Low Fluid Pressure Research Articles

Linking the spatiotemporal distribution of static stress drops to source faults in a fluid-driven earthquake swarm, northeastern Noto Peninsula, central Japan

We investigated stress drops during an earthquake swarm in northeastern Noto Peninsula, central Japan, which is characterized by ongoing seismic activity in four clusters. We focused on the spatiotemporal distribution of the static stress drop and its relationship with the source faults of the earthquake swarm. Employing the empirical Green’s function method, we estimated static stress drops for 90 earthquakes of MJMA 3.0–5.4. We obtained logarithmic mean stress drops of 13 MPa and 19 MPa from P-wave and S-wave analyses, respectively, which were typical values for crustal earthquakes. We comprehensively analyzed the spatiotemporal distribution of static stress drops in the northern cluster due to the abundance of available data and clarity of fault structures there. We observed larger static stress drops for earthquakes along shallow portions of the source faults, as defined by the hypocentral distribution during a given period. Conversely, we observed smaller static stress drops for earthquakes at medial parts along the faults. These results suggest higher fault strength at shallower parts along the faults and reduced fault strength at medial parts. We attribute the high fault strength at shallow parts to low pore fluid pressure after only limited fluid diffusion near the fault terminus. In contrast, we attribute the reduction in fault strength at medial parts to high pore fluid pressure within the fault following penetration by migrating fluids.Graphical

Experimental study of true triaxial high pressure subcritical water impact fracturing

A new fluid alternative to slick water for fracturing shale gas can reduce the waste of water resources and improve the extraction efficiency, enabling volumetric fracturing. For the new fracturing technique, the experiments of different release pressures under pre-injection and for pre-injection were conducted using a self-designed true triaxial experimental system, and the pressure pulse curves were plotted to analyze the fracturing principle. The experimental results showed that: (1) the pressure rise curve in the reactor can be divided into five stages: initial reaction, linear pressure rise, rate slowdown, instantaneous pressure release, and residual pressure stages; (2) Pre-filling fracturing requires a smaller expansion ratio, weaker pressure degradation, resulting in better fracturing effect; (3) The increase in the initial fracture length leads to an increase in the pressure required to extend the fracture, and high-pressure subcritical water impact fracturing achieved fracture extension at a lower fluid pressure; (4) The fractal dimension has a strong linear relationship with fracture complexity, which is a new option when evaluating the fracturing effect. Volumetric fracturing allows for the creation of more tiny trenches that increase reservoir permeability, leading to better recovery of the reservoir’s energy resources.

An alternative to the fault-valve model

This paper proposes a mechanism called the mode-switching model that is presented as an alternative to the fault-valve model. This mechanism is relevant to open-flow, low-porosity, fluid-saturated systems deforming by pressure solution creep. As opposed to most constitutive models discussed in the geological literature, the yield envelope is capped at high normal stresses, as demonstrated by experimental studies. A low-permeability rock has relatively high pore fluid pressure for a given input fluid flux. This increases the dissolution rate for quartz that in turn leads to a higher-permeability rock, low fluid pressure for the same flux and decreased quartz solubility and deposition, returning to a low permeability. This cycle continues indefinitely so long as the rock mass is stressed, a fluid flux is applied, and pressure solution operates. The high fluid pressure drives the Mohr stress circle to the tensile end of the yield envelope resulting in crack-seal and extensional veins. The low fluid pressure drives the Mohr stress circle to the cap end of the yield envelope resulting in laminated veins in rocks undergoing mineral reactions with large net volume losses coupled with solute transfer. Failure at the cap end of the yield envelope results in displacement discontinuities inclined at high angles to σ 1. Previously, these orientations have been taken to represent reactivated normal faults, an integral component of the fault-valve process. In the model presented, the yield surface prohibits the system ever reaching super-lithostatic pressures. The process of effective stress-driven switching between tensile and cap ends of the yield envelope arises from competition between dissolution and deposition, and is independent of any seismic events, fault reactivation or the episodic breaching of an impermeable seal. It provides a unifying, self-consistent concept for the interpretation of joints, faults and veins in hydrothermal systems.

