88%, which shows that the method is fast, accurate, and effective for fracture identification and location, and classification of complex fractured rock masses.

Research on the Failure Evolution Process of Rock Mass Base on the Acoustic Emission Parameters

Fracture mechanics behavior and acoustic emission (AE) characteristics of fractured rock mass are related to underground engineering safety construction, disaster prediction, and early warning. In this study, the failure evolution characteristics of intact and fracture (e.g., single fracture, parallel fractures, cross fractures, and mixed fractures) coal were studied and contrasted with each other on the basis of the distribution of max amplitude of AE. The study revealed some meaningful results, where the value of b (i.e., the distribution characteristic of max amplitude of AE) could represent the failure evolution process of intact and fractured coal. The maximum amplitude distribution of AE events was characterized by Gaussian normal distribution, and the probability of the maximum amplitude of AE events corresponding to 35∼50 dB was the largest. In the stress range of 60∼80%, AE events and maximum amplitude increased rapidly, and the corresponding b value decreased. The energy of AE events showed a downward trend after reaching the maximum value at about 80% stress level. Under the same stress level, the more complex the fracture was, the larger the b value of coal–rock mass was, and the stronger the inhibition effect on the fracture expansion caused by the internal fracture distribution was. Due to the anisotropy of coal–rock mass with a single crack, the distribution of the b value was more discrete, while the anisotropy of coal–rock mass with mixed crack decreased, and the dispersion of the b value decreased. The deformation of cracked coal mainly caused by the adjustment of cracks during the initial loading b value experienced a trend of decreasing first, then increasing, and then decreasing in the loading process. When the load reached 0.8 times of the peak strength, the b value had a secondary decreasing trend, indicating the macroscopic failure of the sample, which could be used as a precursor criterion for the complete failure of coal–rock mass.

GOLD POTENTIAL OF MANITANYRDSKY AREA AND PROSPECTS FOR RESOURCE POTENTIAL RAMP-UP, POLAR URALS

General information on geological structure and gold potential of Manitanyrdsky area located in the western Polar Urals is provided. Niyayuskaya ore zone is revealed, Verkhneniyayuskoye-2, Niyakhoiskoye-2 and Yagodnoye mineral occurrences are characterized. It is shown that, aside from vein gold-sulfide-quartz mineralization, disseminated and veinlet-disseminated mineralization separated from quartz veins is widespread, which is also of economic interest. Gold in ores occurs as ultrafine inclusions in arsenopyrite and pyrite; it is also found in cross fractures as overgrown with later sulfides, which suggests two ore formation stages - gold-pyrite-arsenopyrite and gold-chalcopyritesphalerite-galena. Lithochemical prospecting results are presented, gold-enriched areas are identified. Prospects for the area’s resource potential ramp-up are assessed.

Proppant Transportation in Cross Fractures: Some Findings and Suggestions for Field Engineering

The proppant transportation is a typical two-phase flow process in a complex cross fracture network during hydraulic fracturing. In this paper, the proppant transportation in cross fractures is investigated by the computational fluid dynamics (CFD) method. The Euler–Euler two-phase flow model and the kinetic theory of granular flow (KTGF) are adopted. The dimensionless controlling parameters are derived by dimensional analysis. The equilibrium proppant height (EPH) and the ratio of the proppant mass (RPM) in the secondary fracture to that in the whole cross fracture network are used to describe the movement and settlement of proppants in the cross fractures. The main features of the proppant transportation in the cross fractures are given, and several relative suggestions are presented for engineering application in the field. The main controlling dimensionless parameters for relative EPH are the proppant Reynolds number and the inlet proppant volume fraction. The dominating dimensionless parameters for RPM are the relative width of the primary and the secondary fracture. Transportation of the proppants with a certain particle size grading into the cross fractures may be a good way for supporting the hydraulic fractures.

A comparative study on simulating flow-induced fracture deformation in subsurface media by means of extended FEM and FVM

Accurate and efficient simulation on the fluid flow and deformation in porous media is of increasing importance in a diverse range of engineering fields. At present, there are only several methods can be used to simulate the deformation of fractured porous media. It is very important to know their application scopes, advantages, and disadvantages for solving the practical problems correctly. Therefore, in this paper, we compared two numerical simulation methods for flow-induced fracture deformation in porous media. One is the Extended Finite Element Method (XFEM), which is based on the classical finite element method and can simulate strong or weak discontinuous problems. The other is developed within the finite-volume framework, termed Extended Finite Volume Method (XFVM). We designed three test cases, including single fracture, cross fractures and eight discrete fractures, to investigate the accuracy and efficiency of XFEM and XFVM. The reference solutions were provided by the commercial software, COMSOL, where the standard finite element method is implemented. The research findings showed that the accuracy of the XFEM was slightly higher than that of the XFVM, but the latter was more efficient. These results are likely to be useful in decision making regarding choice of solving methods for the multi-field coupling problem in fractured porous media.

The flowback and production analysis in sub-saturated fractured shale reservoirs

The initial water saturation of shale reservoirs is in general lower than the irreducible water saturation. However, the hydraulic fracture network region is highly saturated by fracturing fluid, which results in a two-phase flow inside fractured region. A long-time two-phase flow period is always observed in the fields even after a high flowback rate, which cannot be explained by existing models. In this paper, we develop a 2D gas-water flow model coupling multi-scale discrete fractures and continuum framework to describe the flowback process of a multi-stage fractured horizontal well in shale gas reservoirs. The model considers the non-linear flow mechanism of free gas, desorption of adsorbed gas and the co-existence of single gas flow and gas-water flow. The capillary pressure model and relative permeability model are both employed with different parameter values for matrix and fractures, respectively. The permeability of naturally fractured shale is described using a fractal model. To overcome non-convergence problem of finite element method for solving two-phase flow in cross fractures, the model is solved by finite element method and finite volume method. The model is verified through a simple two-phase flow simulation and a single gas phase simulation with some simplifications. Then the model is used for history matching as well as production prediction of a well in Changning block, Sichuan, China. The simulation results show that the high water saturation induced “skin factor” in the vicinity of hydraulic fractures significantly delays the gas production for a long time because some of the fracturing fluids become irreducible water and lower the gas phase permeability. Due to the great influence of fracture interference on water flow, the fracturing fluids concentrate on regions near hydraulic fractures and their junctions. The long-term gas production of the complex fracture network is slightly higher than that of the simple fracture network, which is different with the simulation results of single gas flow model.

Numerical Analysis of Crack Propagation Evolution of Specimens with Different Dip Angles of Cross Fractures

In order to study the failure modes and crack propagation laws of uniaxial compression specimens with different dip angles, a mathematical model was established using the discrete element program PFC2D. The crack initiation, expansion and extension of the simulated specimens were analyzed by tuning the horizontal angle under various working conditions. The effects of tensile cracks and shear cracks on crack propagation were comprehensively analyzed using the failure mode combined with the displacement field and the stress–strain curve. The results showed that the failure mode was first shear failure, then composite failure and finally tensile failure as the horizontal angle increased. Meanwhile, the stress decreased, increased and decreased again as the horizontal angle increased. The study of compressive failure of cross-cracked specimens at different dip angles serves as an important guide for roadway support and underground space excavation.

Therapeutic efficacy of Kocher-Langenbeck approach without cutting hip spin short muscle tendons for treatment of acetabular posterior column fractures

Objective To investigate the feasibility and clinical effect of Kocher-Langebeck(K-L)approach without cutting the short external rotator muscles of hip for treatment of cetabular posterior column fractures. Methods A retrospective case series study was performed on 28 patients with acetabular fractures admitted from June 2015 to February 2017.There were 23 males and 5 females, averagely aged 43.6 years (range, 26-71 years). According to the Letournel classification, there were 9 patients with simple fractures, 14 back wall and back pillar fractures and 5 cross fractures. All patients were combined with posterior dislocation. The patients were given tibial tubercle traction after hip joint reduction in the hospital. During the surgery, K-L approach was adopted without cutting the short external rotator muscles of hip. Operation duration and intraoperative bleeding were recorded. Visual analogue score (VAS) was recorded before and after operation. The X-ray at 1 day, 6 weeks, 3 months and 1 year after operation were reviewed to evaluate fracture healing time. Matta criteria were used to assess the surface flatness of joint. The modified Merle d'Aubigne-Postel scoring system was applied to evaluate the function of hip joint. Intraoperative and postoperative complications were recorded. Results The operation duration was 76-120 min (mean, 94 min), and the intraoperative blood loss was 120-320 ml (mean, 265 ml). All patients were followed up for average 16 months (range, 10-24 months). Preoperative VAS was 5-10 points[(7.5±1.3)points], and 0-3 points[(0.9±0.8)points] 6 months after operation (P<0.01). All fractures healed and the average healing duration was 10.1 weeks (range, 6-12 weeks). According to the Matta criteria, 24 patients were graded excellent and 4 good, with excellence rate of 100%. According to the modified Merle d'Aubigne-Postel grading system, clinical results were graded excellent in 23 patients, good in 5, with excellence rate of 100%. Transient sciatic nerve injury occurred in 3 patients after surgery, and the nerve function of these patients fully recovered within 3 months after operation. There were no infection, heterotopic ossification and other complications. The wound healing was good, without liquefaction or cracking. Sciatic nerve injury was found in five patients, three of whom were performed with a transient lesion and recovered within 3 months. Conclusion In the posterior pathway surgery for acetabular posterior column fractures, the K-L approach without cutting the short external rotator muscles of hip can be used to complete the internal fixation with a high rate of fracture healing, sound reduction outcome, satisfactory functional recovery and few complications. Key words: Acetabulum; Hip fractures; Fracture fixation, internal