New insight into fracturing characterization of shale under cyclic soft stimulation: A lab-scale investigation

The cyclic soft stimulation (CSS) is a new method of reservoir reforming for which the mechanism of fracturing crack propagation is ambiguous with regard to the alternating fluid pressure. This study aims to provide a comprehensive understanding of the fracturing mechanical characterizations of CSS under different magnitudes and amplitudes of the alternating fluid pressure. Acoustic emission (AE) is recorded to investigate the damage evolution under CSS based on the b value analysis of AE. Experimental results reveal the difference of pressure in a crack under different cyclic fluid pressure conditions. The AE results show that the maximum radiated energy under CSS tends to be reduced with the increase in the amplitude and magnitude of the alternating fluid pressure. The finishing crucial touch is that the crack extending criterion under CSS is proposed, which combines the injection parameters, the rock properties and in-situ stress. According to the crack extending criterion, the fluctuation fluid pressure causes the reduction of a critical crack extending pressure, and the CSS causes the crack to initiate and propagate under low fluid pressure. Under a higher-value magnitude of alternating fluid pressure, the cyclic times of CSS is less for the crack initiation. In supplement to the crack extending criterion, a distinct relationship between the radiated energy and the cyclic fluid pressure also is established based on the energy dissipation criterion. These new findings provide an insight into the determination of crack extending criterion under CSS for efficiently implementing shale fracturing.

Fluid Pressure Changes Recorded by Trace Elements in Quartz

AbstractFluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion abundance. Because Si‐Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al‐Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL‐brown, Al‐Li‐poor rims to CL‐blue, Al‐Li‐rich quartz rims. CL‐blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL‐brown rims grow at a slower pace, lower Si supersaturation, and higher fluid pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes.

Failure modes in fluid saturated rocks: deformation processes and mode-switching

AbstractWe consider the implications of adding a cap to the yield surface for elastic–plastic and elastic–visco-plastic solids with coupling between deformation, fluid flow and mineral reactions. For a suitable combination of (low) permeability and strain rate, opening-mode veins can form in compression. Such behaviour is enhanced by dissolution and by simultaneous mineral reactions with negative ΔV. These are the opening-mode equivalents of compaction bands in rocks with high permeability. Stylolite parallel veins are considered as forming in this way; such veins are commonly the laminated or ribbon-quartz veins associated with intense gold mineralization. Axial plane veins and melt segregations are in this class also. The addition of a yield surface cap limits the permissible stress states portrayed by a failure-mode diagram and has implications for breccia formation. Failure discontinuities that form at the cap require adecreasein fluid pressure to form as opposed to extension joints and veins that require anincreasein fluid pressure; discontinuities that form at the cap are in orientations that are commonly interpreted as reactivated early discontinuities. The switching between high fluid pressure and low fluid pressure, which we callmode-switching, arises from competition between mineral dissolution and deposition. This is an alternative to the fault-valve mechanism and does not require fault reactivation or failure at a ‘seal’ linked to seismicity or fault reactivation. The capped yield surface concept provides a unifying self-consistent approach for vein/breccia formation and for the kinematics of brittle and visco-plastic rocks.

Megathrust reflectivity reveals the updip limit of the 2014 Iquique earthquake rupture

The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus controls the extent of earthquake rupture. Generalizing these results, seismic reflection data can provide insights into the physical state of the shallow plate boundary and help to assess the potential for future shallow rupture in the absence of direct measurements of interplate deformation from most outermost forearc slopes.

A new insight into casing shear failure induced by natural fracture and artificial fracture slip

Casing deformation is often encountered due to hydraulic fracturing in the unconventional reservoir. Analysis of 3D seismic, microseismic and logging data shows that the casing shear failure is the primary casing failure mode during hydraulic fracturing. The casing deformation is closely related to the fracture system near the wellbore, but the mechanism and law behind this phenomenon are not completely clear. The slip mechanism of the fracture system and the characteristics of casing deformation were investigated to prevent casing deformation and identify the fracture with large slip distance. Based on the Mohr-Coulomb and maximum circumferential stress theory, the slip mode of natural fracture and the slip mechanism of artificial fracture were analyzed, respectively. Then, two numerical models were established to calculate the slip distance with finite fracture height and corresponding casing deformation. Furthermore, a monitoring model of casing deformation was presented based on microseismic data and real-time injection parameters. The result shows three slip modes in the fracture system crossing wellbore under different fluid pressures, including shear-slip under low fluid pressure, open-slip under medium fluid pressure and propagation-slip under high fluid pressure. The slip distance is associated with fracture length, fracture height, fluid pressure in the fracture and fracture angle. Hydraulic fracturing not only activates natural fractures and faults but also causes the slip of artificial fractures created by hydraulic fracturing, which is responsible for the casing shear deformation after fracturing and during production. The greater the stress deflection angle, the longer and higher the artificial fracture, the more likely the casing deformation in the fracturing section. Therefore, the scale of artificial fracture should be controlled during hydraulic fracturing. This study will provide guidance for predicting fracture slip distance and casing deformation prevention.

Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

AbstractSubduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward.

Effect of tread design and hardness on interfacial fluid force and friction in artificially worn shoes

Shoe wear is a major contributor to slip risk. Knowledge of the impact of shoe design features on wear progression and its impact on friction performance is emerging. This study investigated the effect of tread geometric design and shoe hardness on the shoe’s friction response to wear. Three tread designs (two designs using large tread lugs and one tread design using small-patterned tread features) each at three different hardness levels underwent available coefficient of friction (ACOF) and under-shoe fluid pressure testing on a vinyl composite tile contaminated with 90% glycerol solution. These tests occurred at baseline and after each of six iterations of an accelerated wear procedure. The results of this study revealed that tread design influenced under-shoe fluid force, friction, and their response to wear. One of the lug designs experienced high fluid pressures and low friction performance even in its new condition although its performance improved with wear. The other lug design exhibited good drainage, good friction performance, and resilience to wear. The small-patterned tread experienced high ACOF and low fluid pressures at baseline and declined in performance with wear. Thus, lug designs offer a wide range of friction performance and resilience. No effects were observed due to hardness. This study determined that tread geometry is an important factor for designing shoes with durable traction performance.

Анализ траектории трещины гидроразрыва около пластовой выработки и условия ее прямолинейного роста

Model of the geomechanical state of a disc hydraulic fracture propagating in the solid rocks near the seam working is based on the methods of solid mechanics and fracture mechanics. Stress field in the coal-rock mass containing in-seam working and growing hydraulic fracture was constructed as a result of solving an elastoplastic problem, in which the area of plasticity is the extremely stressed zones of the edge parts of the seam. The stress field in the edge parts was determined in the course of the numerical solution of three boundary value problems of the seam limiting state. The criteria for the onset of the limiting state are the general criterion of the Coulomb — Mohr limiting state for the formation and a special criterion for the limiting state for its contact with the rock mass. By replacing the extremely stressed zones with the stresses acting on their contact with the rock mass, the elastoplastic problem is reduced to the second external boundary value problem of the theory of elasticity, which is solved by the method of boundary integral equations. At the relatively low fluid pressures in the pumping unit, the trajectory of the hydraulic fracture is a smooth curved line of the small length with a significant deviation of its ends from the direction of the seed crack. With increasing fluid pressure, the crack length increases, and the deviation from the direction of the seed crack decreases. There are fluid pressures at which the crack propagates in a straight line and practically does not change its original direction. The straight-line trajectory of the crack in the vicinity of the working located at different depths corresponds to a point on the graph of the dependence of the relative length of the crack on the fluid relative pressure. This graph is a straight line.

Rock characteristics, internal structure and physical-chemical properties of Qianning segment in Xianshuihe fault zone

断裂带的变形行为和断层滑移机制是目前地震研究关注的热点，断裂带岩石特征、内部结构与物理化学性质是确定断层蠕滑或粘滑行为以及断层滑移机制的基础和关键。本文以鲜水河断裂带乾宁段地表出露的断裂岩为研究对象，通过野外地质调查、室内光学显微镜、扫描电镜、粒度统计、粉末X射线衍射分析（XRD）和薄片X射线荧光光谱分析（XRF）等多种研究方法，对鲜水河断裂带岩石特征、结构构造、物性、矿物成分及化学元素分布开展了详细的分析，并探讨了相关变形行为和滑移机制。分析表明：（1）断裂带核部主要由黑色断层泥、浅黄色及黄色断层角砾岩和灰色碎裂岩、灰色断层角砾岩组成，呈单核对称结构；（2）黑色断层泥厚3~5cm，具有快速滑动结构特征，表现为断层粘滑行为。断层泥可划分出13个滑移带，最窄滑移带厚约40μm，至少代表13期古地震事件；（3）断层泥主要由伊利石、高岭石和石英等矿物组成，其中边部伊蒙混层含量异常高，为最新一次古地震的主滑移带。由于伊利石和伊蒙混层（或蒙脱石）为主要黏土矿物的断层泥渗透率低、孔隙流体压力大，以及发现断层泥楔入脉，表明地震过程中断层滑动存在热增压弱化机制；（4）从断层泥不同滑动带中碎块蚀变程度和矿物分布特征来看，地震主滑动带有向碎裂岩方向迁移的趋势。推测断层在滑动过程中，更趋向于向弱矿物含量高（如伊利石、伊蒙混层）、强矿物含量低（如方解石、高岭石）的围岩一侧迁移。;The deformation behavior and slipping mechanism of fault zone are the focus of current seismic research. Rock characteristics, internal structure and physical-chemical properties of fault zone are the basis and key to determine creep- or stick-slip behavior and fault slipping mechanism. This study focuses on fault rocks from Qianning segment in Xianshuihe fault zone. Rock characteristics, structure, physical properties, mineral composition and chemical element distribution of the Xianshuihe fault zone have been analyzed in detail through field geological survey, observation of optical microscope and scanning electron microscope, particle-size statistics, powder X-ray diffraction analysis (XRD) and thin section X-ray fluorescence spectrum analysis (XRF). Related deformation behaviors and slipping mechanisms have been discussed. The results show that: (1) the core of fault zone is mainly composed of black fault gouge, light yellow and yellow fault breccia, gray cataclasite, and gray fault breccia, presenting a single-core-symmetrical structure; (2) the black fault gouge is 3~5cm thick and shows rapid-slip structure, indicating stick-slip behavior of the fault zone. The fault gouge can be divided into 13 slip zones, and the narrowest slip zone is about 40μm thick, representing at least 13 paleoearthquake events; (3) the fault gouge is mainly composed of illite, kaolinite, and quartz. The content of illite-smectite (I/S) mixed layer at the edge of fault gouge is abnormally high, which presents the primary slip zone of the latest paleoearthquake. The low permeability and high pore fluid pressure caused by main clay minerals of illite and I/S mixed layer (or smectite) in fault gouge and the discovery of fault gouge injection veins indicate that there is a thermal pressurization weakening mechanism in the fault zone sliding during the earthquake process; (4) according to the degree of fragment alteration and mineral distribution characteristics in different slip zones of fault gouge, the main seismic slip zone has a tendency to migrate towards cataclasite. It is speculated that during the sliding process, the fault tends to migrate to the side of the surrounding rock with high content of weak minerals (such as illite and I/S mixed layer) and low content of strong minerals (such as calcite and kaolinite).

Mechanisms of melt extraction during lower crustal partial melting

AbstractProgressive vapour‐absent partial melting of a closed rock system increases melt pressure due to an expansion in the volume of the mineral plus melt assemblage. For a locally closed system, we quantify the melt pressure increase per increment of partial melting of a metapelite using phase equilibria modelling and combine it with Mohr–Coulomb theory to examine the interplay between melt pressure and fracture behaviour. It is shown that very small increments of vapour‐absent partial melting (<1%) increase melt pore pressure by tens of MPa leading to inevitable brittle failure of locally closed systems. Fracturing will affect these systems, even if initially limited to the scale of a few grains, and a connected microfracture network will enhance permeability as partial melting progresses. This will lead to a conditionally open system, potentially limiting accumulation of melt in the source. Repeated and cyclic fracture as temperature progressively increases will drive migration of the melt into sites of low fluid pressure at all scales. Crystal‐plastic creep processes create deformation‐induced dilatancy gradients that dominate over buoyancy forces at all scales in the melt source. Brittle and ductile deformation therefore cooperate in the extraction of melt. Enhanced porosity and permeability in ductile shear zones result in lower fluid pressure, providing a potentially important driving force for melt migration and drainage ‘up’ shear zones and along larger scale fluid pressure gradients in the crust.

The effect of axial stress in maximum sustainable fluid pressure in Andersonian and non-Andersonian crust: A field-based numerical study from the Southern Andes (39°S)

Fracture opening at low differential stress controls maximum sustainable fluid pressure (λ) within cohesive brittle crust. Standard Andersonian stress states occur when two conditions are met: (1) one of the principal stresses σ1≥σ2≥σ3 is vertical, and (2) failure occurs at optimal orientations so that the stress tensor shape ratio φ=(σ2-σ3)/(σ1- σ3) is irrelevant. Here we explore the role of φ-values (axial compression, triaxial stress and axial tension) on sustainable fluid pressure driving rock failure under general stress states. We analyzed two exposures representing tectonics of the Southern Andes. Calculated failure curves in λ-depth space indicate that the hydrostructural behavior of general stress states is governed by the steepest of the principal stresses and the φ-value. Generally, hydrostructural behavior falls within standard Andersonian λ-depth conditions. However, field examples suggest that non-Andersonian axial stresses may sustain fluid pressures that depart from the standard Andersonian condition: the lowest fluid pressures occur under subvertical axial compression and subhorizontal axial tension; and the highest fluid pressures occur under subvertical axial tension and subhorizontal axial compression. Since around 15% of global stress compilations correspond to one of these categories, it follows that a significant portion of tectonic regimes potentially define a hydrostructural infrastructure different from standard Andersonian crust.