Numerical simulation on solid-liquid two-phase flow in cross fractures

This paper presents a series of numerical simulation on solid-liquid two-phase flow in cross fractures based on the Two-Fluid Model and the kinetic theory of granular flow (KTGF). First, the model is validated by previous experimental data. Second, the dimensionless controlling parameters are derived to describe the particle-laden flow in cross fractures, including the angle between the main and branch slots (bypass angle) θ, inlet particle volume fraction αs0, the ratio of particle size to branch slot width ds/wb, the Archimedes number Ar and the Reynolds number Re. Third, the effects of the dimensionless parameters are investigated. The results show that particles tend to accumulate at the intersection between the main slot and the branch slot. Larger bypass angle between the main slot and branch slot leads to less particle’s flow into the branch slot. The distance of the branch fracture from the inlet of the main fracture induces different particle-flow characteristics into the branch slot. Particle volume fraction at the stable stage increases with the decrease of ds/wb. The deposition thickness of particles increases with the increase of the inlet volume fraction and Ar number, while decreases with the increase of Re number.

Origin and Geological Significance of the Cross Fractures in the Upper Triassic Yanchang Formation, Ordos Basin, China

Cross fractures are well developed in the Upper Triassic Yanchang Formation in the Ordos Basin. According to the data on outcrops and cores and to our mechanical experiments, the origin of these cross fractures is discussed and their influence on petroleum exploration and development is analyzed. Four assemblages of fractures were formed in the Upper Triassic Yanchang Formation within the Ordos Basin. But in a certain place, there are generally distributed two assemblages of cross fractures, i.e. E-W and S-N fractures, at the center of the basin, and NE-SW and SW-SE fractures in the southwest of the basin. Fractures here are mainly high-angle shear fractures that are nearly vertical to the beds that were formed during the Yanshan (135Ma) and Himalayan (65–23.3Ma) periods due to horizontal tectonic compression, deep burial and intense uplift or denudation. The rock anisotropy is the prime reason for impacts wrought on the development of different assemblages of cross fractures in some geological periods, which usually restricts the development of one assemblage of conjugate shear fracture. Theoretically, there should have been formed two assemblages of shear fractures in either tectonic period, and there should have developed four assemblages of shear fractures here. But, due to the effect of a strong rock anisotropy, in each period developed only one assemblage of fracture. Thus two assemblages of cross fractures are usually developed in each part of the Ordos Basin at present. The cross fractures are important pathway system for oil migration in the low-permeability sandstone reservoirs of the Upper Triassic Yanchang Formation. At the same time, they also serve as main channels for fluid flow during the water-injection development process and impact the development of such reservoirs.

Advances in compression nails - principles and biomechanical photoelastic evaluation

The nail-osteosynthesis is an established concept for shaft-fractures of long bones. The compression-nail ICN enables a favourable primary-stability and also a former full weight-bearing over a pre-compression of the nail-bone-system. The indications of the compression-nail include beside more diaphyseal cross fractures and short oblique fractures non-unions and correction-osteotomies. Newer modular nail-types like the tandem compression nail (TCN) can improve the biomechanical prerequisites of the compression nail principle.

Orthogonal cross joints: do they imply a regional stress rotation?

Orthogonal cross joints extend across intervals between systematic joints in brittle sedimentary strata and abut the systematic joints at about 90° angles. These joints typically form a ‘ladder-like’ pattern if viewed on a bedding surface. A common interpretation is that orthogonal cross joints define the orientation of the regional stress field during their formation: least compressive stress perpendicular to the joints. It follows that they indicate a rotation of regional principal stresses by 90° after the formation of the systematic joints. Using a three-dimensional boundary element code (Poly3D), we considered a simple geologic case of vertical systematic fractures developing in horizontal strata under a triaxial remote load with: the maximum principal tensile stress being horizontal and perpendicular to the strike of the fractures, the intermediate principal stress being horizontal and parallel to the strike of the fractures, and the least principal tensile stress (i.e. maximum compressive stress) being vertical. The results show that the local maximum principal stress is first perpendicular, and then parallel to, the strike of the systematic fractures as the ratio of fracture spacing to height changes from greater than to less than a critical value when the horizontal remote principal stress ratio, the ratio of the intermediate remote principal stress to the maximum remote principal stress under the sign convention of positive for tensile stresses, is greater than a threshold value (∼0.2). Thus, the fracturing process changes from infilling of systematic fractures to the formation of orthogonal cross fractures. This provides an alternative mechanism for the formation of orthogonal cross joints that does not require a systematic rotation of the regional stress field by 90°. The critical spacing to height ratio for the local principal stress switch is independent of the least remote principal stress (i.e. overburden). It increases nonlinearly with increasing ratio of the horizontal remote principal stresses, and decreases nonlinearly with increasing Poisson's ratio of the material.