Heat and fluid flow dynamics of a stratovolcano: The Tacaná Volcanic Complex, Mexico-Guatemala

Mexico is the world's sixth-largest geothermal energy producer. To pursue its transition to renewable energy, other potential geothermal areas are under exploration. The Tacaná hydrothermal system is one of them. In this work, we developed a preliminary 2D numerical mass and heat flow model of the natural state of the Tacaná volcano in order to gain insight into fluid and rock properties at depth. A comprehensive set of simulations was performed involving various rock permeabilities, geometries, and upper and basal boundary conditions until the simulated data matched the measured data, including spring and fumarole temperatures at the surface and thermal spring discharge. Simulation results show that the temperature structure and the phase distribution at depth are partly controlled by the permeability distribution inside the volcano together with a low topography-driven downflow and the absence of elevated fluid pressures at shallow levels. The low topography-driven recharge may be related to a low-permeability layer, whereas low fluid pressures most probably related to a relatively deep water table. The deep thermal fluid source must be connected to the San Antonio fumarolic field by a high-permeability conduit to enhance the fluid upflow and the development of a boiling zone in the uppermost part of the cone. Furthermore, our results suggest that the (almost) total absence of hydrothermal manifestation in the NE part (Guatemala) of the volcano is controlled by the tilting of the basement rocks. Our model will be used as initial conditions for the modelling of several geothermal exploitation scenarios in future work.

Effect of Fluid Pressure and Pore Structure on Tight Sand Gas Saturation—Evidence from Micro-CT Simulation Experiment

Summary Numerous technical papers have been published on the reservoir evaluation of tight sand gas in recent decades. It is believed that tight sand gas is characterized by low permeability, complex pore structure, abnormal pressure, and low gas saturation. Researchers have characterized tight gas in terms of, for example, statistics; and some of them are increasingly interested in the effect of fluid pressure and pore structure on gas saturation. However, there is still a lack of effective experimental and mechanism analysis. The focus of this paper is to investigate the effect of two factors on controlling the tight gas saturation. A method was proposed for determining the in-situ injection pressure of the DB 2 gas reservoir in the Kuqa area during the gas-accumulation period, and the result was 1.2 MPa. A polyetheretherketone carbon-fiber core holder (maximum temperature 180 °C, maximum confining pressure 30 MPa) was designed for the microcomputed-tomography (CT) simulation experiment. After that, four physical simulation experiments of different fluid pressures (0.1, 0.5, 1.5, and 5.0 MPa) were implemented using X-ray CT and the new core holder to study the effect of fluid pressure and pore structure on gas saturation. The results show that the fluid pressure and micropore connectivity are important factors for gas-saturation increase; the number of gas clusters (connected pores containing gas) increases with the increase of fluid pressure and the maximum volume of gas clusters increases with the increase of pore connectivity. The connected pores with the largest volume are an important source of gas-saturation contribution. The three types of modes (the blowing balloon mode, stitching mode, and composite mode) of microscopic gas-cluster formation are discussed. The blowing balloon mode is key to gas-saturation growth in the period of low fluid pressure or the early stage of the injection process. The stitching mode is key to gas-saturation growth in the period of high fluid pressure or the late stage of the injection process. The above research system investigates the quantitative control of fluid parameters (pressure) and reservoir parameters (pore structure) on gas saturation, which might change the situation where previous research was too focused on the pore-structure and fluid-pressure characterization, with a lack of quantitative control analysis of reservoir gas saturation. It provides a valuable reference and framework for the reader to conduct further quantitative research on tight gas saturation.