Heat source for Tongonan Geothermal Field

Abstract The primary mineral and whole‐rock chemistry of 46 core samples from the host rocks of the Tongonan Geothermal Field (the Philippines) have been used to infer the likely composition of the heat source for the system. The host rocks consist of andesite lavas (with intercalated fossiliferous early to mid–Miocene shales and limestone), and a plutonic rock basement ranging in composition from gabbro to granite. The whole rock TiO2, Fe2O3 (total iron), MgO, P2O5 and V data for volcanic and plutonic rocks are colinear on conventional Harker diagrams. This, along with similar hornblende chemistry, age and close spatial relationship suggests that the basement and cover rocks are cogenetic and evolved by low‐pressure crystal fractionation. Crystal fractionation models indicate that separation of 60% plagioclase and 30% hornblende from original magma controlled the chemistry of the host rocks. The original Miocene magma chambers beneath the Tongonan field crystallized inwards from the walls at approximately 750°C and 1 kb pressure (3–4 km depth) thus forming a series of plutons or a batholith at drilled depths. A supercritical hydrothermal fluid trapped in the crystallizing, hornblende‐granite‐pegmatite core of a crystallized Miocene diorite batholith was gradually being released to shallower levels through antithetic cross fractures during creep and uplift along the main branches of the Philippine Fault from the Pliocene. This ascending fluid is now thought to be responsible for the present thermal activity of the field.

Cannel coals: implications for classification and terminology

Cannel coals are tough, massive coals with a dull luster that break with an even, compact grain and conchoidal cross fractures. Historically, cannel coals were of significant economic and social importance during the early industrialization of the USA. They were the source for a sophisticated petrochemical industry and a fuel source for heating, gas making, coke making and oil generation. Petrographic data, proximate analysis and ultimate analysis data were obtained for 68 cannel coals from Kentucky to decide how well the cannel coals fit the generally-accepted definitions of a cannel coal. Four maceral assemblages were found—sporinite with abundant medium-grained vitrinite and inertinite, sporinite with abundant fine-grained vitrinite and inertinite of which micrinite is abundant (bituminite may be present), alginite with abundant fine-grained vitrinite and inertinite, and alginite and bituminite with minor vitrinite and inertinite. Volatile matter and H/C ratios are highest for those samples with abundant liptinite with a sample from Breckenridge having the highest value of each. Five samples were analyzed by the Rock-Eval method. The S 2 and HI indices show that the coals would make excellent source rocks. Discrepancies between the values for the five samples are related to maceral composition and there may be a good correlation between source rock potential and importance as a feedstock for synthetic crude oil production in the 1800s. The maceral composition of the microlithotypes is related to the environment of deposition. Coal composed of assemblage (i) formed in humic peat swamps in which unusually large numbers of spores were introduced, possibly because of unusual, climatically controlled reproductive cycles. Coals composed of assemblages (ii) to (iv) formed as sapropelic coals in lakes at some stage of peat swamp development.

Freeze-fracture of sperm of Plumbago zeylanica L. in pollen and in vitro

Sperm of Plumbago zeylanica are dimorphic with regard to numbers of mitochondria and plastids. In most cases examined, the plastid-rich sperm fused with the egg while the sperm with fewer plastids fused with the central cell. However, plastids cannot be directly responsible for fusion because fusion occurs between the plasma membranes of egg and sperm. The question is whether sperm cell membranes are distinctive and possibly dimorphic. Sperm in whole pollen grains and isolated sperm were freeze-fractured. In pollen, freeze-fractured sperm appeared only in cross fractures. No extended membrane fracture faces of sperm were found. Among isolated sperm, two sizes of sperm with different organelles were observed. Isolated sperm were assigned to two categories based on cell diameter and on size and density of organelles. Membrane particles on most sperm were arranged without distinctive pattern. Some hexagonal arrays were observed. In sperm that had been maintained at 4°C, particle-free areas, a probable consequence of lipid phase separations, appeared on plasma membrane fracture faces. No unique fracture patterns and no patterns of dimorphism were detected on freeze-fractured plasma membranes of Plumbago sperm.