Developing a low-fluid pressure safety valve design through a numerical analysis approach

PurposeThis study aims to develop a new design for the fluid-safety valve to make it more environmentally friendly.Design/methodology/approachComputational fluid dynamics is carried out to analyse the behaviour of flow in both traditional and new safety valves.FindingsThe possibility of failure in the new design under the maximum allowable working pressure is analysed using finite element analysis.Originality/valueInvestigating a new low-fluid pressure safety valve design.

Flapping motion of a permeable flag in uniform flow

The flapping motion of a permeable square-shaped flag is studied using an incompressible smoothed particle hydrodynamics method together with a newly proposed permeable flag model. Wind tunnel experiments are also performed, and the results are compared. The present model is successful in simulating two characteristic states depending on the uniform velocity, namely the ‘stretched-straight state’ and the ‘flapping state.’ Both the oscillation periods and the tail-end trajectories quantitatively agree with the experiments. However, the bistable region, which appears in the experiments, is not seen in the computational results. Except for the high-permeability case, the flags oscillate two dimensionally and no flow separation is observed irrespective of the permeability conditions. This might be due the flexibility of the flag in which the flag can change its shape in response to the low fluid pressure acting on the surface. A pair of longitudinal vortices rotating in opposite directions is generated at both spanwise ends of the flapping flag. Their strengths vary in accordance with the instantaneous lift force. When the flag is impermeable, these longitudinal vortices are connected to the spanwise vortex shed from the trailing edge and deform into a V-shaped vortex downstream. Conversely, the longitudinal vortices generated from a permeable flag simply move downstream without reconnecting. In addition, it is found that fluid permeation takes place at the location where the work by the fluid on the flag is concentrated.

An in-situ method of identifying ingredients of zeotropic refrigerants by inversion problem technique

An inversion method is applied to identify ingredients of zeotropic refrigerants in a circular duct. In the case of low Reynolds number and constant fluid pressure, the temperature distribution of direct heat transfer problem can be solved numerically. The thermophysical parameters of zeotropic refrigerants are determined by using inversion problem technique, the ingredients of refrigerants can be identified eventually. An in-situ experimental apparatus was proposed and three test samples with different composition refrigerants were conducted in this study. The experiment results show that the relative error of ingredients identification can be limited within 8.33%.

Supercritical CO2 behaviour during water displacement in a sandstone core sample

CO2 injection into underground formations involves the flow of CO2 in subsurface rocks which already contain water. The flow of CO2 into the target formation is governed mainly by capillary forces, viscous forces and interfacial interactions. Any change in subsurface conditions of pressure and temperature during injection will have an impact on the capillary and viscous forces and the interfacial interactions, which, in turn, will have an influence the injection, displacement, migration, and storage capacity and security of CO2. In this study, an experimental investigation has been designed to explore the impact of fluid pressure (74–90 bar), temperature (33–55 °C), and injection rate (0.1–1 ml/min) on the dynamic pressure evolution and displacement efficiency when supercritical CO2 is injected into a water-saturated sandstone core sample. The study also highlights the impact of the capillary forces and viscous forces on the two-phase flow characteristics and shows the conditions where capillary forces or viscous forces become dominant. The authors are not aware of similar experimental studies conducted in the literature so far. The results revealed a moderate to considerable impact of the parameters investigated on the differential pressure profile, cumulative produced volumes, endpoint CO2 relative (effective) permeability and residual water saturation. The extent of the impact of each parameter (e.g. fluid pressure) was a function of the associated parameters (e.g. temperature and injection rate). Increasing fluid pressure caused the differential pressure profile of supercritical CO2-water displacement to transform to the likeness of liquid CO2-water displacement, while, increasing temperature transforms it to the likeness of gaseous CO2-water displacement. Increasing fluid pressure caused a considerable reduction in the maximum and quasi-differential pressures, an increase in the endpoint CO2 relative permeability (KrCO2) and a reduction in the residual water saturation (Swr) and cumulative produced volumes. Overall, the impact of temperature is opposite to that of fluid pressure. However, with increasing temperature, the KrCO2 showed a declining trend at high-fluid pressures (90 bar) but an increasing trend at low-fluid pressures (75 bar). Increasing injection rate caused a considerable increase in the maximum and quasi-differential pressures, a rise in the KrCO2, a reduction in the Swr, and an increase in the cumulative produced volumes. The Swr was in range of 0.34-0.41 while KrCO2 was less than 0.37, depending on the operational conditions. Changing the operational conditions caused a higher impact on KrCO2 than that on Swr. The results indicate that capillary forces dominate the multiphase flow characteristics as fluid pressure and temperature are increased.